METHOD, SYSTEM AND TOOL FOR RESERVOIR EVALUATION AND WELL TESTING DURING DRILLING OPERATIONS
Reference to Related Patent Applications
This application claims priority from U.S. provisional application serial no. 60/233,847 filed September 20, 2000.
Field of The Invention
The present invention relates generally to testing and evaluating a section of
reservoir intersected during the well construction process. More particularly, the present invention relates to methods, systems and tools used in testing and evaluation of a subsurface well formation during drilling of the wellbore.
Setting of the Invention
A reservoir is formed of one or more subsurface rock formations containing a
liquid and/or gaseous hydrocarbon. The reservoir rock is porous and permeable. The
degree of porosity relates to the volume of liquid contained within the reservoir. The
permeability relates to the reservoir fluids' ability to move through the rock and be
recovered for sale. A reservoir is an invisible, complex physical system that must be
understood in order to determine the value of the contained hydrocarbons.
The characteristics of a reservoir are extrapolated from the small portion of a
formation exposed during the well drilling and construction process. It is particularly
important to obtain an evaluation of the quality of rock (formation) intersected during well construction. Even though a large body of data may have been compiled regarding the characteristics of a specific reservoir, it is important to understand the characteristics of the rock intersected by a specific wellbore and to recognize, as soon
as possible during the process of well construction, the effective permeability and
permeability differences of the formation intersected during well construction.
The present invention is primarily directed to wellbore and formation evaluation while drilling "underbalanced." Underbalanced drilling is a well construction process
defined as a state in which the pressure induced by the weight of the drilling fluid
(hydrostatic pressure) is less than the actual pressure within the pore spaces of the
reservoir rock (formation pressure). In a more conventional process, the well is
typically drilled "overbalanced." In an overbalanced drilling process, the pressure
induced by the weight of the drilling fluid (hydrostatic pressure) is greater than the
actual pore pressure of the reservoir rock.
During underbalanced well construction, the fluids within the pore spaces of the reservoir rock flow into the wellbore. Because flow is allowed to enter the wellbore, the fluid flow characteristics of the formation are more easily observed and measured.
During overbalanced drilling, the drilling fluid may enter the formation from the
wellbore. While this overbalanced flow may be evaluated to assess formation
properties, it is more difficult to quantify fluid losses to the formation then it is to
quantify fluid gains from the formation.
There are significant benefits obtained from the application of underbalanced
well construction techniques. The rate of penetration or speed of well construction is
increased. The incidence of drill pipe sticking is decreased. Underbalanced operations prevent the loss of expensive drilling fluids.
An understanding of the reservoir being penetrated during the well construction process requires direct and indirect analysis of the information obtained in and from the well. Core analysis and pressure, volume, temperature (PVT) analyses of the reservoir
fluids are measurements and testing performed in a laboratory after the wellbore has
been drilled. This process of formation evaluating is both costly and time-consuming.
Also, it is not practical to perform core analysis and PVT studies on every well
constructed within a reservoir. During drilling of a wellbore, important information can be determined by
evaluating the fluids flowing to the well surface from the formation penetrated by the
wellbore. The amount of gas included in the surface flow is particularly important in
evaluating the formation producing the gas. The volume of gas per unit of time, or flow rate, is a critical parameter. The rate of gas flow from the formation is affected by the back-pressure exerted through the wellbore. The information desired for a particular
formation or layer is the flow rate capacity during expected flowing production pressure. The best measure of this flow rate occurs at the flowing production pressure, however,
conventional gas flow measuring instruments require flow restricting orifices in
performing flow measurements. Instruments using differential orifices as the basis for flow management are accurate only within a relatively narrow range of flow. Sporadic
flow changes associated with penetration of different pressured or flowing formations
can produce flow rates outside the accuracy limits of the measuring instrument. Surface
measurements of gas flow are, consequently, performed at pressures that are different
from normal flowing pressures and the results do not accurately indicate the gas flow
potential of the formation. The procedures commonly employed to measure surface
flow during drilling or constructing a well that restrict the flow as a part of the gas flow
rate measurement reduce the accuracy of evaluations of formation capacity based upon such measurements. Conventional instruments that measure flow without
restricting the flow are typically incapable of making precise measurements. These
instruments, which generally use a Venturi tube in the flow line, produce unduly broad
indications of flow rates.
