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APPARATUS AND METHOD FOR ACCURATELY MEASURING FORMATION PRESSURES
FIELD OF THE INVENTION 5
This invention relates to apparatus and method for accurately, and quickly measuring the formation pressure, and permeability in an oil or gas producing formation. The apparatus can be borne by cable, or a drill string. The tool, inter alia, thus relates, in particular to 10 an improved wireline testing tool, and method for testing the formation pressure, formation permeability, and other values of oil or gas producing formations.
Wireline formation testers, tools for the extraction of formation fluids from the wall of an open borehole full of mud, have been known for many years; and tools of this class are used extensively in oil and gas exploration. Typically, a tool of this type includes a fluid entry port, 20 or tubular probe cooperatively arranged with a wallengaging pad, or packer, which is used for isolating the fluid entry port, or tubular probe from the drilling fluid, mud, or wellbore fluids during the test. The tool, in operating position, is stabilized via the packer mecha- 25 nism within the wellbore with the fluid entry port, or tubular probe, pressed against the wall of the subsurface formation to be tested. Gas, or other fluid, or both, is passed from the tested formation into the fluid entry port, or tubular probe via a flowline to a sample cham- 30 ber of defined volume and collected while the pressure is measured by a suitable pressure transducer. Measurements are made and the signals electrically transmitted to the surface via leads carried by the cable supporting the tool. Generally, the fluid pressure in the formation 35 at the wall of the wellbore is monitored until equilibrium pressure is reached, and the data is recorded at the surface on analog or digital scales, or both.
The tools in present use have generally performed satisfactorily in measuring formation pressures, and 40 permeability determinations, when testing medium permeability, consolidated formations. This is not the case however, when testing tight (low permeability) or unconsolidated (very high permeability) formations. Clay particles, naturally occurring or introduced by the dril- 45 ling fluid, exist in the wall pore space. In low permeability formations, these particles often adversely affect tests run at conventional flow rates by blocking the pore throats. In a tight zone, high permeability streaks release fluids and produce a buildup. The chances of set- 50 ting the tool in a tight spot are always large, resulting in a "dry" test. The flowing pressure drops rapidly to zero and stays there. Most of the time no buildup occurs. When a slow buildup is recorded, starting next to zero pressure, it may be the result of fluid flow from the 55 formation, but most likely it is due to a small leakage between the pad and the sandface. The pressure creeps slowly up, with a buildup-like shape, since the leakage decreases with the differential pressure between the borehole and the flowline. If the tester is left in place 60 long enough, the pressure may go up to mud pressure. Needless to say, the buildup curve is meaningless in such a case.
Tight formation testing is also complicated by the supercharging effect. The mud filtrate, which is forced 65 through the mudcake, is injected into the formation. This injection of mud filtrate causes a pressure buildup in the formation. The sandface pressure, pressure mea
sured immediately behind the mudcake, may exceed the formation pressure by up to several hundred psi depending on the mudcake and formation permeabilities.
In medium permeability, shaly formations, damage due to both mud filtrate invasion and drilling fluid small particles invasion may render the invaded zone quasiimpervious. The filtrate damaged zone may extend several feet deep, and the particles damaged zone, up to Jth of an inch deep. Such a formation behaves like a tight zone; dry tests, slow buildups, and pad leakage may be experienced.
In very soft formations such as those encountered in the Gulf Coast, even when using sophisticated snorkel tubes and filters, the formation craters during the flow period and the seal is lost. The pressure in the flowline jumps to hydrostatic mud pressure and no buildup is recorded.
It is, accordingly, the primary objective of this invention to provide an improved apparatus, and method, for testing the formation pressure in oil or gas producing formations; particularly in low permeability and high permeability formations.
In particular, it is an object to provide an improved testing apparatus for lowering into wellbores, via attachment to the end of a cable or drill string, for determining formation pressure, formation permeability, supercharging and mudcake characteristics.
These objects and others are achieved in accordance with this invention, an apparatus embodiment of which includes, preferably as a component of a wireline test tool, or generally similar apparatus borne by a drill string, an extended drawdown subassembly, or formation pressure test unit, which comprises
a pair of interconnected liquid filled chambers, each connected through a controlled valve opening with the passageway and pressure gauge, a first chamber containing a reciprocably mounted piston to provide a variable volume chamber for controlling, when the valve is opened, the pressure applied upon the pressure gauge for stabilization of pressure during testings, and a second chamber for measuring, when the valve is opened, the pressure drawdown rate of the penetrated formation as flowline pressure drops below formation pressure, providing a means for determining very quickly formation pressure and formation permeability.
