Apparatus and method for accurately measuring formation pressures

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.

BACKGROUND 15

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

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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.

OBJECTS

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.

The Invention

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

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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,