Accelerated Downhole Pressure Testing

The present invention relates to the field of oil well completion and formation testing and, more particularly, to an improved method and apparatus for determining the shut-in pressure of a hydrocarbon formation. Still more particularly, the present invention includes a method and apparatus for measuring the shut-in pressure of a hydrocarbon formation perforated by a perforating gun suspended from a tubing string.

One method for testing a formation in a cased well includes running an electric line casing gun perforator in mud of sufficient weight to control the well pressure, perforating the casing adjacent to the zone to be tested, and then withdrawing the perforating gun. Test tools are then run into the well on a pipe string with well pressure being controlled with casing fluid of appropriate weight. A packer is set to close the annulus and a valve is opened in the pipe string to permit fluids from the formation to flow through the pipe string to the surface.

A second method for testing a formation includes running a tool string on drill pipe into the cased borehole with the tool string including full opening test tools with a full opening valve, and a packer disposed on the tool string for packing off the annulus. The casing adjacent to the zone to be tested is packed off with the packer and the full opening valve is then opened, providing fluid communication between the flow bore of the pipe string and the lower packed off portion of the casing. A small through-tubing perforating gun is lowered on an electric line through the test tools, and the casing adjacent the zone is perforated. The wireline perforating gun is then lubricated out of the well.

Another formation testing method is disclosed in the U.S. Pat. No. 2,169,559. In the '559 patent, a formation tester, sub, packer, perforated pipe, perforating gun, and bull plug are all suspended on the end of a drill pipe string. The formation tester includes a limited opening valve and a mandrel for opening the valve. A number of passageways in the sub permit fluid flow from a point beneath the sub into the formation tester. The bull plug below the perforating gun may include a pressure recording apparatus. In operation, the packer is set to seal the lower portion of the well from the portion above the packer and the drill pipe is rotated and lowered causing the mandrel to open the valve in the formation tester. This actuates the perforating gun which detonates and perforates the formation. Any fluid in the formation then flows through the perforations and through the perforated pipe above the perforating gun. This fluid must then pass through the limited openings of the passageways in the sub and of the valve and into the drill pipe. After a sufficient length of time, the drill pipe is lifted to allow the valve to close. When the valve closes, a sample of the fluid from the formation is entrapped in the drill pipe. The packer is then released and the entire assembly is removed from the well with the entrapped sample.

Various drill stem test procedures may be used in determining the potential productivity of a subsurface hydrocarbon formation which has previously been perforated. The typical procedure begins by including in the drill string various test apparatus. After the packer is set to seal the casing annulus below the packer, the well is perforated and the formation is then permitted to produce through the drill string to provide an indication of the ability of the formation to produce without the use of enhancement techniques.

After a specified time interval, a tester valve positioned in the drill string above the packer is closed, thereby closing-in the hydrocarbon formation. Pressure measuring apparatus within the closed-in portion of the drill string records the rate of pressure build-up and the ultimate shut-in pressure of the hydrocarbon formation. This data also provides an accurate basis for evaluating the hydrocarbon formation, including, for example, the well's production capability, transmissibility, actual flow capacity, permeability, and formation damage. After the test sequence is completed, pressure above and below the packer is equalized, the packer is unseated, formation fluid is flushed from the drill string by reverse circulation, and the drill pipe is pulled from the well. The drill stem test procedure is further described in II Composite Catalog of Oil Field Equipment & Services, 3286-88 (1978-1979) and U.S. Pat. No. 3,970,147 to Jessup et al.

Well completion is typically accomplished by running a small through-tubing perforating gun on a wireline through the tubing string suspended and packed off in the cased borehole. The borehole is filled with drilling mud or some other appropriate fluid so as to prevent the blow-out of the perforating gun upon detonation. However, this drilling fluid also prevents the flow of hydrocarbon fluids into the borehole at the time of perforation. Generally, the through-tubing perforating gun is actuated electrically. After the through-tubing gun is lubricated out of the well, the control fluid in the tubing string is removed to bring in the well.