The apparatus includes, in particular, as part of the apparatus combination, the usual tool body, and passageway into the drill body housing test components which includes a pressure gauge, and at least one test chamber for adjusting, or regulating the flow rate of connate fluids introduced into the passageway from the subsurface formation. The tool also contains the usual means for affixing and stabilizing the tool body in the wellbore at the level of the formation to be tested, this including an extensible packer assembly, and pad with pad opening adapted for sealing engagement and alignment of the pad opening with the passageway into the tool to isolate same from wellbore fluids, and to establish a path for fluid communication between the subsurface formation and the tool body passageway. The improvement in the overall apparatus combination further requires the presence of the extended drawdown subassembly, or pressure formation test unit, constituted of
a pair of interconnected liquid filled chambers, each connected through a controlled valve opening with the passageway and pressure gauge, a first chamber the volume of which can be varied by the presence of a reciprocably mounted piston for adjusting, regulating, 5 and controlling, when the valve is opened, the pressure applied upon the pressure gauge during testing, and a second chamber for measuring, when the valve is opened, the pressure drawdown rate of the penetrated formation as the flowline pressure drops below forma- 10 tion pressure, providing a means for determining very quickly formation pressure and formation permeability.
The use of the extended drawdown subassembly, or formation pressure test unit, as part and parcel of the overall combination makes it feasible to accurately, and 15 quickly test formations, particularly low permeability and high permeability formations, to determine formation pressure, formation permeability, supercharging and mudcake characteristics. By using a very slow rate of pressure decrease in the tool flowline, the formation 20 pressure and permeability can be determined quite quickly, generally within the first minute of testing. No pressure buildup is necessary, as required in accordance with conventional techniques. In low permeability formations, corrections can be made for the supercharging effect using the data collected. In high permeability and soft formations, the formation pressure can be determined even if the seal is lost during the flow period. A simple mathematical model can be used for determining 3Q formation pressure, formation permeability, supercharging and mudcake characteristics.
A preferred apparatus, method, and the principles of operation of said apparatus, and method, will be more fully understood by reference to the following detailed 35 description, and to the drawings to which reference is made in the description. The various features and components in the drawings are referred to by numbers, similar components being represented in the different views by similar numbers. Subscripts are used in some instances with members where there are duplicate parts or components, or to designate a sub-feature or component of a larger assembly.
REFERENCE TO THE DRAWINGS
In the drawings:
FIG. 1 depicts a novel, improved type of wireline testing tool useful for testing the formation pressure, and formation permeability, in a subsurface formation. The tool in this instance is suspended via a cable within 50 a wellbore, after having been lowered from the surface through a number of formations.
FIG. 2 depicts, in a somewhat enlarged sectional view, the wireline testing tool with an external wall removed to expose various sub-assemblies, particularly 55 the extended drawdown sub-assembly, or test unit, for measuring formation pressure, and permeability. In this figure the tool is set in place within the wellbore at the wall of the formation to be tested.
FIG. 3, or more specifically FIGS. 3A, 3B, 3C and 60 3D are a series of fragmentary views representative of the positioning, and functioning of the extended drawdown sub-assembly, or test unit, as employed in the measurement of formation pressure, and permeability.
FIG. 4 graphically depicts the early drawdown per- 65 iod initiating a cycle of operation of the extended drawdown sub-assembly, or test unit, which becomes essentially a straight line function, decreasing gradually from
a higher value for mud pressure, Pm, and ending with a lower value for formation pressure, Pe.
FIG. 5 graphically depicts the balance of the curve, typical of a cycle of operation of the extended drawdown sub-assembly, or test unit, employed in the measurement of formation pressure, and permeability.
FIG. 6 depicts a tool as previously described, except that in this instance, the tool per se is incorporated in a drill string just above the bit and borne by the drill string. The drill string is thus used to lower and raise the tool in the wellbore, and carries the required electronic circuitry for transmitting signals, and commands from the surface to the tool, and vice versa.
FIG. 7 is a cross-section taken through Section 7—7 of FIG. 6.
FIG. 8 is a cross-section taken through Section 8—8 of FIG. 6.