Indirect analysis of information requires reference to well logs that are recorded
during well construction. A well log is a recording, usually continuous, of a
characteristic of a formation intersected by a borehole during the well construction
process. Generally, well logs are utilized to distinguish lithology, porosity, and
saturations of water oil and gas within the formation. Permeability values for the
formation are not obtained in typical indirect analysis. An instrument for repeated
formation tests (RFT) also exists. The RFT instrument can indicate potentially provided
permeability within an order of magnitude of correctness. Well logging can account for
as much as 5 to 15 percent of the total well construction cost.
Another means of formation testing and evaluation is the process of drill stem testing. Drill stem testing requires the stopping of the drilling process, logging to
identify possible reservoirs that may have been intersected, isolating each formation
of each intersected reservoir with packers and flowing each formation in an effort to
determine the flow potential of the individual formation. Drill stem testing can be very time consuming and the analysis is often indeterminate or incomplete. Generally,
during drill string testing, the packers are set and reset to isolate each reservoir
intersected. This may lead to equipment failures or a failure to accurately obtain
information about a specific formation.
Because each formation is tested as a whole, the values or data obtained
provide an average formation value. Discrete characteristics within the formation must
be obtained in another manner. The discrete characteristics within a layer of the
formation are generally inferred from traditional well logging techniques and/or from
core analysis. Well logging and core analyses are expensive and time-consuming.
The extensive time involved in determining the permeability (productability) of each intersected reservoir layer in a wellbore through multiple packer movements and multiple flow and pressure buildup measurements required during a drill stem test make the process expensive and undesirable.
Summary of the Invention
It is the primary object of the present invention to provide a method, system and
tool for obtaining information about a formation while constructing a wellbore designed
to intersect the formation. One characteristic of the formation that determines the
productability of the well is permeability. During production, the fluid flows through the
medium of the reservoir rock pores with greater or lesser difficulty, depending on the characteristics of the porous medium. The parameter of "permeability" is a manager
used to describe the ability of the rock to allow a fluid to flow through its pores.
Permeability is expressed as an area. However, the customary unit of permeability is
the miilidarcy, 1 mD = 0.987 x 10"15 m2. Permeability is related to geometric shape of
flow passages, flow rate, differential pressure, and fluid viscosity.
Parameters such as bottomhole temperature and pressure are acquired through a bottomhole assembly during actual drilling operations and the acquired values are transmitted to the surface. In the first method of the invention, the drilling assembly drills the wellbore to a point above the formation of interest. The measuring instruments in subsurface instruments carried by the drilling assembly are calibrated with surface measuring instruments at the well surface. The calibration is performed by evaluating injected and
return fluids circulated through the closed flow system provided by the drill string
assembly and the wellbore annulus. Precise qualitative and quantitative measuring instruments are provided in the calibrated system to produce accurate measurements of fluid composition, flow rates, volumes and condition of fluids injected into the drill string from the surface and fluids returning in the annulus from both the drill string and
the formation.
An important feature of the present invention is the use of an ultrasonic gas flow meter in the surface measurements of gas being produced from the formation to permit unrestricted flow measurements that accurately reflect the formation's flow
characteristics. A chromatograph is used in the surface measurements of annular fluid
flow to precisely identify constituents of the flow. The results of the measurement
assist in making well construction decisions as the well is being drilled.
