GB2342669A

GB2342669A - Dual purpose borehole sub assembly for pressure measurement and chemical injection

Info

Publication number: GB2342669A
Application number: GB9922444A
Authority: GB
Inventors: Mark Buyers; Simon Benedict Fraser; David Forsyth
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-10-06
Filing date: 1999-09-23
Publication date: 2000-04-19
Anticipated expiration: 2019-09-23
Also published as: GB9821632D0; GB2342669B; GB9922444D0

Abstract

Bottom hole pressure of a wellbore is monitored using a control line hydraulic tube 23, a pressure entry sub 20 and a floating isolation piston 21 having bottom hole well pressure on its lower side. The control line tube 23 leads to the surface clamped to tubing 13 and is connected to a pressure indicating gauge and pressure recording equipment. When it is required to inject chemicals downhole, the piston 21 is displaced to the bottom of sub 20 where an injection valve will be pushed open to allow the chemicals to be injected via injection point 27.

Description

DUAL PURPOSE BOREHOLE SUB This invention relates to a dual purpose borehole sub for use in a method and apparatus for monitoring bottom hole pressure of a wellbore and for carrying out additional function (s).

In the petrochemical industries, oil, gas and water wells allow the extraction of products from below the surface of the earth. The wells may be drilled to a great depths and are periodically cased with a large diameter metal pipe in order to avoid collapse of the newly drilled hole. The wells are normally completed with a pressure sealing conduit called"tubing"which runs from the wellhead or xmas tree valve assembly on surface to near the bottom of the well. Both ends are sealed in order to provide a pressure tight communication between the surface facilities and the producing formation. The wellhead will seal this annulus at the surface and a device called a packer will seal pressure downhole near the producing formations. Often there will be many producing formations at different depths in the wellbores below the packer. These producing formations each may have differing concentrations of oil/ gas and water and may contain these elements at a variety of pressures. In order to prevent or minimise flow from one zone to another and to assist efficient flow to surface, the wellbore and production facilities must be properly managed.

Efficient extraction of wellbore fluids and gasses may be significantly assisted by monitoring such parameters as flow rate, gas/oil/water ratio, wellhead pressure and bottom hole pressure. With the exception of bottom hole pressure, most other measurements are available on surface relatively easily. Bottom hole pressure may be recorded on a wireline set and retrievable pressure gauge which records pressure and temperature for a limited period. This provides useful information but is costly and intermittent.

A permanent system exists comprising of a downhole mandrel which houses an electronic means of measuring pressure, an electronic system to transmit the pressure data, an electric armoured cable clamped to the tubing and running up the annulus to the surface and a surface display/recording unit. All the downhole apparatus is shielded from the ambient hydrostatic and well pressure. Known disadvantages of this system are the unreliability of the electronics systems-especially when associated with high temperatures found at the bottom of some wellbores, fragility of the electric cable and other components and the tendency of the connections top and bottom to pressure ingress. The systems are also costly to purchase and install.

Wellbores may also periodically require the use of chemicals to be placed downhole. These may be to inhibit corrosion of the production tubing, to inhibit scale precipitation on the production tubing, to prevent the formation of paraffin's, asphaltines, hydrates, to prevent crude from foaming or for other purposes. Dosage rates vary and the chemicals are presently inserted by the three following means. Bulk chemicals may be pumped down the wellbore using large capacity pumps at great expense.

Treatments of this sort can only economically be performed very periodically. An injection valve (check valve) may be placed in the wellbore at the well construction phase and the annulus filled with chemicals. Additional chemicals pumped into the annulus from surface will displace those around the injection valve into the wellbore providing a continuous metered facility. Known disadvantages of this system are the deleterious effects of temperature on the treating chemicals affecting their performance (the large volumes necessitating long retention times) and the likelihood of the dirty annular cavity environment providing debris which may clog or plug the injection valve. Also, a large volume of chemical is required to begin the process which may be expensive. A chemical injection sub may also be used which is directly linked to surface by a hydraulic line. This sub is located downhole during the well construction phase and also contains a check valve. Small dosages of chemicals may be accurately and economically placed in the wellbore. Plugging problems are less likely as all fluids may be filtered prior to use.