Another method for completing oil and/or gas wells, now well known in the art, includes lowering into the cased borehole and tubing string and perforating the well by shooting perforations through the casing and cement into the hydrocarbon formation to permit the hydrocarbons to flow into the cased borehole and up to the surface. U.S. Pat. No. 3,706,344 to Vann discloses the method of suspending a packer and perforating gun on a tubing string, setting the packer to isolate the production zone, releasing the trapped pressure below the packer by opening the tubing string to fluid flow, actuating the perforating gun through the tubing string, and immediately producing the well through the tubing string upon perforation. One means for actuating the perforating gun includes dropping a bar through the tubing string to impact the firing head of the perforating gun.

Vann's completion technique exhibits several advantages, as are described in the above-named patent, over prior art completion techniques. Vann's technique does not, however, provide for any particular means for evaluating the potential of the formation. Use of the drill stem test procedure with Vann's technique would require unseating the packer and pulling the entire tubing string from the borehole, thereby defeating one of the advantages of the Vann method. Hence, it would be useful to devise a method and apparatus for evaluating and testing a hydrocarbon formation, the method and apparatus being particularly useful in conjunction with a formation testing or well completion method where the perforating gun is suspended on the tubing string.

The present invention includes a method and apparatus for measuring downhole pressure and/or temperature of an oil or gas well and is particularly useful where the well has been perforated by a perforating gun suspended on a tubing string. A tubing string having a valve, pressure sensing means, an on/off sub, blanking plug sub, packer vent assembly, release coupling, and perforating gun are lowered and suspended within the well with the gun adjacent to the formation to be tested or completed. The valve is located above, but relatively near to, the perforating gun, particularly as compared to the distance between the gun and the surface.

After the perforating gun has been detonated to perforate the formation, and hydrocarbon fluids have been permitted to flow into the tubing string via the vent assembly to clean out the well, the valve is closed to prevent further hydrocarbon flow through the tubing string to the surface. Closing the valve shuts-in the well. The pressure from the formation then builds up in the lower borehole annulus below the valve and packer. The shut-in pressure is then sensed by the pressure sensing means.

The pressure sensing means may take the form of several embodiments. It should be understood that each of the embodiments may function separately and alone to accomplish the objectives of the invention and no individual embodiment is preferred over another. Each embodiment of the sensing means includes a small housing disposed on the exterior of the tubing string below the valve. The housing forms a chamber therewithin. A port or aperture through the wall of the tubing string provides fluid communication between the flow bore of the tubing string below the valve and the chamber of the housing.

In one embodiment, the sensing means includes a small diameter pressure tubing extending from the housing up the tubing/casing annulus to the surface. The lower end of the pressure tubing is in fluid communication with the chamber of the housing and the other end is connected to a pressure monitor device. The pressure monitor device includes a constant, regulated flow of fluid through a flowmeter and pressure sensor and into the pressure tubing. The pressure sensor at the surface provides output signals which are communicated to a recording device.

In operation, the shut-in pressure will vary the pressure at the pressure sensor as the flow rate of the fluid passing through the pressure tubing, chamber, and into the flow bore of the tubing string is held constant. The shut-in pressure is thus measured at the surface. It should be understood that the discharge pressure at the surface is held constant the shut-in pressure is determined from the change in flow rate at the surface.

One variation of this embodiment is the disposal of a diaphragm on the lower end of the small diameter pressure tubing (within the chamber) to prevent commingling of the fluid in the pressure tubing with the hydrocarbon fluids in the tubing string. In this embodiment, the fluid does not flow through the pressure tubing but merely transmits the variation in the downhole pressure as the diaphragm flexes in response to the downhole pressure.

Still another embodiment is a self-contained pressure sensing and measuring means which is disposed within the chamber of the housing. The pressure tubing described for previous embodiments is not present in this embodiment. This embodiment requires the retrieval of the pressure sensing and measuring means.