A second method of the present invention utilizes a downhole device to obtain
downhole flow rates. These downhole flow rates can be compared to the flow rates
determined from well surface operations. The direct measurement of downhole flow
permits a more accurate permeability calculation on a foot-by-foot basis of the wellbore
penetration through the formation. The need for a complex mathematical model to
convert surface rates and flow properties to downhole conditions is eliminated when
accurate bottomhole flow rates are obtained with a directly measuring tool.
In the methods of the invention, the bottomhole temperature and pressure may
be used to determine density and/or viscosity of the produced fluids. To determine initial reservoir pressure, the drilling operation may be stopped and the well shut in to
allow the pressure to buildup. Additionally, a series of flows at different differential
pressure may be used to extrapolate to the initial reservoir pressure. Using these
parameters, an effective permeability can be calculated for the section of formation
contributing to the flow.
The measured parameters at the bit are transmitted to the well surface using fluid pulse telemetry or other suitable means. Generally, the downhole data
transmission rate, relative to the rate of penetration in a reservoir, is such that the data
acquisition at the bit downhole or at the surface is considered to be "real-time" data.
Another means of obtaining the necessary data for these novel methods of formation evaluation is to have the downhole measurements taken and stored in a
subsurface memory device during actual well construction operations. After the data
is acquired and stored in the memory device, it may be retrieved at a later time such
as during the replacement of a worn out drill bit. This recorded data is considered "near real-time" because it is not transmitted to the surface from downhole. This near real-time data from downhole is synchronized and merged with either surface measurements of hydrocarbon production or downhole measurements from the subsurface measurement instrument and used to compute the permeability and
productivity of the formation intersected during the well construction process. Near
real-time methods are utilized when the added expense of real-time is not warranted.
The choice is usually based upon required placement accuracy of the wellbore, or
when the real-time transmission is technically not feasible, or when the general
economics of the reservoir prohibit use of real-time methodology.
A novel downhole flow measuring tool comprises a part of the present invention. The downhole tool connects between the drill string and bit. Blades on the tool provide
external longitudinal recesses that channel fluid across transducers mounted on the
blades. The tool structure functions as a drilling stabilizer and, while rotating, positively
directs the well fluid into the fluid recesses where various transducers carried by the
tool are used to assist in determining flow rate and other parameters of the well fluid.
This latter feature is particularly useful in horizontal drilling application where the well fluids may tend to stratify vertically.
In the preferred embodiment of the tool, several types of transducers are
deployed along the tool's external surface to provide a large number of different well
fluid measurements. The increased number of measurements permits significant
improvement in the accuracy of the flow rate measurements and other measurements made by the tool.
Brief Description of the Drawings
Figure 1 is a schematic illustration of a system of the present invention used to evaluate a subsurface formation being intersected by a wellbore during well construction;
Figure 2 is an elevation of an integral blade stabilizer body having energy measurement transducers used for subsurface measurements while drilling;
Figure 3 is a partial cross section taken along the line 2-2 of Figure 1 illustrating
the placement of three different types of energy transducers or sensors integrated into
the drilling stabilizer of Figure 1 ;
Figure 3A is an enlarged view of a focusing notch employed with the induction
transmitters of the present invention; and
Figure 3B is an enlarged view of illustrating details in the construction of the
capacitance transducers of the present invention.
Detailed Description of the Illustrated Embodiments
Figure 1 illustrates a system of the present invention indicated generally at 10. The system 10 is employed to determine the permeability of a formation F that is to be penetrated by a wellbore B. A drilling assembly comprising a bit 11 , drilling stabilizer 12, subsurface measuring and recording instrument 13 and drill string 14 extend from the wellbore B to the wellbore surface T. Only a portion of the bottomhole assembly is
illustrated in Figure 1. The projected wellbore trajectory is indicated by a dotted line
section 15.