It is the purpose of this invention to continuously monitor the bottom hole pressure of a wellbore using a control line hydraulic tube, downhole pressure entry sub and to use the same downhole equipment for the purpose of chemical injection means.

According to one aspect of the invention there is provided a method of monitoring bottom hole pressure of a wellbore using a control line hydraulic tube, downhole pressure entry sub and hydraulically operable assembly, in which the assembly is also utilised to carry out injection of chemicals into a required region of the wellbore.

According to a further aspect of the invention there is provided apparatus for monitoring bottom hole pressure of a wellbore using a control line hydraulic tube, downhole pressure entry sub and hydraulically operable assembly, in which the assembly is also capable of being utilised to carry out injection of chemicals into a required region of the wellbore.

Preferably, a hydraulically operable sleeve or floating piston assembly is positioned inside a pressure bearing downhole sub, for the purpose of carrying out injection of chemicals into the wellbore.

According to a preferred embodiment of the invention, a hydraulic line or control line of 1/3"outside diameter or thereabouts will run from a point on the surface to a hydraulically operated sleeve or piston assembly which is positioned inside a pressure bearing downhole sub. This is located preferably near to the producing formation or at any convenient point. The choice between piston or sleeve type mechanism will be dictated by the outside diameter of the downhole sub containing the piston/sleeve, relative to the casing inside diameter and the required internal diameter of the sub. Where internal diameter through the tubing is unimportant, the piston assembly may be utilised. Where a large diameter access is required to wellbore components below the sub, the sleeve mechanism may be used. The control line is clamped to the tubing and runs from the sub, up the annulus to a pressure tight feed through mechanism at the surface. The surface equipment features a pressure indication gauge, pressure recording means and a facility for pumping chemicals and/or pumping clean hydraulic oil.

The sleeve or piston will be floating in normal operation.

It will sense well pressure below by means of a communication port to the production tubing and will transmit this pressure to the hydraulic oil above which in turn will transmit the pressure along the hydraulic control line to surface to be measured. The production tubing will be isolated from the main wellbore annulus by a packer, as is normal. The hydraulic control line may be filled with a light hydraulic oil selected for its low specific gravity and bulk modulus properties. More than one system may be installed in a well by using packers equipped with multiple hydraulic feed throughs and a similar treatment at surface.

In this way, a bundle of hydraulic lines connected to various producing zones, each hydraulically isolated from the other, may monitor the bottom hole pressure in several places in a wellbore.

Most oil and gas reservoirs feature a downhole pressure in excess of the fluid gradient pressure. It is this feature which allows an oil well to flow to surface.

Indeed, some wells may have a wellhead pressure of thousands of psi, for example, a 10,000 ft deep well may have a bottom hole pressure of 6,500 psi. (A water column this high will measure 4,330 psi at its base). A static column of hydraulic oil with a specific gravity of 0.870 10,000 ft deep will exert a bottom hole pressure of 3,770 psi. If a floating isolation piston were placed between the well fluid and the hydraulic fluid at 10,000 ft depth, an imbalance of 2,730 psi would register on the surface of the hydraulic fluid line. When, in any particular position except for top and bottom stroke, an imbalance between the formation pressure and the hydrostatic head of hydraulic control line pressure will cause a positive to be read at surface. This pressure plus the known hydrostatic pressure for the given oil used will equal the bottom hole pressure. The bottom hole pressure would be calculated at 2,730 psi system surface pressure plus the hydrostatic of 3,770 psi = 6,500 psi. The wellhead or tubing head pressure may be even higher than the system surface pressure due to the gas content of the column lightening the hydrostatic pressure.

The wellhead pressure is unreliable as a base means of estimating bottom hole pressure as the gas content and free gas pressure cannot be quantified in a dynamic system. When the well is shut in, the free gas may wholly or partially go into solution in the oil, the ratio of which would be unknown. Certain gas and artificial lift wells which feature low bottom hole pressure may not be suitable for the system as described as sufficient pressure must exist to lift the hydrostatic pressure of hydraulic oil.

Calibration and accuracy checks may be performed on the pressure measurement system by using wireline techniques to place an accurate pressure memory gauge in the wellbore at the same point as the piston/sleeve and noting both pressures simultaneously. This may provide a useful comparison and assist calibration of the system. Two of the borehole subs when run together with a restriction of a known size between could form the basis of a flowmeter.

