NO317270B1 - Method and apparatus for testing a formation fluid sample obtained from a geological formation pierced by a well - Google Patents

Description

This invention generally relates to a method and apparatus for geological formation testing, and more particularly to methods and apparatus for providing real-time simultaneous measurements of the response to changes in pressure, volume and/or temperature in fluid samples from geological formations pierced by boreholes for performing on-site tests for determining factors that affect the formations' potential productivity, and for obtaining samples of formation fluid in sample tanks under conditions of controlled pressure for later laboratory analyses. Such on-site investigations include determination of fluid bubble point pressure at formation depth, and under formation conditions, and determination of fluid compressibility and use of same for reservoir characterization using plots of bubble point pressure and fluid compressibility profiles.

Background for the invention

The sampling of fluids or fluids found in geological formations provides a method for formation testing and zones that may be of interest by obtaining a sample of any fluids present for later analysis in a laboratory environment, while doing minimal damage to the tested formations. Formation sampling is primarily a spot test of a geological formation's possible productivity. In addition, a continuous recording of the sequence of events at the surface is made. From this recording, valuable information can be obtained for reservoir analysis on formation pressure and permeability data in addition to the determination of fluid compressibility and relative viscosity.

Previous instruments for formation fluid sampling, such as that described in US Patent No. 2,674,313, were not commercially successful because they were limited to a single test for each trip downhole. Later instruments were suitable for several types of tests, however, the success of these tests depended to some extent on the characteristics of the particular formations to be tested. For example, a different sampling device had to be used if the soil formations were unconsolidated than if they were consolidated.

Multitest instruments for downhole use have been developed with sampling probes that can be extended to engage the borehole wall at the formation of interest to extract fluid from it and measure pressure. In instruments of this type for downhole use, it is typical to provide an internal drawdown piston that is hydraulically or electrically driven after the attachment to the borehole wall, and to increase the internal volume of the fluid receiving chamber inside the instrument. This action reduces the pressure in the contact area between the formation and the instrument, causing fluid to flow from the formation into the tool's fluid receiving chamber. Until now, the pistons have only achieved suction when they have moved in one direction. On the return stroke, the piston simply sends the formation fluid back through the same orifice through which it was drawn in, thus achieving no pumping activity. In addition, one-way piston pump systems of this type can only move the pumped fluid in one direction, which makes the sampling operation relatively slow.

Previous multitest instruments for use downhole were not equipped with options for essentially continuous pumping of formation fluid. Even tools with large capacity have so far been limited to a maximum drawdown collection possibility of around lOOOcc and have not so far had the option of selectively pumping different fluids to and from the formation, to and from boreholes, from boreholes to the formation or from formation to boreholes.

US patent 4,513,612 describes an apparatus and method for formation testing at multiple flow rates which allows relatively small tapped volumes to be sent down the well or to be pushed back into the formation. The use of "passive" valves in this method precludes backflow. This method allows limited or one-time backflow, much in the same way as an injection cannula, but transfer of large volumes of fluid between two reservoirs in a near continuous manner cannot be achieved with this method. It is therefore desirable to provide a sampling tool for use downhole with an increased pumping capability with an unlimited possibility of passing formation fluid down the well and with the possibility of achieving two-way fluid pumping to allow flow in the opposite direction, thus allowing fluid can be transferred to and from a formation. It is also desirable to provide a test instrument for use downhole with the possibility of selective pumping of different fluids such as formation fluid, known oil, known water, known mixtures of oil and water, known gas/liquid mixtures and/or completion fluid for this to allow in-situ determination of formation permeability and relative viscosity and to control the effect of a particular formation treatment fluid on the producibility of formation fluids present in the formation.

In all cases known to date, multitest sampling devices for use in boreholes comprise a fluid circuit for the sampling system that requires formation fluid extracted from the formation, together with foreign elements such as fine sand, rock, filter cake, etc. which is struck by the sampling probe, to be tapped into a chamber of relative volume, and which is sent into the borehole when the tool is closed, as in US Patent 4,416,152. Before it closes, a sample can be allowed to flow into a sampling tank through a separate but parallel circuit. Other methods provide the sample to be sampled through the same fluid circuit.

US Patent No. 3,813,936 describes "valve element 55" in column 11, lines 10-25 which forces trapped well fluids in a "reverse flow" through a filter element when "valve element 55" is withdrawn. This limited volume of reverse flow is intended to wash the filter element and cannot be compared to the bi-directional flow in this specification due to the limited volume.

In US patent 3,209,588 a known sampling system is described which shows a well tool and a well wall probe and a well wall gasket. The well tool has test fluid chambers and piston pumps to inject test fluids into the well wall, then draw formation fluid into a test chamber while performing simple tests on it. The well tool does not have a two-way piston pump.

The following publications show additional prior art examples: US Patent No. 4,782,695 to Glotin et al., US Patent No. 3,492,946 to Martin, US Patent No. 3,611,799 to Davis, US Patent No. 3,657,925 to Gross, GB 2,022,554 to McConnachie, and WO 91/12411 to Massie et al.

The mud filtrate is forced into the formation during the drilling process. This filtrate must be flushed out of the formation before a true, uncontaminated sample of formation fluid can be collected. Previously known sampling devices have a first sampling tank for collecting filtrate, and a second for formation fluid. The problem with this procedure is that the volume of filtrate to be removed is not known. For this reason, it is desirable to pump formation fluid that is contaminated with filtrate from the formation until uncontaminated formation fluid can be identified and produced. Conventional borehole testing instruments do not have an unlimited possibility of fluid pumping and therefore cannot ensure complete flushing of filtrate contamination before sampling.

Estimates for formation permeability are routinely carried out with pressure changes produced with one or more drawdown pistons. These analyzes require that the viscosity of the fluid moving during pumping is known. This is most easily achieved by injecting a fluid of known viscosity into the formation, and comparing this viscosity with the treated formation fluid.

A reversible pump direction will also make it possible to inject the fluid from the tool or borehole into the formation. For example, treatment fluid stored in an internal tank or chamber in the instrument or tapped from the well may be injected into the formation. After the injection, further tapping and/or sampling can be done to determine the effect of the treated fluid or completion fluid on the producibility of the formation. Early formation sampling instruments have not been equipped with features to determine optimal sampling pressures. The present invention also provides a positive method for overcoming uneven sealing at the gasket.

Determining whether thin formation layers are truly connected is an important issue when completing an oil/gas well for production. This is currently done using pressure-gradient plotting, however, when the formations are at the same depth, the pressure-gradient plots are not necessarily unambiguous.

An alternative to increasing the accuracy of the depth measurements is to examine other physical characteristics that may also be correlated with joint or separate formations. The development of pressure, volume and temperature measurements down in the boreholes using devices and methods as described here can produce these characteristics in an efficient way.

Bubble point pressure is a physical property of the formation fluid which is defined as the pressure at which gas begins to be released in a liquid/gas mixture and a gas bubble begins to form in a fluid at constant temperature. The bubble point pressure is determined by keeping a fluid at a known temperature and by observing the pressure changes when the volume of the fluid changes. A plot of volume versus pressure will indicate the fluid's phase change from one phase (liquid) to two phases {liquid and gas). The intersection of the fitted (best fit) "volume" and "pressure" lines in the plot indicates the bubble point pressure of the sampled fluid.

As soon as an oil field is discovered, it is important to know details about the fluid's content and the geological boundaries of the reservoir in order to maximize the extraction of oil products from the field. Most oil and gas fields are stratified and consist of several layers. Some producible oil fields can be 300 meters thick and others as thin as 30 cm. The different layers in the reservoir may contain the same fluid or may contain fluids of different origin, driven by the same pressure source or by different pressure sources. Determining which of these conditions apply before completing a well for production is very important for efficient extraction of hydrocarbons from the formation or formations drilled by the well.

Bubble point pressure data has been used for fluid content analysis using recombined fluid samples. Such data have so far not been used for reservoir characterization, principally due to the lack of opportunity to obtain representative samples in quantities, qualities and in a quantity that is necessary for interpretation of the reservoir. It is therefore desirable to obtain a PVT multitester instrument for use in boreholes and a method for performing bubble point fluid analysis and determination of fluid compressibility at formation depth and to use the results thereof for reservoir characterization.

The present invention overcomes the weaknesses of the prior art in on-site pressure, volume and temperature (PVT) measurement, and bubble point pressure and fluid compressibility analysis. These features are achieved using a PVT multitester instrument that has a double-acting, bi-directional fluid control system comprising a double-acting, bi-directional piston pump that can achieve pumping action in each direction of its linear motion and can, by its reciprocating pumping motion and valve action, a two-way flow. The PVT instrument for use downhole can also selectively empty formation fluid into the well or into containers for samples or pump fluid from the well or sample container into the formation.

For bubble point and fluid compressibility analysis, at least one of the pump chambers in the PVT multitester instrument is designed as a bubble point or fluid compressibility test chamber for valve-controlled space with a known volume of formation fluid. By accurately measuring the temperature and pressure of the trapped fluid, for example by using precision temperature and pressure sensors in contact with the test chamber, and by selectively changing the volume of the chamber by piston movement and accurate detection of the test chamber's volume changes, for example by using precision-controlled linear potentiometers, electronic signals representing fluid temperature and volume changes can be easily obtained and used for bubble point pressure profile plots for use in reservoir characterization.