A measuring system 20 used in the evaluation of a formation F is equipped with
an inlet fluid measuring section 21 , an outlet measurement section 22 and a calibrated
instrument analysis section 23. The measuring system 20 measures and evaluates the fluids flowing into the wellbore B through the drill string 14 and measures and evaluates the fluids returning to the top or surface of the wellbore T through an annulus A formed between the drill string and the wellbore. As used herein, reference to measuring or
evaluating "flow" of a fluid is intended to include measurement or evaluation of
characteristics of the fluid such as temperature, pressure, resistivity, density, composition, volume, rate of flow and other variable characteristics or parameters of the fluid.
The calibrated analysis section 23 may be supplemented with subsurface parameter values obtained from a subsurface values section 24. The data from the section 24 are delivered from either a data resource 25 or from an actual downhole
measurements section 26. Data provided by the data resource section 25 may be data
taken from historical data sources 25a, such as analogous or similar wells or the data
may be derived from a computer data model 25b that performs mathematical
calculations, or determines data from other inferential processes. The actual downhole measurements are provided through a real-time system section 27 or a near real-time system section 28.
In applying the method of the present invention to a system in which subsurface
flow values are to be inferred or deduced from measurements or assumed values of related parameters, the system 20 is calibrated and checked before the wellbore B is
extended into the formation F. This step in the procedure assists in determining system
noise and in determining circulating system responses to changes in the back-pressure
in the annulus A.
The system calibration process and checking are preferably performed between
5 and 25 meters above the anticipated top of the formation F. The top of the formation F may be determined using a geological marker from an offset well, seismic data or reservoir contour mapping. During the calibration process, a closed fluid flow system
is established by the drilling assembly in the wellbore B such that fluids introduced into the drill string 14 travel through the drilling assembly 14, 13, 12, 11 , and exit the drilling assembly through the bit 11 where they are returned to the well surface T through the
annulus A. Only fluids introduced into the drill string 14 flow through the closed system
during the calibration and checking process.
The calibration performed by circulating a known quantity and density of a
known fluid (gases included) while the drilling assembly and any downhole sensing equipment carried in the drilling assembly are deployed within the wellbore B. A
material balance relating the injected fluids to the returned fluids is preferably employed
in the calibration process. The calibration process is employed to establish a standard
or control to detect or determine changes in measurements that result from
encountering a productive formation environment.
In a preferred method of calibration, the following parameters are measured for a minimum of three different back-pressure values obtained at the annulus A while
fluids are circulating through the system:
I) injection: pressures, temperatures and rates;
II) bottomhole: annulus pressures and temperatures;
III) return: pressures, temperatures and rates; and
IV) C1 to C6 hydrocarbon percentage over a period of 1.1 to 15 wellbore circulation volumes.
The time required for the fluid to complete circulation through the drilling assembly and return to the surface through the annulus is monitored and recorded. In a preferred method, a circulation time measurement is performed with the assistance
of a tracer added to the injection fluid stream entering the drill pipe 11 at the well
surface T. The elapsed time from injection ofthe tracer until reappearance of the tracer
in the fluid returns at the well surface annulus indicates the circulation time. The tracer
material may be a carbide, or an inert substance such as neon gas, or a short half-life
radioactive material or other suitable material.
After calibration and system checking are performed, the drilling operation is resumed and the drilling assembly is used to extend the wellbore into the formation F.
During extension of the wellbore, the rate of penetration is preferably maintained at a rate below 25 meters per hour. The weight on bit and rotary or bit motor speeds are maintained as constant as possible to enhance the accuracy of the results of the system measurements.
In performing the method of the present invention during underbalanced drilling
conditions, it is preferable to maintain an underbalanced bottomhole pressure between
100 and 2000 psi below the anticipated pressure of the formation F. The bottomhole pressure can be adjusted by manipulation of the drilling fluid densities, pump rates and
annular back-pressures.
The point at which the drill bit 11 encounters the top of the formation F may be determined by closely monitoring the system 20 for any significant change in the
bottomhole pressure, bottomhole temperature, C1 or surface flow rates. Once the top
of the formation F has been traversed, an additional 1 to 5 meters of wellbore depth is drilled into the formation and the drilling is stopped as fluid circulation is maintained.