FIG. 9 is a partial cross-sectional view of the tool described by reference to FIGS. 1 through 5 except that in this instance the tool is drill string borne, and includes ducts for transport of drilling fluid from the drill string to the bit. Activation of the tool as required in its operation, and function, can be made by the transmission of mud pressure signals sent from the surface, while data is transmitted to the surface by mud pressure signals. These and other electronic communication techniques per se are well within the skill of the present art.
Referring first to FIG. 1, there is shown a wireline testing tool 10, as the tool would appear after it had been lowered from the surface through a series of subsurface formations and wellbore casing 5 on a multiconductor cable 11 into a fluid or mud filed wellbore 12, or borehole, to a level opposite a specific subsurface formation 13 to be tested. The tool 10 is suspended in the mud filled borehole 12 from the lower end of the multiconductor cable 11 that is conventionally spooled at the surface on a suitable winch and coupled to a tool control system, recording and indicating apparatus, and power supply, not shown. Control signals are electrically transmitted from the surface, and measurements made with the tool 10 are transduced into electrical signals and transmitted as data via the multiconductor cable 11 to the surface recording and indicating apparatus; this generally including both analog and high resolution digital scales. Control from the surface permits operators to place the tool 10 at any of a number of operating positions, and to selectively cycle the tool from one position to another as may be required. These control mechanisms per se for control and manipulation of the tool from the surface are conventional, as are the data gathering and recording techniques.
Continuing the reference to FIG. 1, and also to FIG. 2, the tool 10 is constituted of an elongated body formed by an enclosing wall 15. At locations just above and just below the mid section, respectively, and on one side of the elongated body there is located a pair of selectively extendible anchoring pistons I61, 17i and on the opposite side thereof a packer assembly 20, which includes a pad 21 which is also extendible outwardly from the surface of the body 15 via a pair of laterally movable pistons 23], 241. The simultaneous extension of the pistons I61,17i and pad 21 from within the body of the tool 10, via actuation of pistons 23i, 24i, for contact with the surrounding wall 12 of the subsurface formation 13, as shown by reference to FIG. 2, lock and stabilize the tool 10 in place for operative analysis. So positioned, the pad 21 provides a means for sealing off a selected portion of the wall of borehole 12 from the wellbore fluid,
or mud, and for establishing a passageway between the chamber 51 is thus provided via use of a cyclinder-pis
tool 10 and subsurface formation 13 so that fluid may be ton unit; a unit constituted of a housing 52, or wall
transferred from inside the formation 13 into the tool for surrounding a cylindrical shaped opening within which
analysis. is fitted a reciprocably mounted piston 53. The piston
A hydraulic system, which includes a motor 9, pump 5 53, suitably, is mounted on the upper end of a threaded 8 and reservoir 6, per se of conventional design is opera- shaft 54, which in turn is mounted, via threadable tively connected to a manifold, through multiport means, within a rotatable body 55, coupled with a valved connections, provide the hydraulic power re- motor gear drive. With valve 58 open, on withdrawal of quired for actuation of the pistons 16j, 17i, and pistons the piston 53 the chamber 51 can be opened and its 23i, 24i of the packer assembly 20. The pistons 16i, 171 10 volume progressively increased. Conversely, on adare components of hydraulically actuated cylinder-pis- vancing the piston 53 upwardly into the chamber 51, ton units 16, 17. Hydraulic fluid, under pressure, intro- the chamber volume can thus be progressively deduced via lines I62, 172 into the rearward ends of the creased, and closed. Thus, activation of the motor M, housings of the cylinder-piston units 16, 17 produce moves the gears 57,56 in one direction to raise the shaft extension of the pistons 161, 171 from within their en- 15 54 which carries the piston 53 into the cylindrical openclosing housings, or cylinder I63, 173. The helical ing of the housing 52, this progressively decreasing, and springs seated in the forward ends of the cylinder-piston closing the chamber; or alternatively activation of the units 16, 17 are compressed on extension of the pistons motor M to move the gears 57, 56 in the opposite direcI61,17i so that on reversal of the applied pressure, and tion withdraws the piston 53 from the housing 52 to release of the applied pressure, the pistons 161, 171 are 20 open the chamber. A potentiometer circuit 59 is prowithdrawn or retracted into their respective cylinders vided to monitor, and record the position of the piston or housings. Suitably, double acting cylinder-piston 53 within the housing 52; and via electrical circuitry, units can be employed, i.e., hydraulic fluid could be not shown, the signal can be carried to the surface, and alternately applied to the two ends of a cylinder I63, read at the surface. The motor M, and the pressure 173, respectively, to extend and retract a piston I61,17i, 25 gauge 40 are also provided with electrical circuitry, and respectively. leads for control from the surface. The second chamber