It is a principle feature of the present invention to provide a new method for making pressure, volume and temperature measurements downhole on formation fluids extracted from a selected formation and for exerting pressure changes on the formation fluid for plotting PVT relationship curves which can be used for to determine the reservoir type for a sample is obtained.

It is also a feature of the invention to produce a new method for pressure, volume and temperature measurements in formation fluids present in geological formations of interest in which the bubble point pressure of the fluid is determined in order to set up the optimal sampling pressure for collecting samples representative of the state of the reservoir.

It is another object of this invention to provide a new method and apparatus for performing downhole testing of geological formations to enable identification of the fluid prior to obtaining samples, to determine bubble point pressures for optimal sampling conditions for determining the properties of the fluid under pressure -transient analysis, and for the acquisition of pressure, volume and temperature data downhole to check the validity of the samples at the surface, either on site or in reservoir fluid laboratories.

It is also an object of the invention to provide a PVT multitester instrument for downhole use which contains a defined test chamber and signal acquisition system for bubble point pressure and fluid compressibility analysis at formation depth to obtain effective bubble point and fluid compressibility profile plots for use in reservoir characterization.

It is another important feature of this invention to provide a controlled drawdown pressure or sample flow pressure to improve pressure transient analyzes in a drawdown and build-up test.

It is another important feature of this invention to provide control of injection pressure in fluid injected into the formation to improve pressure-transient analysis in an injection test.

It is another important feature of this invention to achieve pressure-transient analysis on the spot in the environment down borehole.

It is another important feature of this invention to determine the fracturing pressure of the formation, which is critical data for the stimulation of the formation.

It is another important feature of this invention to produce a new test apparatus for use down boreholes comprising a double-acting, two-way pump which is placed inside a drawdown module which has a pump-through option and in which the pump speed can be controlled and thus a controlled pressure drop, where total volume and pressure drop rate are such that the test being carried out is adapted to the formation's characteristics.

It is another feature of this invention to produce a new apparatus for downhole testing which includes a pump and valve mechanism which enables selective pumping of the fluid from the formation to the borehole so that all filtrate can be removed from the formation before sampling or obtaining other formation data such as such as pressure, volume and temperature (PVT), resistivity, compressibility, bubble point, relative viscosity, etc.

It is a further feature of this invention to provide a new downhole test apparatus comprising a double-acting, bi-directional pumping mechanism capable of reversing its pumping direction to allow fluid, such as completion fluid, to be injected from the instrument or downhole into the formation and which can make drawdown from the formation for sampling form to determine the effect of the fluid that is injected into the formation.

It is another feature of the invention to provide a new formation testing instrument with downhole draw down and pump through capabilities and having a double acting, two way pump and valve arrangement which can achieve selective pumping of unlimited volumes of formation fluid from the formation to the borehole or for collection tanks for cleaning or rinsing out debris such as sand, stone, filtrate etc. in the formation surrounding the well or on the interface between the sampling probe and the wall of the well to ensure the collection of clean, uncontaminated samples for testing.

It is a further feature of this invention to provide a new formation test instrument having a downhole double-acting, two-way drawdown pump mechanism which allows fluid to be injected from the tool or downhole into the formation to determine the effect of the completion fluid on the formation and its constituents.

In short, the various features of the invention are achieved by using the formation test instrument for use downhole which can pressurize an initially small, for example around 70cc, sample with fluid in a large pressure range, for example in the order of 20,000 psi. The sampling instrument can also depressurize the fluid sample in a controlled manner. This pressurization and depressurization capability allows the construction of PVT correlation curves that can be used to determine the type and other desirable characteristics of the reservoir fluid before a sample is obtained. PVT tests can be repeated an unlimited number of times until a clean formation sample is obtained.

The bubble point pressure and compressibility of the formation fluid are determined to set up the optimal sampling pressure for obtaining a sample that is representative of the reservoir conditions. At least one of the chambers in the two-way piston pumps in the instrument is provided with a shut-off valve to keep the fluid in the chamber, and is designated as a chamber for bubble point pressure and compressibility testing with a precision temperature sensor and a precision pressure gauge connected. The piston in the test chamber is connected to a precision potentiometer for the detection of volume changes in the test chamber. By thus selectively changing the volume of the test chamber, and the formation fluid that is in the chamber, and by observing the fluid's pressure when the volume changes, the fluid's bubble point can be observed using representative electrical signals. These signals can thus be used in the preparation of bubble point pressure and fluid compressibility profile plots that can be used for reservoir characterization.

The compressibility and viscosity of the sampled fluids are calculated from PVT curves, and are used in the calculation of the fluid flow parameters in the reservoir.

The main features of the PVT services are (1) to identify the type of fluid present before obtaining the samples, (2) to obtain the determination of the bubble point pressure downhole for optimal sampling conditions, (3) to produce bubble point pressure and fluid compressibility data to enable the preparation of bubble point pressure and fluid compressibility profile plot for use in reservoir characterization, (4) to achieve overpressure in the formation fluid sample to avoid phase separation on cooling and thereby open to determination of the fluid properties of the formation fluid for pressure-transient analysis, and (5) obtain PVT data from the borehole to verify the validity of the samples at the surface, either on site or in reservoir fluid laboratories.

The apparatus in the present invention performs pressure-volume-temperature (PVT) measurements downhole with a sampling probe in the wireline formation tester attached to the formation of interest. One of the purposes of the invention is to determine the bubble point of formation liquid/gas samples obtained from the formation. Another purpose is to perform bubble point pressure and fluid compressibility analysis on formation depth and conditions to prepare bubble point pressure and fluid compressibility profile plots for reservoir characterization. Before or after a sufficient amount of formation fluid has been drawn from the formation into either a tank or a borehole, the instrument performs measurements of pressure, volume and temperature in a limited sample of formation fluid. This feature can be achieved by a pump-through option achieved by a draw-down system with a pump-through option. This can also be done with a separate piston that does not pump through. The basic element of the drawdown system of the instrument is a two-way piston pump consisting of a double-acting piston and valves to control pumping with positive displacement, taking into account the direction and volume being pumped. At least one of the cylinders of the reciprocating pump is provided with a shut-off valve to trap a fluid and is designed as a test chamber for performing bubble point and fluid compressibility analysis downhole at formation depth and for use in reservoir characterization. Precision temperature and pressure sensors are connected to the test chamber and a precision linear potentiometer is connected to the piston for the detection of volume changes in the test chamber and the formation fluid trapped in the chamber during testing. The sensors and potentiometer output an electrical signal that is used in bubble point pressure and compressibility profile plots that are interpreted for reservoir characterization. Hydraulic pressure supplied from an onboard hydraulic pump is used to power the pump for piston movement. Solenoid valves, pilot valves and/or control valves are used to control the operation and direction of the piston pump and to control the fluid capture for the bubble point pressure and fluid compressibility analysis.

The primary pump element is a double-acting, two-way, positive displacement pump, which includes a reciprocating piston that provides pumping action in both directions of stroke. This differs from existing constructions of draining pistons where the draining piston only provides pumping action in one direction. This capability allows significantly faster pumping action when compared to conventional one-way pumping action. Conventional draining units provide pumping action in one of the piston's directions of movement, and an ineffective return for the other piston direction.

The pump unit can be set to pump from bottom to top, or from top to bottom. This pumping direction is achieved on site by controllable positioning of a four-way valve. This valve is controlled by an "on-board" {"vessel"-)

computer which is located in the downhole instrument.

The apparatus according to the present invention achieves a controlled draining rate. The pump displacement rate can be controlled, thus allowing a controlled rate of pressure drop, total volume or volume/second. This control allows improved test data by adapting the test to the formation characteristic.

The apparatus of this invention can also pump fluid from the formation of interest to the borehole. This can be advantageous when removing all filtrate, i.e. filter cake, from the formation before sampling or obtaining other data, such as PVT. Resistivity and/or capacitance can also be measured during pumping. The pump can be set to control the withdrawal rate of fluid in the drain module. The device can also pump in the opposite direction to the normal one, that is, from top to bottom, instead of from bottom to top.

The piston displacement can be measured acceptably in a number of ways available in the art. For example, the displacement of the piston can be measured with a position sensor by measuring the movements of the piston with a variable resistance or an inductive coil at the time an acoustic pulse needs to be reflected from one surface of the piston or at the phase change of a light beam that is reflected at one surface of the piston . In addition, the piston pump's volume changes can be calibrated in the environment down in the borehole and in relation to the mechanical limitations of the piston's movement in both directions. By accurately measuring piston displacement, full control over the pump can be achieved. The possibility of back pumping allows the pumping of an unlimited volume of fluid out of the sampling probe for the cleaning of residues such as fine sand, rock, filtrate, etc. that may be in the sampling probe. The reversible pumping direction also allows fluid to be injected from the tool or borehole into the formation. An example could be injecting completion fluid stored in a tank, or fluid taken from the well, or fluid that has previously been pumped from the formation, into the formation. After the injection of completion fluid, further tapping and/or sampling can be done to determine the effect of the completion fluid on the formation of interest.

The above-mentioned advantages and distinctive features are achieved with a method and an apparatus according to the patent claims set out below.

The present invention differs in particular from prior art in that it relates to the combination of a bidirectional pump, control circuit and control valves in the method and apparatus.