In an underbalanced condition, reservoir flow and pressure response are established while injecting fluid into the drill string 14 from the surface and combining the injected fluids with fluids flowing from the reservoir F into the wellbore B. The
combined injection and formation fluids flow through the annulus A to the well surface T. During this step, the following sensor point measurements are performed:
I) injection: pressures, temperatures and rates;
II) bottomhole: annulus pressures and temperatures;
III) return: pressures, temperatures and rates; and
IV) C1 to C6 hydrocarbon percentage over a period of 1.1 to 15 wellbore circulation volumes.
The measurements l)-IV) are made and recorded for a preferred period of time
equivalent to 1.5 to 15 times the "bottoms up" time. "Bottoms up" time is the time required to flow fluid at the bottom of the wellbore to the well surface. Once a stabilized annular flow through the annulus A has been established, the back-pressure in the
annulus is increased to achieve a second underbalanced flowing condition. If the
annular flow does not stabilize at this increased back-pressure, the back-pressure is reduced by 25 percent and the annular flow is maintained for 1.5 to 15 times the bottoms up time to test for stabilization of the annular flow.
The next step in the method is to reduce the circulating back-pressure or
bottomhole pressure by 30 to 40 percent, preferably not to exceed 35 percent of the
draw down on the bottomhole pressure (BHP) for a period of time of from 1.5 to 15
times the bottoms up time, depending on the annular flow conditions. The time of each
back-pressure change is recorded, to be correlated with the flow measurements. The
back-pressure is increased, using either a surface choke or by increasing the
bottomhole pressure, to a safe drilling level and then stabilized over a period of from 1.5 to 15 times the bottoms up time.
Drilling is resumed and the borehole B is extended to the formation F at a steady
drilling rate of preferably 10-20 meters per hour. During the resumption of drilling, the
sensor points variable measurements l)-IV) are continuously monitored and recorded.
Drilling is continued until the formation F has been fully traversed. Once the wellbore extends below the bottom of the formation by 2 to 10 meters, drilling is stopped. Fluid
flow through the annulus is continued for a time of from 2 to 15 times the bottoms up
time. If the back-pressure in the annulus A cannot be increased without killing the well, the annulus back-pressure is decreased by 15-20 percent from the initial pressure value occurring following initial penetration of the formation bottom. If the back¬
pressure in the annulus A is still high enough to kill the well, the annulus back-pressure
is decreased 30-40 percent from the initial pressure value.
Once the measurements have been completed following the application of the different back-pressures in the annulus A, the original back-pressure existing at the penetration of formation bottom is restored and the wellbore drilling is continued, or the drilling assembly is pulled from the well if the total well depth has been reached. The flow rates and corresponding bottomhole pressures obtained from the
foregoing process are plotted to form Inflow Production (IPR) curves. The IPR curves
are extrapolated to determine the virgin reservoir pressure P* of the formation F or a
specific portion of the formation or layer of interest. This method is an alternative
technique for determining the formation pressure P* without using direct measurement
process of stopping circulating through the well, shutting in the well and then allowing
the pressure from the formation to build up to a stabilized level indicative of P*.
With the collected data, Darcy's Radial Flow equation is used to solve for matrix permeability "k," or fracture transmissibility "kh." Skin effect S is assumed to be zero where underbalanced drilling conditions are used since the absence of drilling fluid flow into the formation exerts minimal skin damage to the formation. P* is taken from the
IPR curves or shut in pressure buildup determination. These calculations can
conveniently be used to provide a graphical presentation of flow rate versus drilling
depth.
Evaluation of the formation F using the measurements and data obtained in the described process may be enhanced with the use of a computer model 29 of the reservoir. The computer model can account for variances attributable to multiple
formation layers, partial penetration of a zone, dual porosity of the formation and the occurrence of vertical, horizontal or high angle wellbores as well as other variations in
parameters. The computer model may be employed to more accurately project well
production and reserve estimates. Presentation of the evaluation and activation of
alarms is made by an evaluation section 30. A kick alarm 31 provides early warning of an influx of formation fluids into the wellbore.