Although the above embodiments have been described as method and apparatus for measuring the shut-in pressure, they may also be used to provide a continuous monitoring of the downhole pressure. Thus, the present invention is not limited to closing the flow bore of the tubing string but can be used to measure downhole pressure of a producing well with hydrocarbon fluids flowing to the surface during pressure measurement.

The present invention overcomes the problems of the prior art by providing an efficient technique and apparatus by the use of which a well may be perforated with a gun suspended on a tubing string, with its inherent advantages, and subsequently tested and evaluated without defeating the advantages of such a technique, and without introducing additional time into the perforating process. These and other objects and advantages of the present invention will become readily apparent to those skilled in the art on reading the following detailed description and claims and by referring to the accompanying drawings.

For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings wherein:

FIG. 1 shows a part diagrammatical, part schematical, cross-sectional view of a cased borehole having disposed therein a string of tubing arranged in accordance with the principles of the present invention and illustrating multiple embodiments of the invention;

FIG. 2 shows a portion of the borehole of FIG. 1 in a later stage of operation;

FIG. 3 shows the borehole of FIG. 1 at a point in time later than that of FIG. 2;

FIG. 4 shows an enlarged, cross-sectional view taken along line 4--4 of FIG. 1;

FIG. 5 shows a graph which illustrates two downhole variables of the borehole of FIG. 1;

FIG. 6 shows the borehole of FIG. 1 including an alternative embodiment of the present invention;

FIG. 7 shows a broken view of a part of the tubing string depicted in FIG. 6; and

FIG. 8 shows an enlarged, cross-sectional view of a part of FIG. 6.

Perforation of a cased borehole and testing of the surrounding hydrocarbon formation are key steps in the completion of an oil well. Evaluation of the test results provides information regarding the size of the reservoir, the type of fluid or gas within the formation, whether the well will free-flow or will require some type of artificial lift, the permeability of the formation, formation damage, and other important characteristics. Measurements of temperature and pressure, particularly shut-in pressure, are key indicators of the aforenoted characteristics.

Shut-in pressure is the pressure developed within the wellbore subsequent to perforating the casing and hydrocarbon formation and sealing off (closing-in or shutting-in) the well. The present invention provides a method and apparatus for determining the shut-in pressure, as well as temperature and other pressure measurements, when the well is perforated by means of a perforating gun affixed to the lower end of a string of production tubing.

Referring now to FIG. 1, there is shown a borehole, shown generally at 10, extending generally vertically from the surface 14 of the ground beneath a drilling rig 12 into a hydrocarbon formation 18. The interior of the borehole 10 is lined with a casing 16, typically formed of a cement mixture, for securing the integrity of borehole 10. A string of production tubing 20 extends concentrically within the cased borehole from the surface 14 to a point adjacent hydrocarbon formation 18 and supports on the lower end thereof a perforating gun 28.

Tubing string 20 includes an upper tubing portion 42 extending from surface 14 to a valve 30, a medial tubing portion 44 extending from valve 30 to an on/off sub 34, and a lower tubing portion 46 extending from the on/off sub 34 to the perforating gun 28. The medial tubing portion includes a pressure sensing means, indicated at 32 and 74, mounted on the exterior of tubing string 20, and projecting into the annulus formed by tubing string 20 and casing 16. The lower tubing portion includes a packer 22, blanking plug sub 36, vent assembly 38, releasable coupling 40 and perforating gun 28.

Perforating gun 28 may be any apparatus suitable for forming perforations or channels through the casing 16 and into the surrounding geological formation, so as to permit the flow of hydrocarbon fluids from formation 18 into tubing string 20, via vent assembly 38. Perforating guns which meet the aforestated requirement are disclosed in greater detail in U.S. Pat. No. 3,706,344 and U.S. Pat. No. 3,717,095, which are hereby incorporated by reference.