For a better understanding of the above-mentioned features of, and advantages and purposes of, the present invention, a more detailed description of the invention will be given with reference to the embodiments illustrated with the attached drawings.

It should be noted that the attached drawings only illustrate typical embodiments of this invention and therefore cannot be considered as limiting its scope, since the invention also allows other, equally effective embodiments. Figure 1 is a view, partially in section, of a formation test instrument constructed in accordance with the present invention and placed in a borehole in sampling position with respect to the formation of interest, and further shows a surface processing and control system using a schematic block diagram. Figure 2 shows a section of a portion of the downhole test instrument shown in Figure 1, and illustrates the two-way piston drawdown unit thereof, in position within the pump section in the main body of the instrument. Figure 3 shows part of the two-way piston pump mechanism i

figure 2.

Figure 4A is a partial view of a pilot-operated four-way formation fluid control valve assembly for selecting and reversing the pump direction of the two-way pump mechanism of Figure 3 shown in the normal position. Figure 4B is a partial view of the pilot-operated four-way fluid control valve shown in Figure 4A, with the valve mechanism shown in the pilot-operated position thereof. Figure 5 is a hydraulic schematic illustration of the two-way piston pump mechanism in Figure 3 together with a hydraulic control circuit for changing the direction of pumping while the sampling instrument is positioned in the borehole. Figure 6 is a partial hydraulic schematic illustration representing the addition of a pilot-controlled valve for delimiting a sample for PVT testing and illustrates a solenoid valve control for positioning the valve. Figure 7 is a hydraulic schematic illustration of the fluid circuit for operation and control of the double-acting, two-way formation tapping and pumping system of this invention. Figure 8 is a graphical presentation illustrating the use of pressure/volume plots to determine the fluid's bubble point and compressibility. Figure 9 is a graphical presentation illustrating the use of bubble point pressure/formation depth plots to identify different geological production zones. Figure 10 is a graphical presentation illustrating how adding compressibility/depth plots can be used to identify different geologic fluid production zones with the same fluid pressure. Figure 11 is a mechanical and electrical schematic illustration of a part of the two-way piston pump mechanism according to the invention which consists of a test chamber and system for performing bubble point pressure and fluid compressibility analysis at formation depth.

With reference to the drawings in more detail, in particular figure 1, where there is schematically illustrated a section of a borehole 10 that pierces the soil formations 11, shown in vertical section. A sampler and measuring instrument 13 is placed inside the borehole 10 by means of cable or wireline. The sampler and measuring instrument comprises a hydraulic power supply system 14, a section 15 for storing the fluid sample, and a section 16 for the sampling mechanism. The sampling mechanism section 16 includes a selectively extendable arm 17 with a pad to be brought into contact with the borehole wall, a selectively extendable arm with a fluid-acquiring sampling probe 18, and a bi-directional pump device 19. The pump device 19 can also be placed over the arm with the sampling probe 18 if is desirable. The sampling probe 18 will comprise one or more gaskets, as shown schematically at 25 in Figure 7, for connection to the well wall and isolation of the passage for the fluid intake from the borehole pressure, which is typically significantly higher than the formation pressure. Any other suitable packing systems can be used to isolate the sampling probe from the borehole pressure and in this way ensure that the formation pressure, or pressure that is only slightly different from it, can be collected through the sampling probe to the various fluid circuits in the multitest tool.

In use, the sampling and measuring instrument 13 is positioned inside the borehole 10 by unwinding or unwinding a wire 12 with a winch 20, around which the cable 12 is wound. Depth information from the depth indicator 21 is connected to the signal processor 22 and the recorder 23 when the instrument 13 is placed next to the soil formation to be investigated. Electrical control signals from the control circuits 24 are transmitted through electrical conductors inside the cable 12 to the instrument 13.

These electrical control signals activate an operational hydraulic pump within the hydraulic power supply system 14, shown schematically in Figure 7, which supplies hydraulic power that moves the pad-provided arm 17 and the fluid acquisition arm 18 laterally from the instrument 13 to engage the soil formation 11 and the two-way pump device 19. The fluid-acquiring arm or sampling probe 18 can then be moved in with a fluid connection to the soil formation 11 isolated from the borehole pressure. After such a probe/formation connection is established, electrical control signals from the control circuits 24 will selectively activate the magnetic valves within the instrument 13 obtaining samples from the formation.

Referring to the partial section shown in Figure 2, there is illustrated a bidirectional pump section with two ends, in the sampling instrument, shown generally at 30, which would typically comprise a pump section 19 in the multitest instrument. The pump section 30 comprises a double-acting, two-way fluid control system with the possibility of injecting a fluid medium such as, for example, completion fluid into the formation in question, and for draining formation fluid from

the formation. This feature allows downhole testing of the effect of a particular completion fluid on the formation

which is intended to be completed for production. This apparatus can also pump injection fluid either from the well or from a reservoir inside the instrument and inject the fluid into the formation. In addition, the double-acting, two-way fluid control system can draw formation fluid from the relevant formation and selectively discharge the fluid into the well, for example to clean the system of contamination, such as fine sand, rock, filtrate or other foreign elements or selectively pass the obtained formation fluid through the instrument's sampling system. The two-way fluid control system is also designed to define a test system for performing bubble point pressure and compressibility tests at formation depth. These and other features of the invention will become clear when the preferred embodiment of the invention is described in detail below.

The pump section 30 defines an elongate main body 32 which is intended to be used alone or in conjunction with other sampling instrument components. This main part will contain suitable cavities and passages to accommodate the pump circuit illustrated in figures 5, 6 and 7.

As shown in Figure 2, the main part 32 defines a pump cavity 34 within which is placed a two-way piston pump shown generally in 36 and shown in more detail in Figure 3. The piston pump 36 comprises an elongated pump part 38 with at its upper and lower ends connecting members 40 and 42 are sealed relative to the end members 72 and 74 by annular seals 41 and 43. The end connecting members 40 and 42 are prevented from moving relative to their sealed connection to the end members by the main member 32 or by other suitable means, and are connected to tubular projections 44 and 4 6 thereof, which are connected to corresponding parts of the main part 32 in a tight manner. The pump member 36 is also supported within the cavity or space 34 by means of support clamps 48 and 50 which are received within reduced diameter portions 52 and 54 of the pump cylinder 38 and are attached to the main member 32. Each of the support clamps act as connecting rings and act as supports for quick disconnects 56 and 58 in sealed connection with the main part 32 and the pump cylinder 38 so that a fluid connection between the pump part-operated fluid ports 60 and 62 and corresponding fluid passages 59 and 61 in the main part 32 as shown in Figure 2. The middle part of the pump cylinder 36 is also attached to the main part 32 by by means of a fastening screw or bolt 64 which extends through a bolt opening in the main part 32 and is threadedly engaged with the pump cylinder 38.

The pump cylinder 38 comprises an intermediate part 66 which is most clearly shown in figure 3 with internally threaded end parts 68 and 70, to which threaded parts of the end parts 72 and 74 are attached. The end parts are sealed in relation to the middle part 66 by means of ring-shaped seals 76 and 78 respectively.

The intermediate part 66 and the end part 74 together form an elongated internal pump chamber 80 with a cylindrical pump chamber surface which forms a piston pump chamber. A piston 86 divides the pump chamber 80 into two pump chambers 83 or 84, with variable volume on the respective sides of the piston 86. The piston is sealed against the cylindrical pump chamber surface 82 by means of an annular seal 88 and is moved back and forth. within the pump chamber of hydraulic fluid selectively injected through pump chamber ports 60 and 62 into respective pump chambers 83 and 84. On opposite sides of piston 86 extend piston rods 90 and 92, each with respective high pressure seals 94 and 96 sealing against respective internal cylinder surfaces 98 and 100 which respectively define pump chambers 102 and 104. These pump chambers with respective tubular coupling elements or projections 4 6 and 106 which provide tight fluid exchange between the respective pump chambers in the piston pump and the formation fluid flow passages defined by the instrument body 32. One of the pump chambers may be designated as a valve controlled test chamber , such as illustrated rt in more detail in Figure 11, for capturing a known volume of formation fluid and performing a bubble point pressure and compressibility test on the fluid so that the test results can be used in bubble point pressure and compressibility profile plots for reservoir characterization. The tubular connecting element 106 is sealed with respect to the pump part and the main part by means of annular seals. In this way, when hydraulic fluid causes the piston 86, and the piston rods 90 and 92, to reciprocate, the piston provides a suction and pumping effect on the formation fluid depending on the direction of the piston movement. When a piston in its working stroke to push formation fluid out of its chamber, the opposite piston will move so that it sucks, drawing formation fluid into the pump chamber for subsequent pump displacement.

The end portion 72 defines an internal position sensor chamber 110 which is sealed with respect to the pump chamber 102 by means of a partial seal assembly 112. A position indicator rod 114 extends from the end of the piston rod 90 and extends in a sealed manner through the partial seal assembly 112 into a internal reception defined by a position-sensing potentiometer or other suitable piston position sensor. The output signal from the precision potentiometer effectively determines the known test chamber volume and volume change for bubble point pressure and fluid compressibility testing. Electronic signals representing the position of the piston rod 114 within the potentiometer are transmitted via electrical conductors leading through the tubular coupling projection 44 to a suitable electrical circuit within the instrument body 32. In this way, as the piston 86 is moved back and forth within the chamber 84, the movements of the piston measured and transmitted electronically to where it can be used for pump control for pump calibration and for precision measurements of volume, such as for determining known volume and volume changes in

the test chamber.