The methods of the present invention may also be practiced in a system using data obtained directly with downhole flow measurement instruments that comprise a
part of the drilling assembly. In a directly measuring downhole system, the requirement
for initial system calibration is reduced or becomes unnecessary. With the exception ofthe initial calibration step, the steps used in performance of the method when using
direct downhole flow measurement instruments are substantially the same as those employed when downhole flow parameters are determined inferentially or are obtained
from indirect measurements or a data resource. Using actually determined subsurface
flow measurements eliminates the requirement for the computer model 29 or the data
model 25b and otherwise reduces the need for extensive mathematical correlations and
calculations to obtain accurate formation values. Direct measurements also enable
rapid warning of a kick to initiate an alarm from the measuring component 31.
Figures 2 and 3 illustrate details in a preferred subsurface measurement tool,
indicated generally at 50, for assisting in determining permeability of the formation F.
The measurement tool 50 is illustrated connected to a drill bit 51 to function as part of
a near-bit stabilizer. It will be appreciated that the tool 50 may be employed at other near-bit locations within a bottomhole drilling assembly and need not necessarily be
connected immediately to the bit, the objective being to provide a stabilizing
relationship between the bit and the tool 50. The instrument tool 50 includes three separate types of detection devices in the vicinity of the drill bit permitting a large number of combinations of signals to be analyzed thereby producing increased
flexibility and accuracy in both measurement while drilling (MWD) and formation
analysis operations.
The instrument tool 50 is equipped with an axially extending body 52 having a central, axially developed passage 55 for conveying fluid between a first axial tool end
56 and a second axial tool end 57. Radially and axially extending, circumferentially spaced blades 60, 61 and 62 extend from an external tool surface 65. The instrument
tool 50 is connected at its first end 56 to a bit 51 and at its second axial end 57 to a
monitoring and recording tool 66 that processes and records the measurements taken
by the instrument tool 50. The tool 66 records and/or transmits measurements to the well surface. Recorded measurements are retained in the recorded memory until the
drilling assembly is retrieved to the well surface or the measurements may be transmitted to the surface through fluid pulse telemetry or other suitable communication means.
The tools 50 and 66 are connected with the measuring system 20 for real-time
or near real-time measurements that permit formation evaluation. Analog to digital
converters in the measuring system 20 process signals detected at the transducer
receivers and capacitive energy transducers and supply numerical representations to
a microprocessor system within the components 23, 29 and 30. The measuring system
20 of the present invention employs a microprocessor and digital-to-analog converters
to enable the production of many different types of signals with the acoustic transducers or electromagnetic antenna systems. Both high and low frequency signals can be created. In addition, fast rise time and slow fall time "saw tooth" signals may be
employed to provide specific, more discrete rates of change in electronic signaling as compared to older techniques employing continuous variations of sine waves.
The output signals from the energy transducers employed in the present invention are calibrated and the programming employed in the measuring system is modified to counter intrinsic tool inductance and capacitance that would normally distort the output signals. Reduction in distortion and the presence of discreetly rising and
falling signals contribute to greater accuracy in the measurement of the inductance of
the fluids. Broad variations in times of signal changes are employed to cause
attenuations or reinforcements of signals depending upon gas bubble sizes or oil droplet diameters and volumes. The combinations of frequencies ranging from high to
low, and varying rates of change within signals assist in sorting smaller and larger
bubbles and globules. The dimensions of water concentrations between other fluid
contacts also alters the broad range of signals in different ways. Significant fluid geometry information is extractable from the many signals being altered by the flowing fluids and then detected at the receivers of the present invention.