Calculations based o the pressure drop across the restriction relative to the restriction size provide a flowrate. This technique will be obvious to anyone skilled in the art.

It is a known disadvantage of downhole systems featuring a hydraulic control line that gas will eventually ingress the system through the seals and the hydraulic oil will degrade and change its properties with time. This is a known problem with downhole safety valves. The pressure measuring system may therefore be provided with a facility to dump the contents of the control line through the pressure sensing piston. This is achieved by displacing the piston to bottom stroke where an injection valve will be pushed open and clean filtered fluid injected from surface will displace the system contents through the piston and into the wellbore. It can be seen that by use of this pressure measurement system, accurate doses of chemicals may be periodically injected followed by clean hydraulic oil in order to treat the well for a variety of conditions, thus achieving the two aims of the system, namely chemical injection and pressure measurement. The sleeve or piston may also be equipped with one or more check valves upstream of the injection valve for safety purposes in the event that the hydraulic control line becomes damaged. The check valve (s) will prevent well pressure and fluids from entering the annulus should leaks occur in the hydraulic control line. The check valves are calibrated to open at around 100 psi differential pressure. Prior to the valves opening, the increased hydraulic pressure above the piston or sleeve will displace the fluid below the piston and push the piston to bottom stroke where the valve is mechanically opened.

As seals and packing can grip cylinders and pistons, the hydraulic operating sleeve or piston may feature as few as two low friction seals in order to minimise the system friction. A"dither"function may be included whereby the valve is occasionally fractionally repositioned by injection of small amounts of fluid from surface in order to prevent frictional increases caused by the seals binding or locking with time. Provision of an internal profile inside the sub allows retrofitting of a sleeve inside the tubing which can straddle and close off the pressure sensing port and seal pressure should any part of the system become damaged or leak. Failure of any part of the system should not compromise the well safety.

The stroke of the downhole piston is important in that it must contain sufficient volume to compress the total volume of the control line and system for a given pressure change. A particular hydraulic oil to be used has a bulk modulus which gives a 0.5% change in volume per 1,000 psi pressure change. The system volume for a 15,000ft deep well using 1/4"control line with a 0.12" internal diameter is approximately 7 gallons. Approximately 1/3 pint of fluid is required to be injected into this system to provide a pressure increase of 1,000 psi. If a downhole pressure range of 2,000 psi was anticipated, it would be sensible to size the swept volume of the downhole piston/sleeve to be 1 pint, thus allowing for a 50% considering.

Preferred embodiments of method and apparatus according to the invention have been described above, and are also shown in the accompanying drawings, in which: Figure 1 is a schematic illustration of a general type of oil well to which the invention may be applied, utilising a dual function assembly downhole, which is capable of carrying out continuous monitoring of bottom hole pressure, and also of being utilised to carry out injection of chemicals, when required, into required regions of the wellbore; Figure 2 is a detailed view of a first embodiment of dual purpose borehole sub, of piston type, for use in a method and apparatus according to the invention, and in particular with the general arrangement shown in Figure 1; Figure 3 is a detailed view of a second embodiment of dual purpose bore hole sub of the sleeve type, also capable of being used in the method and apparatus shown in Figure 1; and Figure 4 is a detailed view of the sleeve type of piston assembly.

Referring first to Figure 1 of the drawings, there is shown a schematic illustration of a general type of oil well installation 10 to which a method and apparatus according to the invention may be applied, for carrying out measurement of bottom hole pressure in a wellbore production system using a control line hydraulic tube 11, downhole pressure entry sub and hydraulically operable assembly. This assembly is also utilised to carry out injection of chemicals into a required region of the wellbore.

The illustration of Figure 1 shows usual casing 12, tubing 13 running from a wellhead or Christmas tree 14 at the surface down to the producing formations 15, a packer 16, a dual purpose sub 17, a surface pressure measurement/ recorder 18 and surface chemical/hydraulic oil pump 19 for introducing chemicals into the wellbore.