The pumping action achieved by pistons 90 and 92 is relatively small, but pumping action can be achieved at a very high pressure, for example on the order of 20,000 psi, by controlling the relative pressure-responsive dimensions of pump 86 and pump cylinder floats 98 and 100. In the case of lower pressure where a higher pump volume is desired, a double-acting pump mechanism with a larger diameter of the pump cavity and pistons than that shown in Figure 3 can be used. In fact, changing the pump capacity of the modular pump section can be easily accomplished under field conditions, since the double-acting, two-way pump section 36 is preferably releasably fixed within the cavity or receptacle. After the protective cover 120 has been removed, the pump part 36 will emerge, and can be easily and quickly removed using readily available tools before being replaced with a pump with a different pump capacity. Thus, if a different pump capacity is desired or if there is a need for repair or replacement for other reasons, this can be easily and efficiently done even under field conditions in just a few minutes, using commonly available tools. This feature also allows a downhole multitest instrument without bubble point pressure testing capability to be easily converted simply by replacing the bidirectional pump mechanism. Sealed fluid connection lines between the pump part 36 and respective fluid passages in the main part 32 can be of the plug-in type so that the installation and removal procedure for the pump is simplified. Each of the end connection parts 40 and 42 is provided with plug-in type connection projections as shown at 44 and 46 which are each provided with annular sealing elements for producing a seal inside the respective holes in the main part. Similarly, the intermediate part of the piston pump is provided with a tubular coupling element with sealing rings in adapted recesses to form a tight connection between both the intermediate part of the pump and the main part 32.

To enable a reversal of the pumping direction of the two-way piston pump 36, two pilot-operated control valve units 168 and 170 are arranged as illustrated schematically in figure 5, and in section in figures 4A and 4B. These control valves will be referred to herein as "dirty fluid control valves" since they are used to control the directional pumping of sampled formation fluid and injection fluid through various fluid passages in the instrument. Although the control valves 232 and 234 have a slightly different construction, they are mounted in a valve tube which can be moved hydraulically under selective control from the surface, for example, with the control circuits 24 or is moved between operative positions under programmed control with control circuits which are either located within surface-based circuits or within the instrument itself. As a basic design, the dirty fluid control valve assembly is generally of the form shown in conjunction with Figures 4A and 4B. The control valves for dirty fluid shown generally at 168 and 170 in Figure 5 are hydraulically operated four-way control valves whose function is to change the pump-through direction of the sampling instrument. A two-way normally open hydraulically actuated valve 170 shown in Figure 6 is used to enable PVT testing with the on-site instrument.

Referring now to Figure 4A, a representative example of a dirty fluid valve is illustrated, which is a pilot-operated four-way valve. Body 32 defines a plurality of fluid passages marked as indicated, which passages are located in connection with a transverse valve channel 188. At the junction of the respective passages with the valve channel, the valve channel is expanded to define annular fluid conducting recesses as shown at 190 A valve seat sleeve 192 is positioned within the channel 188 and defines a shoulder flange 194 which is mounted against a circular internal stop flange 196 defined by an extension at one end of the channel 188. The valve sleeve is sealed to the main body by means of an annular sealing element 198 which engages with the stop flange 196. At its opposite end, the valve sleeve 192 is sealed with respect to the main part 32 by means of an annular sealing element 200 which is positioned against a part of the cylindrical internal sealing surface defined by the valve channel 188. A valve stop - cover 202 comprises an externally threaded part 204 which is received by an internally threaded d el of the valve sleeve 192 and defines an annular stop flange 206 that engages an internal annular shoulder 208 to lock the valve sleeve in a tight, static position within the valve channel 188. The valve cover is sealed with respect to the valve sleeve 192 by means of a circular sealing member 210 which establishes a tight engagement with one wall of the valve sleeve. The valve sleeve has a cylindrical configuration and defines an internal, cylindrically segmented sealing surface 212 that is interrupted by a plurality of internal recesses 214 for fluid transfer and that form an internal valve chamber 216 within which is positioned a generally cylindrical valve tube member 218. The valve tube member is sealed relative to the internal cylindrical sealing surface segments 212 using a plurality of circular sealing elements 220.

The valve tube element 218 as shown in Figures 4A and 4B is pressed against one direction by means of a compression spring 222 with one end thereof in force transmitting contact with the valve tube element 218 and with the other end retained in the closing and support element 224 which provides a threaded contact with the valve sleeve in 226. An annular sealing member 228 located in a circular recess in a radial sealing flange portion 230 of the closure and support member 224 establishes a tight connection to an annular planar end surface 231 of the valve sleeve. It should be remembered that the valve tube element can be unsprung so that it is not pressed by any mechanical force. Alternatively, a corresponding valve can be equipped with a valve tube element which is hydraulically moved in one or the other direction and which is releasably fixed in each position by means of latches. In the version of the invention shown in Figure 4A, valve tube member 218 defines an internal control valve chamber within which a pair of control valve members 232 and 234 are shown as a schematic illustration. The control valve elements 232 and 234 are fixed inside the valve by means of a pair of control valve stoppers 236 and 238 which are in threaded engagement with external, internally threaded parts of the control valve receivers. The control valve stoppers 236 and 238 are sealed relative to the valve tube element 218 by means of external annular sealing elements provided in suitable sealing recesses.

As also shown in Figure 4A, the valve tube member 218 is shown in position to obtain flow from the packing to the borehole or to a sampling tank or container located internally within the instrument. Therefore, when the piston pump is used, movement of the bidirectional piston in one direction will draw a stream of production fluid from the packing, tank or other sources into the pump chamber. Since the packing can usually be sealed against the formation, this fluid is usually filtrate or formation fluid. If the gasket is not attached, well fluid will be pumped. By attaching a cup-shaped elastomer seal around the tool between the seal and the well drain, drilling mud can be pumped into the two-way piston pump to push the tool up or down the hole. This two-way pump is the first practical way to develop enough pressure (100 psi is roughly equivalent to the weight of the tool in a 6-3/4 hole) to lift or otherwise move a tool that has become stuck (due to pressure differentials ). The output of the piston pump can also go to two inflatable packings before being used to pump fluid to or from the "packing-closed zone". By adding tanks with large volumes, small-scale drill string tests can be made possible. US Patent No. 4,535,843 shows quite generally the pumping of fluid, but shows no practical way of constructing or controlling such a pump. When the direction of the piston is reversed, this recovered formation fluid is either pumped to the well, for example to wash away fine sand, rock, mud cakes or other waste that is present in the connection point between the sampling probe and the well wall at formation depth. After all the waste has been flushed down the well, the pumped flow is changed so that formation fluid from the sampling probe enters one or more sampling containers for on-board storage, to later dispose of it downhole or to finally transfer it to the surface for laboratory testing.

The valve device also makes it possible to achieve pumping of fluid components, such as completion fluid, oil/water mixtures, either from a fluid reservoir in the instrument or drilling mud from the borehole directly into the formation. This feature enables efficient testing of the formation with a completion fluid, so that the effect of the completion fluid can be determined before the actual completion of the well or determination of relative permeability to fluid of known viscosity. Reversal of the direction of the fluid pumping is achieved by applying hydraulic pressure from an operational pressure source located in the instrument on the pump pressure inlet passage shown in the upper left of Figure 4B. Hydraulic fluid under pressure thus pushes the valve tube member 218 downward, compressing the spring 222. Fluid present in the valve chamber 216 will be directed through the valve sleeve to the hydraulic reservoir passage located at the lower left of Figure 4B, for return to the hydraulic pump sump . In the position shown in Figure 4A, operation of the two-way piston pump mechanism will initiate a flow through the packing/sampling passages of the valve mechanism and through the control valve 232 as the pump piston moves one way. When the pump piston turns, the flow is sent through the opposite control valve 234 thereby forcing the collected formation sample through the wellbore or tank passage shown in the lower right of Figure 4A. In the valve position shown in Figure 4B, the pumped fluid will flow through the borehole/tank passages in the valve mechanism and through control valves 232 and 234 to the packing/sample passages in the main body of the instrument as shown in the lower left of Figure 4B. Flow of collected formation sample to the borehole or sample tank is controlled by suitably electronically selected electrical or hydraulic movement of the valves.