The packer assembly 20 is constituted of a sealing pad 45, connected via the valve 46 to the passageway 44 is
21, a support plate 22 on which the pad 21 is mounted, of fixed volume. Its function is to facilitate the retention
and a pair of hydraulically actuated pistons 23i, 24i via of the slow pressure decrease rate. It is also an essential
means of which the pad 21 can be extended, simulta- 30 component of the extended drawdown sub-assembly, or
neously with pistons 16], 17i, into contact with the wall formation pressure test unit 50. Its function, as well as
surface of the borehole 12, to affix and stabilize the tool the function of the extended drawdown sub-assembly
10 within the borehole. Conversely, when required, 50 as a whole will be better understood by the following
these pistons 23, 24 can be retracted simultaneously description of a complete cycle of operation, specific
with pistons I61,17i to release the tool 10 from its previ- 35 reference being made to FIG. 3, or more specifically
ously selected position within the borehole 12. Exten- FIGS. 3A, 3B, 3C and 3D, and by reference to FIGS. 4
sion of the pistons 23j, 24j is accomplished by the intro- and 5 which explain the methodology of the operation,
duction of hydraulic fluid into the rearward ends of the In operation of the tool, with the tool now positioned
housings 23, 24 of these units via lines 232, 242. Retrac- in the wellbore opposite the formation to be tested, the
tion of the pistons 23i, 24j occurs via the introduction of 40 pistons 23i, 24j are projected outwardly, which moves
pressurized hydraulic fluid into the opposite side of the the pad 21 of the packer assembly 20 into contact with
housing of the cylinder-piston units 23, 24. Alterna- the surface of the wellbore. The pad 21 is thus pressed
tively, compressed coil springs can be employed to tightly against the wall of the wellbore opposite the
retract the pistons. Besides this function, in any event, formation to be tested, by virtue of which the interior of
the packer assembly 20, after the tool 10 has been low- 45 the tool 10 is isolated from the wellbore fluid,
ered from the surface to a level opposite a wall of the A complete cycle of operation, beginning just after
targeted subsurface formation 13, is used to seal off lowering the tool to a preselected depth into the bore
from borehole fluid, or mud. a selected portion of the hole, is described as follows: A first step, Step 1, is
borehole wall 12, with its mudcake lining 14, and pro- required where it is necessary to compute supercharg
vide a path, or passageway, for the transfer of connate 50 ing; as occurs in low permeability, or tight formations,
fluid from within the subsurface formation into the tool Thus, in order to keep the formation from producing
for testing. into the borehole the mud hydrostatic pressure must be
The extended drawdown sub-assembly, or formation greater than the sandface pressure, i.e., Pm>PsF- Conpressure test unit 50, is constituted of a pressure gauge sequently, in a tight formation some filtration through 40, inclusive of a pressure sensor (not shown), directly 55 the mudcake will take place and the pressure in the communicated with the passageway 44, and a pair of flowline will decrease slowly. This decrease can be interconnected chambers, 45, 51 each also connected related to filtration rate, a value which can be used later through a controlled valve, 46 and 58, respectively, to correct for supercharging.
with the passageway 44, and pressure gauge 40. The Step 1: Reference is first made to FIG. 3A, and also to first of these chambers, chamber 51, is one the volume 60 FIG. 4. To begin an operation, valves 46, 58 are opened of which can be varied due to the presence of a movable and piston 53 is thrust to its extreme upward position, piston. Retraction, or withdrawal, of the piston from The tool is set in place by pressing the pistons 16], 17] within the housing wall forming the chamber opens the and pad 20 against the wall of the formation. The formachamber; continuing withdrawal of the piston increas- tion 13 is open to the entry 32 of the block 31, constituting the volume of the chamber Upward movement of 65 ing a component of the packer assembly 20, via passagethe piston decreases the volume, and closes the cham- ways 33, 44 past valved equilibrium line 8 to the exber. The variable volume chamber is directly communi- tended drawdown sub-assembly 50 (FIG. 2). So posicated with the passageway 44. The variable volume tioned, the chamber 45 is filled with drilling mud (or