Figure 1 therefore illustrates schematically a general type of oil well installation to which the invention may be applied, and which is capable of carrying out monitoring of bottom hole pressure of a wellbore using a control line hydraulic tube, downhole pressure entry sub and hydraulically operable assembly, but which is also capable of being utilised to carry out injection of chemicals into a required region of the wellbore.

Figure 2 shows first embodiment of dual purpose borehole sub, of piston type, for use in a method and apparatus according to the invention, and in particular with the general arrangements shown in Figure 1.

Figure 3 shows a second embodiment of dual purpose borehole sub, but of the sleeve type, and which is capable of being used in the method and apparatus shown in Figure 1.

Figures 2 and 3 therefore show, respectively, a hydraulically operable floating piston assembly, or sleeve assembly, which is positioned inside a pressure bearing downhole sub, for use in a dual function arrangement, namely monitoring of bottom hole pressure, and also utilization to carry out injection of chemicals into a required region of the wellbore when required.

Figure 2 therefore shows utilization of a dual purpose borehole sub of the piston type, designated generally by reference 20, with a floating isolation piston 21 having bottom hole well pressure on its lower side and preventing ingress of gas to its upper side by use of low friction seals 22. The piston 21 possesses sufficient stroke to accommodate expected pressure variations in the wellbore, and which communicates with surface by way of a continuous hydraulic line placed outside the production tubing 13 but in the wellbore, and with wellhead pressure feed through terminating in pressure measuring means. A control line 23 leads to the surface from the upper end of tubing 24 clamped to the tubing 13, and control line 23 runs from the sub 20 up the annulus to a pressure tight feed through mechanism at the surface. The surface equipment features a pressure indicating gauge, pressure recording means and a facility for pumping chemicals and/or pumping clean hydraulic oil (not shown). The piston 21 will be floating in normal operation, and will sense well pressure 25 below the piston, and will transmit this pressure to hydraulic oil 26 above the piston which in turn will transmit this pressure along the hydraulic control line 23 to surface to be measured.

The production tubing will be isolated from the main wellbore annulus by a packer, as per normal practise. The hydraulic control line may be filled with a light hydraulic oil selected for its low specific gravity and bulk modulus properties.

A lower injection point 27 is shown in Figure 2, through which chemicals can be introduced when required.

Figure 3 of the drawings shows a second embodiment of dual purpose borehole sub, designated generally by reference 28, and which is of the sleeve type. This is particularly suitable for use when large diameter access is required to wellbore components below the sub. The function of the sleeve type of dual purpose borehole sub 28 is generally similar to that described above with reference to the piston type of sub shown in Figure 2. The sub 28 shown in Figure 3 includes a floating piston 29, check/injection valves 30, hydraulic oil 31 above the piston 29, and a communication/injection port 32 below the piston 29 and valves 30.

Figure 4 shows in more detail a further construction of sleeve type piston for use in a dual purpose borehole sub which may be employed in a method and apparatus according to invention. The further embodiment shown in Figure 4 is designated generally by reference 33, and includes check valves 34 and injection valve 35.

Claims

1. A method of monitoring bottom hole pressure of a well bore using a control line hydraulic tube, downhole pressure entry sub and hydraulically operable assembly, in which the assembly is also utilised to carry out injection of chemicals into a required region of the wellbore.

2. A method according to claim 1, in which a hydraulically operable sleeve or floating piston assembly is positioned inside a pressure bearing downhole sub.

3. A method according to claim 1, utilising a downhole sub with floating isolation piston having bottom hole well pressure on its lower side and preventing ingress of gas to its upper side by use of low friction seals, said piston possessing sufficient stroke to accommodate expected pressure variations in the wellbore, which communicates with surface by way of a continuous hydraulic line placed outside the production tubing but in the wellbore, and wellhead pressure feed through terminating in pressure measuring means.

4. A method according to any one of claims 1 to 3, in which the same apparatus is used to carry out pressure measurement and injection of chemicals downhole, and including check and injection valves housed in the piston.

5. A method according to any one of claims 1 to 4, including a plurality of measurement/chemical injection devices arranged in a single wellbore.

6. Apparatus for monitoring bottom hole pressure of a wellbore using a control line hydraulic tube, downhole pressure entry sub and hydraulically operable assembly, in which the assembly is also capable of being utilised to carry out injection of chemicals into a required region of the wellbore.