Now referring to Figure 6 there is shown a simplified, schematic illustration of a portion of the downhole instrument for performing pressure-volume-temperature (PVT) measurements downhole with a wireline formation tester while is attached to the formation. In cases where varying attachment is a problem, the sample can be taken into a tank, after which the tool can be closed and moved slowly up or down the borehole while the PVT analysis is carried out on the fluid in the sampling tank. One of its purposes is to determine the bubble point pressure and fluid compressibility of liquid/gas samples obtained from a selected formation so that the formation or formations can be characterized using this information. Before or after a sufficient amount of formation fluid has been drained from the formation and either into a tank or down a borehole, the multitester instrument can be controlled to perform measurements of a final formation fluid sample's pressure, volume and temperature. This is carried out using the double-acting, two-way pump mechanism with the possibility of pumping through. The simplified schematic illustration in Figure 6 includes a hydraulically controlled pressure supply pump 236 which supplies hydraulic fluid pressure through a pilot pressure supply line 238 under the control of two solenoid valves 240 and 242 and a control valve 243. These normally closed solenoid valves are selectively controlled to direct the flow of hydraulic fluid from the hydraulic pump 236 to a normally open two-way dirty fluid valve, shown generally at 171. A portion of the two-way pump mechanism of Figure 3 is illustrated, showing one of the piston rods 92 which can move back and forth in the piston chamber 104. The pressure and temperature sensors "P" and "T" are connected to the piston chamber 104 to thereby allow on-site measurement of pressure and temperature in the formation fluid inside the piston chamber. Since the dirty fluid valve 171 is a normally open, two-way valve in its open position as shown in Figure 6, the pumped fluid from the piston chamber 104 is delivered through the valve assembly 170 to the control valves 232 and 234. When the piston rod 92, shown in Figure 6, moving to the left, it produces a suction effect in the pump chamber 104 which acts as though through the normally open valve 171, thereby initiating a flow of formation fluid through the sampling probe and packing line from the selected formation and through the control valve 232. When the piston rod 92 of the two-way pump mechanism moving in the opposite direction, the resulting flow is sent through the dirty fluid valve 171 and control valve 234 to the tank or borehole line. The flow of fluid to the sample acquisition tank or well is selected using a magnetic control valve to actuate a two-way dirty fluid valve located in another part of the instrument.

As shown in the simple schematic illustration in Figure 5, the two-way pump mechanism is illustrated generalized at 36 with the pump chambers 102 and 104 connected in fluid communication to the dirty fluid control valves shown generalized at 168 and 170. The two-way piston pump is arranged to pump either from the pump chambers 102 and 104 to the packing line or to the wellbore storage tank line, depending on the position of the dirty fluid control valves, as controlled by the positions of the respective solenoid valves 240 and 242 in the pilot pressure supply line 238. For reciprocating movement of the bidirectional piston pump mechanism, directional control valves 244 and 246 are selectively opened by an electrical control circuit, and pump pressure is selectively directed to pump pressure supply lines 248 and 250. Control valves 252 and 254 are located in the return line circuits to direct hydraulic fluid from the respective variable volume pump chambers 83 and 84 to the hydraulic storage reservoir t il the hydraulic supply pump 236 and is selectively opened by pressure via the dashed pilot pressure lines 253 and 255. A pair of directional flow lines 256 and 258 are connected respectively to the packing line and to the wellbore or supply tank line and effect the routing of formation fluid or completion fluid from the respective pump chambers 102 and 104 in a direction selected by the position of the dirty fluid control valve assembly 168 and 170. By simply changing the pump direction, the two-way two-way pump 36 can pump fluid either into the formation or out of the formation, and pump collected formation fluid either into in a collection container or into the well. These features provide a significant advantage in downhole testing flexibility.

An alternative method of reversing the flow is illustrated in Figure 7. In this case, a four-way valve switches the inlet and outlet lines to permanently mounted dirty fluid control valves. The hydraulic circuit of the wireline downhole test instrument is illustrated schematically in Figure 7 and shows a hydraulic fluid supply line 288 which is the discharge line for the hydraulic fluid pump "P" which is driven by an electric motor "M". The electric motor is driven and controlled through suitable electrical circuits with surface-based equipment as shown in Figure 1. The pump "P" draws its hydraulic fluid from the sump "S" via the suction line 290. The pump and motor are preferably located inside the hydraulic reservoir, which is a sump or sump to act as a cooler, but this is not intended to limit the scope of this invention. The symbol "292" where found in the hydraulic circuit represents the return of the hydraulic fluid to the sample sump "S".

Pressure inside the supply line 288 is limited by a pressure relief valve 294 which relieves excess pressure to the sump. Pressure in the hydraulic supply line 288 is selectively vented to the sump using a normally closed solenoid valve 296. The line pressure in the supply line 288 is selected for the desired pressure level for operating the various hydraulic circuits in the downhole sampling instrument by means of an electrically operated variable flow resistor 298.

For PVT testing of the formation in question, it is desirable to establish a connection between the sampling probe or the access part and the underground geological formation crossing the borehole. The symbol "300" represents the borehole at the formation level. Flow of fluid to or from the well is achieved by means of a sampling circuit 302 with a pilot-operated isolation valve 304 which is supplied with hydraulic pressure by means of a solenoid valve 306 in the branched hydraulic fluid supply line 308.

The fluid receiving part shown generalized at 18 comprises a movable sampling probe 310 which can move laterally from the instrument 13 with connection to a hydraulically activated probe moving piston 312 which has its piston part 314 received in a hydraulic cylinder 315 which is placed inside the instrument, and which is divided into divided into hydraulic chambers 316 and 317. The sampling probe 310 is hydraulically actuated independently of the piston 312. A sampling line 318 connected to the sampling probe's fluid passage is connected to a pilot operated four-way dirty fluid control valve assembly 168. This valve will , when positioned as in Figure 7, establish contact between the line 312 and the suction and discharge lines 327 and 329 via the control valves 334 and 344 depending on the direction of movement of the pump piston 86. The sampling circuit 302 is connected to the suction and discharge lines of the pump 36 via line 345 and control valves 342 and 346 through the control valve assembly for dirty f discussed below and is also connected to at least one sample collection tank 320 under control of a pilot operated valve 322 and an isolation valve 323. The pressure within the sample line 318 is detected by a pressure sensor 324 which may be an absolute pressure gauge as schematically illustrated or which may take another suitable form.

It is desirable that the test and sampling instrument for use downhole has the possibility to vary the pressure of the formation fluid away from the actual pressure in order to carry out certain tests at the location downhole, such as bubble point pressure and fluid compressibility tests, and also to carry out certain tests laboratory tests of formation fluid collected in sample tank 320. For example, by tapping fluid that has not undergone a phase separation from the formation at a pressure above the bubble point pressure, samples can be obtained and delivered to the laboratory for examination by elevating the pressure of the formation fluid to above its bubble point pressure to allow cooling to surface temperature without depressurizing the tank below the bubble point pressure. It is therefore desirable that the instrument can increase the pressure on the collected formation fluid to a pressure difference of approximately 20,000 psi for this purpose. To obtain pressure variations in the sample of formation fluid, also referred to herein as "dirty fluid", the pilot-operated four-way valve shown generalized at 168 is connected by selected, pilot-controlled connection to the sample lines 302 and 318. This valve receives its pilot pressure activation from the supply line 28 9 via the pilot pressure supply line 326 by controlling a solenoid valve 328.

The double-acting, two-way piston pump mechanism illustrated generalized at 36, and described above in connection with Figure 3, is connected to its respective pump chambers 102-104, with connection to the suction-discharge lines 327 and 329, respectively. The pump mechanism's pump pressure is measured by an absolute pressure gauge 324. For PVT analysis, pressure and temperature in the formation are measured by pressure gauge 330 and temperature sensor 332.

With the dirty fluid control valve 168 in its normally open position, as shown in Figure 7, the suction stroke of the pump chamber 102 will induce a flow of formation fluid through the sampling probe 310, the sampling line 318, the valve 168, line 343, the control valve 334, and the fluid flow line 327 to the pump chamber 102. At the same time, sending the double-acting piston pump 36 formation fluid from the pump chamber 104 and through the fluid flow line 329 when the pilot-operated normally closed control valve 336 is opened by means of pressure provided via the line 338 by opening the normally closed solenoid valve 340. The formation fluid, now under pressure due to piston pump system, flows through the control valve 342 and the connecting line 345 to the sampling line 302. With the pilot-operated valve 323 opened by activation of the normally closed solenoid valve 322 via the pressure supply line 325, the formation fluid under pressure will enter the sample tank 320 via the sample tank the opening 321. When the direction of the piston pump is reversed, the pump chamber 104 becomes the suction chamber and the pump chamber 102 becomes the pressure chamber. In this case, the flow of formation fluid from the sampling line 318 goes through the valve 168 and the control valve 34 4 and through the line 329 and the open valve 336 into the chamber 104. At the same time, the piston in the piston pump 36 creates a pressure in the pump chamber 102 and thus emits pressurized formation fluid through line 327 and the control valve 346 to the dirty fluid control valve 168 through line 345. The pressurized formation fluid is directed to the sampling tank 320 in those cases when the pilot-operated valve 323 is opened by means of pilot pressure through the line 325 when the solenoid valve 322 is activated in its open position. In this case, the pilot-operated valve 304 will be closed, thereby isolating the pressurized formation fluid from the formation. For sending out fluid that is pumped through the sampling line 302 to the well, the valve 304 will be opened by means of pilot pressure supplied through the open solenoid valve 306 while the tank supply valve 323 is closed.