As best illustrated in Figure 3, several fluid receiving recesses 70, 71 and 72 are
defined between the circumferentially spaced blades in an area intermediate the
external surface 65 of the tool body and the wellbore wall (not illustrated). The
recesses 70, 71 and 72 are illustrated in Figure 3 between dotted lines 73, 74 and 75, respectively, and external tool surfaces 76, 77 and 78, respectively, of the tool 50.
The primary monitored indicator of flow in the recesses 70, 71 and 72 is
preferably a marker comprising a bubble of gas or a gaseous cluster entrained within
the liquid flowing through the recess being monitored. The electrical sensors, circuitry
and analytical process for correlating the measurements taken by the various
transducers determine a rate of movement of the bubble marker past the transducers.
Energy transducers are carried by the blades for evaluating characteristics of fluid contained in the fluid receiving recesses. The measured characteristics are convertible into a measure of the flow rate of the fluid flowing through the recesses. To this end, acoustic transducer receivers 85 and acoustic transducer transmitters 86 are carried in the blades 61 and 60, respectively. Electromagnetic induction transmitting transducers 90 and electromagnetic receiving transducers 91 are carried
in the blade 60 and 62, respectively. Electrical capacitance transducers 95, 96 and 97
are carried on the tool body between the blades 62 and 61.
Referring to Figure 2, the energy transducers carried by the tool 50 are deployed
at axially spaced locations along the tool body 65 and blades 60, 61 and 62 to enable
the transducers to detect variable parameters associated with axial movement of fluid flowing through the recesses with which the transducers are associated. Accordingly, three acoustic receivers 85a, 85b and 85c are deployed at axially spaced locations
along the blade 61 and three acoustic transducer transmitters 86a, 86b and 86c are deployed at axially spaced locations along the blade 60. Similarly, two electromagnetic
transmitters 90a and 90b are axially deployed along the blade 60 and three electromagnetic receivers 91a, 91b and 91c are axially deployed along the blade 62.
Capacitive transducers are also deployed at circumferentially and axially spaced
locations along the body of the tool 50. Capacitive transducers 95, 96 and 97 are displayed in Figure 3 at only one axial location. Similar arrays of capacitive transducers (not illustrated) are deployed at other axially spaced locations between the
blades 61 and 62. The various transmitters, receivers and capacitance energy
transducers are preferably located high within the protected areas between the
stabilizer blades to avoid the mud and rock cuttings that often accumulate in greatest
qualities on the lower portions of the blades. The blades function to form fluid channeling recesses to confine the fluid being monitored and also provide protective structure for the energy transmitters.
With reference to the detail drawing of the transducer 90 in Figure 32A, the induction transmitting antennas of the transducers 90 are positioned within notches in the blade 62 that have curved shapes with sloping surfaces 90b that slightly increase from a parabolic shape to produce an over focusing from a parallel beam to a concentrated point at the receiving transducers 91. Over focusing of the transmitter signal counteracts dispersion caused by bubbles and rock cuttings in the fluid flowing past the sensors. The angles between the transmitters and receivers are preferably optimized for vector processing relating to typical rotation speeds and expected fluid velocities.
As illustrated in the detail drawing of transducer 95, illustrated in Figure 3B, the capacitance transducers 95, 96 and 97 are preferably provided with concave surface electrode shapes 95a to improve contact with the convex surfaces of bubbles or rounded oil globules entrained within the fluid flowing past the transducers. Gas bubble shapes change sizes as a function of changing depth and pressure within the wellbore. The capacitance transducers preferably protrude slightly radially from the body of the tool body 50 with the concave surface shapes having an increasing curvature toward the top 95b of the tool 50 to permit better contact of the surface with both small and larger bubbles. The larger curvature at the top of the transducers permits improved matching of shapes of the smaller bubbles or oil globules with the transducers. The smaller curvature at the bottom 95a of the transducers forms a better match with the external surfaces of larger bubbles or globules.
ln operation, the acoustic and electromagnetic transducers in the tool 50 and
associated instruments in the recording tool 66 monitor the characteristics of the fluid intercepted in the travel paths of the energy signals traveling between transducers.