As the piston pump 36 is driven, the position of the piston is continuously measured by the position sensor schematically shown at 114-116 in Figure 3. This feature allows measurement of piston displacement versus time of displacement and also allows precision measurement of a final test chamber volume and volume change for determining bubble point pressure and fluid compressibility. for the formation fluid. Precision pressure and temperature gauges 330 and 332 also provide precise pressure and temperature data to provide the instrument with precise measurements of the volume of fluid extracted from the formation. In addition, the double-acting two-way piston can be effectively calibrated under downhole conditions to achieve pumping at a rate determined by the production capability of the formation. To achieve this feature, the actuating pump system of the piston pump 36 receives hydraulic pump pressure via a branched supply line 348 by selectively opening and closing the pump actuating solenoid valves 350 and 352 to thereby selectively pressurize the pump operating supply lines 354 and 356. Of course , when one of the supply lines 354 or 356 is pressurized to pressurize one of the pump chambers 83 or 84 arranged with variable volume, the opposite pump chamber must be opened to avoid fluid displacement. This draft is achieved by means of a ventilation line 358 which is connected to the sump and which is controlled by selective opening of the solenoid valves 360 and 362. Thus the solenoid valves are electronically operated in a cyclic manner to achieve a selective operation of the piston pump 36. Also these valves are electronically selectively controlled to achieve precise reciprocating movement of the piston thereby achieving precision measurements of fluid volume. Since the exact position of the piston is known at all times, data indicating the piston movement is indicative of the rate of fluid pumping into the formation or the rate of fluid withdrawal from the formation. Precision detection of the piston's position also enables calibration of the pump down the borehole by electronic adjustment of the cyclic control and/or adjustment of the hydraulic fluid supply pressure, for example by varying the pressure control of the electronically controlled fluid flow resistor 298. By selective opening of solenoid valve 328 to pressurize pressure supply line 326, dirty fluid four-way control valve 168 will be shifted to its opposite flow direction position, so that pumped fluid under pressure from either pump chamber 102 or 104 will be delivered through control valves 342 and 346, respectively, to the connecting line 345 where it is passed through the switched dirty fluid control valve 168 to then flow through the sampling line 318 to the sampling probe 310 and then into the formation. The fluid that is injected into the formation can be retrieved from the sampling tank 320 via the open valve 323 and the sampling line 302, thereby avoiding the necessity of retrieving the sampling instrument for removal of aggregate samples. Furthermore, with valve 168 in its normal position, as shown in Figure 7, and with valve 323 closed and valve 304 open, samples of formation fluid being pumped can be sent into the well to thereby provide an almost unlimited flushing opportunity for to remove filtrate from the formation. By reversing the dirty fluid control valve 168 and with valve 304 open, collected formation fluid, perhaps with some test fluids such as completion fluid, can be retrieved from the well and fed back into the formation by the two-way pump mechanism through sampling probe 310. For this reason, formation fluid, sampling fluid or a combination of these, such as filtrate, is removed by pumping it into the formation into which it can be placed and into the fluid medium that the formation contains. Furthermore, the instrument can pump virtually unlimited amounts of test fluid from on-board tanks into the formation and then retrieve treated formation fluid for on-site testing and/or for laboratory testing.

For operation of the sampling probe, the activated probe-activating piston is driven by injection of pressurized fluid into the respective piston chambers 316 or 317 via respective supply lines 364 and 366. For this purpose, hydraulic pressure is directed via supply line 289 which contains pressure-regulated hydraulic oil. The flow resistor 298 regulates the pressure in the supply line 289 from the surface setting via the control circuit 24 and electronic feedback control to control the pressure detected by the supply line pressure gauge 400 in the branched supply lines 368 and 370 via the magnetically actuated control valves 372 and 374. When one of the piston chambers is supplied with a fluid under pressure, of course, the opposite chamber must be ventilated. For this reason, the vent circuit is connected across lines 368 and 370 and is vented to the hydraulic sump by selective opening of solenoid valves 378 and 380. A pair of pad actuating cylinders is provided, each with internal chambers connected to the supply lines. 386 and 388 for simultaneously supplying a selected cylinder chamber 385 or 387 with hydraulic pressure. The piston rods 390 and 392 on the pad-activating cylinders simultaneously provide power for operating the pad-equipped arm 17 which engages the well wall in Figure 1.

With the valve set as in Figure 4A, small changes in volume can be made (in the range of 5cc to 20cc, and preferably around 10cc). This change could be called a "draw down" test. In this test, which is usually carried out with conventional formation pressure testers, the pressure in the instrument can be measured with the gauge 324 shown at the bottom right of Figure 7 where the pressure is first lowered to below the formation pressure and then increased or "built up" towards formation pressure as flow from the formation changes the pressure in the volume of fluid located between the formation and the piston chamber 104 or 102. The larger this volume, the longer it takes for the pressure to "build up" to formation pressure. It is desirable to reduce the total time used for pressure testing, the volume between the formation and the displacement must be as small as possible. Practical trade-offs for the physical distance between the gasket and the pump means that a valve is needed immediately after the pressure gauge 324 and the gasket 25. By adding a pilot-controlled normally open two-way valve and a solenoid control valve 404, it is possible to stop the draining immediately flow from the formation has begun, allowing the pressure to recompress only the fluid between the valve 402 and the formation. The pressure gauge 324 is in this line to accurately measure the formation pressure. Since the piston displacement is also measured in relation to time and can be correlated with pressure, all data is available for a pressure-transient analysis when the valve 404 is closed.

Since the two-way piston pump can be used to repeat tests, one method may be to tap off in several very small volumes, such as lcc, and close valve 402 after each volume and see if the pressure increases or not. If not, the next increment could be fetched until build-up is observed. In each case, the volume to be built up is minimized so that the build-up time is minimized.

An alternative method of detecting that a flow has started is to monitor the pressure time response with gauge 324 until a plot of the derivative of the pressure with respect to time shows that a spherical flow has started (a negative slope is observed). The valve 402 is then closed, the pump stopped, and the pressure build-up observed. This ensures the acquisition of valid data with minimum disturbance from the formation.

With the sampling probe 310 in fluid-communicating connection to the relevant formation, the two-way pumping mechanism is operative and selectively controllable to extract formation fluid from the formation, to inject a fluid medium into the formation, pump the formation fluid thus obtained into a sampling tank or pump formation fluid into the well . The reversible piston pump 36 can retrieve a fluid from on-board fluid storage or from the well and inject it into the formation. This feature is particularly important when it comes to testing the formation with regard to the effect of a test fluid, such as completion fluid. This invention allows for the efficient pumping of fluids of different viscosity with the purpose of evaluating the formation characteristics as determined by the relative viscosity of the formation fluid and the injected fluid. The fluid's relative viscosity can be very important when it comes to formation productivity, including secondary fluid recovery from the relevant formation. The two-way piston pump according to this invention can also achieve pumping to or from the formation or well. Continuous pumping from the formation into the well can be done to achieve cleaning of the formation's interface for waste, such as filter cake, fine sand, stone etc. and thus allow the collection of a clean sample for testing down the borehole. All of these features can be achieved without removing the formation test instrument from the well. One or more important features of this invention is the ability to investigate formation pressure as a function of time, and the ability to correlate this with volume as a function of time. This possibility has so far not been available in test instruments for use down the borehole.

With reference to figure 8, the bubble point pressure is determined by closing fluid inside a room of known volume, and by observing the pressure variations when the volume of the fluid sample is changed. A plot of fluid volume versus pressure will indicate the transition from one phase (liquid), as shown at line 410, into two phases (liquid and gas), as shown at line 412. The intersection point between two best fits lines, such as shown at 414, indicate the bubble point pressure of the sampled fluid. Line 411 is representative of the variation in the compressibility of the fluid.

Fluid compressibility is calculated as follows:

there:

VI = Volume at high pressure

V2 = Volume at low pressure

Pl = High pressure

P2 = Low pressure

A typical room temperature value for water is 3.3xl0~<6>psi"<1> and for crude oil 60xl0~<6>psi<-1>, which illustrates that the compressibility can also be different for oil and water.

Now with reference to figure 9, which illustrates that when the bubble point pressure is determined at several depths in the oil well it will be expected that a plot of bubble point pressure against depth will be constant. When this is true, it is likely that the tests were carried out on the same fluid. However, if the bubble point pressure is different, then different types of fluids were tested and therefore two or more different reservoirs exist. For example, as shown in Figure 9, the bubble point pressure tests identify fluid from three different production zones A, B and C.

Now referring to Figure 10, where the graphical representation illustrates three different profile plots of the same geological formation in comparison, thus showing how the reservoir characterization can be incorrect or incomplete depending on the profile plot system used. By comparing the curves in Figure 10, a reservoir with several zones can be identified. The figure also indicates how plotting fluid compressibility against depth can additionally identify different geological production zones that have the same bubble point pressure. Curve "A" is representative of a conventional pressure gradient plot, which essentially defines a straight line, thereby establishing a pressure trend indicating the presence of a single production formation from which the sample fluid is flowing. It is easy to see that, starting from curve "A" alone, the true character of the reservoir cannot be recognized. Curve "B" represents the bubble-point pressure profile, which defines a sharply drawn offset at 420, thus providing evidence that the production fluid is flowing from two independent geological formations. Curve "C" represents a fluid compressibility (one phase) and indicates at sharply defined offsets at 422 and 424 that three different fluids are flowing from formations that define this particular formation interval.

Figure 11 shows a partial section of one of the cylinder and piston units in the present invention which represents a bubble point pressure and fluid compressibility chamber, and which also serves as a piston pump chamber for the pump-through option of the PVT multitester instrument for use downhole. Figure 11 also provides a schematic illustration of the hydraulic and electrical circuit. The supply line or passage 426 to the pump chamber 102 is provided with a magnetically controlled shut-off valve 428 for confining a finite volume of formation fluid in the pump chamber, so that the pump chamber defines a test chamber for bubble point pressure and fluid compressibility. The test chamber is provided with precision pressure and temperature sensors, 430 and 432, respectively, which are connected to electrical signal conductors, 434 and 436, respectively, which provide output signals "T" and "P" in the surface equipment, representing the temperature and pressure of the fluid sample. The precision linear potentiometer 116 is provided with electrical power supply conductors 438 and 440 and includes a position signal output conductor 442 for delivery to the surface of signals representing

the position of the piston rod, and thus the volume of the piston chamber 102, at all times. For testing, the piston can be accurately located within its cylinder after drawing in a finite volume of formation fluid as indicated by the potentiometer position signals. The shut-off valve can be closed to trap the sample. After this has been done, the pumping mechanism can be hydraulically activated to change the volume of the test chamber, and thus the volume of the final volume of trapped fluid. Observation of the pressure changes in the fluid sample during its volume changes will indicate the fluid's bubble point pressure. Although a linear type pump cylinder and piston arrangement for bubble point pressure has been described above, this invention is not intended to be limited thereto. The multitester instrument may be constructed to establish any suitable variable volume test chamber, which volume may be measured in any suitable manner. Any suitable precision temperature and pressure gauges may be used within the scope of this invention.

Bubble point pressure plots and bubble point pressure and compressibility profile plots have been found to be accurate, understandable and more reliable than conventional pressure gradient plots in reservoir characterization. They have a number of significant advantages compared to pressure-gradient plots. They are useful in the identification of vertical reservoir extent within the same reservoir, and indeed more reliably than the conventional pressure-gradient method of vertical reservoir extent. With a pressure gradient profile curve, different types of fluid from different formations can follow the same pressure gradient trend and thus give an incorrect indication of a single formation. With the method of the present invention, although different fluids may follow the same pressure gradient trend and have the same bubble point pressure, the fluid compressibility will indicate that different fluids are present.

The present method and device can also be used to identify the drive mechanism in the formation, such as depletion or gas casing drive, for example. If the formation pressure is higher than the formation fluid's bubble point pressure, the drive mechanism constitutes a depletion drive in which dissolved gas is separated from the fluid and creates a driving influence of the formation's flow towards the well. In such cases, secondary recovery ("recovery") by pressure maintenance is necessary for optimal production from this type of formation. If the formation pressure is lower than the bubble point pressure of the sampled fluid, a gas mantle exists and the flow process that drives the formation fluid towards the well is due to the gas mantle expanding. The system of the present invention can also be used for the identification of low formation pressure, which gives an indication that the formation is substantially depleted and therefore cannot be efficiently produced, even under the conditions of secondary recovery. The present invention also provides information to define optimal flow pressure to control gas separation from the formation fluid, thus enabling the producer to maximize recovery by effective control of production pressure.

On the basis of the foregoing, it is obvious that the present invention is well adapted to achieve all the purposes and features set forth above, together with other purposes and features resulting from the apparatus described herein.

As will be clear to a person skilled in the art, the present invention can be produced in other specific forms without departing from the idea or essential characteristics of the invention. The present embodiment is therefore to be regarded as illustrative, but not limiting, and that the scope of the invention is indicated by the claims, and not by the preceding description, and all changes that come within the meaning and coverage of the equivalence of the claims are intended to be covered by these.

Claims (38)