The capacitive transducers monitor the characteristics of the fluid engaging the reactive surfaces of the transducers. Each of the three acoustic transmitters communicate with
each of the three acoustic receivers to produced nine transmission paths. The paths
are identified as a function of their physical position within the fluid receiving recess.
The electromagnetic transducers function similarly to produce a total of six transmission paths. The radial and axial displacement of transducer paths produces an array of
readings that can be correlated both in time and location to provide the rate of flow of
fluids flowing through the fluid receiving recesses. The change in capacitance along the axial distribution of the capacitive transducers provides a measure of the flow past the monitoring surfaces.
The measuring process performed by the tool 50 is preferably done while the
tool is rotating with the bit in the wellbore. The rotating motion of the tool homogenizes
the liquid and gases into a uniform mixture that enhances the detection capabilities of
the sensors. Rotation of the tool 50 also permits each set of three detection systems
to provide full borehole coverage. The blades ofthe tool protect the measuring devices
from impact with borehole walls and also afford protection from impact with solids in the returning well fluids.
Rotation of the tool produces centrifuging of certain fluids that enter the fluid
receiving recesses of the tool. Gas, oil and water are inclined to be differentially
concentrated by centrifuging. As a result, methane and other gases may be more easily detected as they are concentrated within the receiving recesses by the spinning
motion, pushing denser liquids to the outer edges of the blades. The spinning of the tool also significantly reduces segregation of fluids with respected to the top or bottom
side of an inclined wellbore. Mixtures of liquids commonly encountered in well drilling produce complex combinations of signal frequencies and signal wavelet shapes
transmitted from acoustic and reactive sources to detectors. Analysis ofthe transmitted
signals provides numerous data sets for physically evaluating a slurry having variations
in mixing rules or properties.
The tool 50 may be used as a kick detector during the construction of the well.
The tool's kick detection capability stems from its ability to recognize changes in the
subsurface wellbore conditions and fluids associated with a kick. Subsurface detection
of increased flow rate or other variables can give an early kick warning. If a wellbore influx or kick occurs during drilling, the presence of oil bubbles in the fluid flowing through the recess 72 will slow acoustic travel times between the acoustic sensors 85a, 85b, 85c and 86a, 86b, 86c. Gas bubbles in the recess 72 will cause far greater
increases in acoustic travel time between the energy transducers significant acoustic wave amplitude attenuations will also occur upon the influx of oil or gas into the recess 72. Wave shapes of acoustic signals will be distorted or exhibit complex interference
and dielectric measurements will deviate from drilling mud readings. A predetermined
combination of the described sensor readings causes the software or firmware in the
measurement section 30 to alter mud pulsing priorities and send warnings to the
surface kick detection component 31.
Gas or oil bubbles passing up past the bit during a trip out of the hole are
detected by leaving the power on to the induction and acoustic monitoring systems included in the tool 50. Since mud pulses are not being relayed during tripping, a
waming system as relayed to the drilling crew by changing acoustic pulses to a gas detection indication sequence. A stethoscope type or amplified sound detection and filtering system in the component 31 enables a crewman to hear a kick warning pulse
pattern (e.g., SOS) during a brief quiet period (block lowering time) between pulling
each stand.
The tool 50 may also be used to indicate early wellbore stability problems.
Faster acoustic travel times, some resistivity changes, and some dielectric changes can
indicate increases in quantities of rock cuttings. Mud velocity reductions or other
actions may be taken to reduce excessive "washing out" or widening of the borehole after increased cuttings volumes from weaker formations are detected.
It will be appreciated that various modifications can be made in the design, construction and operation of the present invention without departing from the spirit or scope of such invention. Thus, while the principal preferred construction and mode of
operation of the invention have been explained in what is now considered to represent its best embodiments, which have been illustrated and described herein, it will be understood that within the scope of the appended Claims, the invention may be
practiced otherwise than as specifically illustrated and described.