1. Method for testing a geological underground formation (11) with a well (10) defined therein, and comprising formation fluid, characterized in that the method includes the following steps: a) positioning in the well (10) a formation test instrument (13) with a sampling probe (18), and which includes a two-way piston pump (36) in pumping communication with the sampling probe (18) for selective pumping into and out of said geological formation, and comprising at least one internal sampling tank (320), b) movement of said sampling probe (18) from said formation testing instrument to sampling connection to the well at said geological formation, c) pressing with said two-way piston pump of a quantity of test fluid through the sampling probe (18) and into said geological formation, and d) extraction with said two-way piston pump of a quantity of said formation fluid from said geological formation through said sampling probe and into said formation test instrument. 2. Method according to claim 1, where signal processing circuits are used for processing electronic fluid test signals, characterized in that the method further comprises the following steps: a) performing at least one formation fluid test on said formation fluid inside said test instrument and generating electronic signals that represent the results of said formation fluid test, and b) sending said electronic signals from said formation test instrument to said signal process- ring circuits. 3. Method according to claim 1, where fluid circuits equipped with valves for control thereof are located inside said formation testing instrument, characterized in that the method further comprises the following steps: after said extraction, selective control of said formation fluid at first valve settings from said geological formation into said sampling tank, and at second valve settings control of said fluid from said geological formation through said fluid circuits in said formation test instrument into said well. 4. Method according to claim 1, where said pressing of said amount of test fluid through said sampling probe and into said geological formation is: characterized by the following steps: a) establishment with said two-way piston pump of a predetermined pump direction, pumping of said volume with test fluid into said geological formation using the bidirectional piston pump for mixing with said formation fluid to define formation test fluid-treated formation fluid, b) reversing the bidirectional piston pump's pumping direction from said predetermined pumping direction, and c) pumping said formation test fluid-treated formation fluid from said soil formation into said test instrument by means of said two-way piston pump in its said reversed pumping direction. 5. Method according to claim 1, where said two-way piston pump defines at least one pump chamber (102), where said method is further: characterized in that said pressing of said formation test fluid is achieved by pumping said formation test fluid from said pump chamber in said two-way piston pump in said test instrument into said geological formation. 6. Method according to claim 1, characterized in that said test fluid is pressed by pumping the test fluid from said well through said formation test instrument and into said geological formation. 7. Method according to claim 1, characterized in that a) said pressing of said volume with fluid at step (1) includes establishing pump direction control of said two-way piston pump by selective positioning of the instrument's valves, b) the fluid is pumped from said formation test instrument through said sampling probe and into said geological formation, c) the two-way piston pump's pump direction control is reversed by selective repositioning of the instrument's valves, and d) said retrieval or extraction in step d) comprises pumping formation fluid from said geological formation through said sampling probe and into said formation test instrument by pumping operation of said two-way piston pump. 8. Method according to claim 7, characterized by: a) selective positioning of the instrument's valves after step d) in claim 7, and b) pumping of formation fluid through said formation test instrument to said well for flushing said waste from said geological formation. 9. Method according to claim 7, where said formation test instrument contains a fluid tank (320) with formation test fluid in, characterized in that the method further comprises the following steps: a) selective positioning of the instrument's valves according to step d) in claim 7, and b) pumping, using said two-way piston pump (36), of formation test fluid from said fluid tank through said sampling probe for flushing of waste from said geological formation. 10. Method according to claim 1, where said two-way piston pump in said formation test instrument comprises a double-acting, two-way piston pump (36), characterized in that the method further comprises the following steps: a) extraction of a sample with said formation fluid by means of said double-acting two-way -piston pump, b) changing the pressure in said sample with formation fluid using said double-acting two-way piston pump, and c) carrying out pressure, volume and temperature tests on said sample with formation fluid that has been exposed to pressure changes. 11. Method according to claim 10, characterized by repeated pressure, volume and temperature tests until a selected fluid sample is obtained. 12. Method according to claim 1, where said two-way piston pump comprises a double-acting two-way piston pump mechanism connected to said formation test instrument, characterized in that the method comprises: controllable operation of said piston pump mechanism in one predetermined pumping direction for said press stage c), and controllable operation of said piston pump mechanism in the opposite direction for said withdrawal in step d). 13. Method according to claim 12, wherein said two-way piston pump mechanism has a pump part (38) which defines two pump chambers (102, 104) and has an operating piston chamber (80) located between said pump chambers and further has an operating piston (86) located forward - and return movement inside said operating piston chamber (80) and has pump pistons (94,96) which are attached to, and projecting from, each side of said operating piston and are respectively received for pumping back and forth movement inside said pump chambers, where said method is: characterized by the following steps: a) controllable back and forth movement of said operating piston and pump pistons with said piston pump mechanism set in a selected pumping direction, b) determination of the position of at least one of said operating and pumping pistons inside said pump part, c) generation of electronic position signals representing said specific position, and d) correlation of said electronic po sion signals with the time of piston movement to identify the volume of the formation fluid pumped by said pumping mechanism. 14. Method according to claim 13, characterized by electronic variation of the volume of the formation fluid that is pumped by said piston pump mechanism. 15. Method according to claim 13, characterized by: a) detection of the temperature of the formation fluid which is withdrawn from said geological formation, and b) correlation of the formation fluid's detected temperature with said volume pumping rate to establish the temperature-correlated accurate volume of formation fluid pumped with said piston pump mechanism. 16. Method according to claim 1, characterized by: a) detection of the temperature of the formation fluid that is withdrawn from said geological formation, b) obtaining a selected volume of said formation fluid into said formation test instrument, and c) correlating said final volume of formation fluid with said detected formation fluid temperature for accurate determination of the temperature-correlated volume thereof. 17. Method according to claim 16, where said formation fluid has a bubble point pressure, characterized in that the method further comprises the following steps: a) the mentioned two-way piston pump controllably changes the volume to the selected volume of formation fluid, b) the pressure in the selected volume of formation fluid is observed while the volume is changed to determine the bubble point pressure of the formation fluid, c) the compressibility of the formation fluid is calculated , and d) the formation fluid's bubble point pressure and compressibility are used to determine said geological formation's reservoir characteristics. 18. Method according to claim 1, where said formation fluid has a bubble point pressure and said two-way piston pump (36) defines at least one pump chamber (102) with a pump piston (94) and into which formation fluid is led and from which said formation fluid is driven out by linear movement of said pump piston during the pumping activity, where said pump chamber (102) is further a test chamber for bubble point pressure and fluid compressibility in the formation test instrument, where said formation test instrument further comprises means (332,330,334,116) for measuring the temperature in said formation fluid, the pressure in the formation fluid inside said test chamber and the volume of the test chamber, and with means for closing off the formation fluid inside said test chamber, characterized in that the method further comprises the following steps: a) a final volume of the formation fluid is captured in said test chamber, b) the volume and pressure of the captured formation fluid are compared and representative ele is produced ctronic signals thereof, c) the volume of the trapped formation fluid is changed in a controlled manner, d) the pressure changes in the trapped formation fluid are observed during the volume change, e) the bubble point pressure of the formation fluid is determined by comparing the formation fluid's pressure, volume, volume change and pressure change, and f) use of the aforementioned bubble point pressure from several formation fluid tests for reservoir characterization of said geological formation. 19. Device for performing pressure, volume and temperature tests in a formation fluid obtained from a geological subsurface formation with a well defined therein, and for obtaining samples of formation fluid contained in said geological formation, where the device comprises: a) an instrument main part (32) for positioning at a selected formation depth in said well, which instrument main part has a fluid tank (320) with test fluid, b) a sampling probe (310) which can be extended laterally from said instrument main part for fluid sampling connection to said geological formation, where said sampling probe defines a fluid sampling passage (318) for guiding said formation fluid sample from said geological formation to said instrument and for transferring fluid from said instrument to said geological formation, characterized in that it further comprises: c) a two-way piston pump (36) located inside said instrument main part and with a pump cylinder (80) and a piston (86) movable inside the pump cylinder, and d) solenoid valves (350,352,360,362) which selectively control the piston pump mechanism's pump direction for selective pumping of said test fluid through said sampling probe into said geological formation and pumping of formation fluid from said geological formation through said sampling probe and into said main part. 20. Device according to claim 19, characterized in that: a) said instrument part defines an internal test chamber (102) adapted to contain a finite volume of formation fluid therein, b) means (330,332) for measuring formation pressure and formation fluid temperature, c) means (116) for changing the volume and measuring the volume changes in said final volume with said formation fluid in said test chamber, and d) means (324) for measuring pressure changes in said final volume with said formation fluid during the change in the volume of the fluid to establish the bubble point pressure of the formation fluid. 21. Device according to claim 20, characterized in that it further comprises means (116,324) connected to said test chamber for determining the compressibility of the formation fluid inside said test chamber. 22. Device according to claim 20, characterized in that said piston pump mechanism defines at least one pump chamber mechanism (80) comprising a piston (86) which can move back and forth to achieve pumping activity, where said pump chamber defines said test chamber (104). 23. Device according to claim 22, characterized in that the piston (8 6) defines said means for changing the volume to the final volume with said formation fluid. 24. Device according to claim 23, characterized in that it further comprises: a) a linear potentiometer (116) inside said instrument part (32) arranged for motion-sensitive connection to said piston, where the linear potentiometer provides electronic position indicating signals corresponding to the final volume of formation fluid in the test chamber and the change of final volume of formation fluid inside the test chamber. 25. Device according to claim 22, characterized in that it further comprises: a) a fluid passage (426) defined by said instrument part and which is connected to the fluid sampling passage of the sampling probe and with the pump chamber (102), and b) insulating means (428) arranged in connection to the fluid sampling passage and for selectively operated capturing the final volume of formation fluid inside the test chamber for testing the fluid's bubble point pressure. 26. Device according to claim 22, characterized in that it further comprises: a) a temperature sensor (432) connected to said pump chamber (102) for measuring the formation temperature in said formation fluid in the test chamber, and b) a pressure sensor (430) connected to said pump chamber (102) for measuring the formation pressure in said formation fluid inside the pump chamber (102) and for measuring pressure changes in said final volume with formation fluid during said volume change. 27. Device according to claim 20, characterized in that it further comprises: a) a movable wall (86) which defines a part of said test chamber, and b) a potentiometer (116) which is movably operated by said movable wall, and which produces electronic output signals corresponding to the movable wall's positions and thus the test chamber's volume for determining the aforementioned final volume and the volume change in the formation fluid that is in the test chamber. 28. Device according to claim 19, characterized in that it further comprises: a) means (116) for position measurement to measure the position of said two-way piston pump (36) and generation of electronic piston position signals, and b) means (22) for receiving and processing the electronic piston position signals for identification of the volume of the formation fluid being pumped by the aforementioned two-way piston pump. 29. Device according to claim 28, characterized in that it further comprises: a) means (330,332) for determining pressure and temperature in the formation fluid that flows into the formation test instrument from said geological formation and generating electronic signals corresponding to the measured pressure and temperature, and b) means (22) for processing the electronic pressure and temperature signals in correlation with the electronic piston position signals for calculating pressure- and temperature-corrected volumetric changes in the formation fluid with said piston pump mechanism. 30. Device according to claim 19, characterized in that: a) said two-way piston pump mechanism (19) is a two-way piston pump (36) with opposite pump chambers (102,104), b) said means (168,170) for selective control of the piston pump's pumping direction is a hydraulic fluid pump circuit with control valves which allowing unidirectional fluid flow to and from said pump chambers, and c) a reversing valve (240,242) connected to the hydraulic fluid pump circuit for selecting the direction of fluid flow to and from the control valves. 31. Device according to claim 28, wherein said two-way piston pump defines two pump cylinders, each with a pump piston movably located therein, characterized in that the means (116) for position measurement comprise: a) a position indicator rod (114) which extends from one of said pump pistons, and b) a linear movement potentiometer which receives the position indicator rod and which generates electronic piston position signals corresponding to the position to the position indicator rod relative to the linear movement potentiometer. 32. Device according to claim 19, characterized in that said two-way piston pump mechanism comprises: a) a pump part (36) which defines two pump chambers (102,104) and an operating piston chamber (80) placed between said pump chambers, b) an operating piston (86) located for hydraulically activated forward and return movement inside said operating piston chamber (80), c) two pump pistons (94,96) attached to, and extending from opposite sides of said operating piston (86) and which are received for pumping back and forth movement inside the respective pump chambers ( 102,104), d) a hydraulic pump operating circuit (Fig. 5) for inducing operating reciprocating movement of the operating piston inside the operating piston chamber, e) a formation fluid circuit (Fig. 6) located inside said instrument body with connection to the pumping chambers and which receives formation fluid from the pump chambers by the reciprocating movement of the pump pistons, and f) direction controlling means (Fig. 4) for selectively directing the formations the flow of the ionic fluid inside the formation fluid circuit to or from said geological formation. 33. Device according to claim 32, where said formation fluid circuit (Fig. 6) comprises a fluid source inside said instrument part and a fluid reception inside said instrument part, characterized in that said direction controlling means comprise: a) a flow reversing valve (168) inside the formation the formation fluid circuit with a first flow controlling position which directs formation fluid flow from said geological formation to said pumping chambers (102,104) and from the pumping chambers to a sample receiver (320), wherein said flow reversing valve has a second flow controlling position which directs fluid from said fluid source to the pumping chambers and from said pumping chambers to the geological formation, and b) means (240) for controllably positioning said flow reversing valve at said first and second flow controlling positions. 34. Device according to claim 33, characterized in that said means for controllable positioning of said flow-reversing valve comprise: a) a pilot pressure reservoir (326) operably connected to said flow-reversing valve, and b) a control valve (240) which selectively controls the use of pilot pressure from the pilot - the pressure supply to said reversing valve for selective control of this to said first and second flow controlling positions. 35. Device according to claim 33, characterized in that said flow-reversing valve (168) comprises: a) a valve seat sleeve (192) held in close relationship inside said instrument main part and forming a plurality of internal sealing surface segments, where said valve seat sleeve has separately placed external annular fluid conducting recesses arranged with connection to respective passages for hydraulic fluid and formation fluid in the main body of the instrument, wherein said valve seat sleeve further has a plurality of internal annular fluid conducting recesses (214) which separate said internal sealing surface segments, b) a valve tube element (218) which is movably positioned in tight relation to the valve seat sleeve and defining separately located external annular fluid-conducting depressions with selective connection to said annular internal fluid-conducting depressions in the valve seat sleeve and with internal fluid-conducting passages, and c) means (328) for selective hydraulic movement of said valve tube element from before th flow-controlling position to said second flow-controlling position. 36. Device according to claim 35, where said device comprises at least one storage tank (320) for samples located inside said instrument part, characterized in that said valve pipe element comprises first and second one-way valve-controlled passages (343,345) each of which allows one-way flow of formation fluid and which is selectively operated depending on the position of the valve pipe element to allow pumped flow of formation fluid from or to said geological formation and from or to said well or storage tank for samples. 37. Device according to claim 35, characterized in that it further comprises means (170) inside the instrument part for selectively directing the pumped flow of formation fluid through said flow reversing valve to said well or to said storage tank for samples. 38. Device according to claim 37, characterized in that said means for selected routing further comprise: a) a flow passage (302) which connects the passages of said flow reversing valve together with said well and said storage tank for samples, and b) at least one control valve (168) for selective control of the flow of formation fluid from said reversing valve to the well or to the storage tank for samples.