At a point above and relatively near the perforating gun 28, for example, within ten feet thereof, packer 22 forms a fluid seal between casing 16 and tubing string 20 and thereby divides the annulus formed by casing 16 and tubing string 20 into an upper annulus 24 and a lower annulus 26. Packer 22 prevents the flow of hydrocarbons into upper annulus 24, thereby directing the flow through ports in vent asssembly 38 into tubing string 20 and up to the surface 14. U.S. Pat. Nos. 3,871,448, 3,931,855, 4,040,485, and 4,151,880 show suitable packer and vent assemblies and the actuation thereof in greater detail and are hereby incorporated by reference.

Referring still to FIG. 1, tubing release coupling 40 is connected in series with tubing string 20 between vent assembly 38 and perforating gun 28 by lengths of tubing 48, 50. Actuation of release coupling 40, for example by use of a wireline tool suspended from the surface 14, causes tubing length 50 and perforating gun 28 to detach from coupling 40 and drop into the well.

Valve 30, series connected in tubing string 20, is provided for opening and closing the flow bore of tubing string 20 to the flow of fluids. Valve 30 may, for example, be a ball valve which is opened by a 180.degree. rotation of the upper tubing portion 42 above valve 30. It is essential that the opened flow passage of valve 30 be of sufficient diameter to permit the introduction therethrough of tools into the medial and lower portions of tubing string 20 below valve 30. Valve 30 is preferably positioned within approximately 100 feet of packer 22, but may vary somewhat depending on the apparatus included in that portion of tubing string 20 therebelow.

Referring still to FIG. 1, an on/off sub 34 is serially positioned in tubing string 20 between packer 22 and valve 30 in the length of medial tubing portion 44 between valve 30 and on/off sub 34. On/off sub 34 permits the entire length of tubing string 20 above sub 34, namely upper and medial tubing portions 42, 44 (sometimes hereafter referred to as the tool string) to be removed and replaced, for example, with a string of tubing more suitable for producing hydrocarbon fluids due to the absence of certain test apparatus, such as valve 30, found in the tool string.

Blanking plug sub 36 is serially positioned in tubing string 20 below on/off sub 34 and supports about its exterior packer apparatus 22. The upper end of blanking plug sub 36 is connected through a tubing string section 46 to on/off sub 34. The lower end of blanking plug sub 36 supports vent assembly 38. Blanking plug sub 36 includes an interior profile (not shown) arranged to receive therein a blanking plug which can seal off the flow bore of tubing string 20.

It should be noted for the purpose of understanding the operation of the present invention that the length (denoted by l.sub.2) of tubing string 20 below valve 30, namely medial and lower tubing portions 44, 46, is negligible (approximately within 110 feet) in comparison with the length (denoted l.sub.1) of upper tubing portion 42, which may be, for example, between 2,000 and 20,000 feet.

In accordance with the principles of the present invention, tubing string 20 includes an improved apparatus for providing a report of the shut-in pressure of a well which has been perforated by means of a perforating gun or a tubing string. The apparatus may also provide a readout of well temperature and pressure over an extended period of time.

The present invention provides method and apparatus for measuring the temperature and pressure, particularly the shut-in pressure, of a well which has been perforated by means of a perforating gun suspended on a tubing string, as for example described in U.S. Pat. No. 3,706,344, without having to unseat the packer and pull the entire tubing string from the borehole, such as would be required if prior art drill stem test procedures were used to evaluate the well.

Referring now to FIGS. 1-4 showing the preferred embodiments of the present invention, the pressure sensing means, indicated at 32 and 74, is mounted on the exterior of medial tubing portion 44 and projecting into upper annulus 24. Although it is not essential that the pressure sensing means be located on the exterior of tubing string 20, such a question is preferred because locating pressure sensing means in the flow bore of tubing string 20 may cause interference with through-tubing operations, such as the passage therethrough of tools. The pressure sensing means may include recording means at the surface as illustrated with respect to means 32 or may include recording means downhole as illustrated with respect to means 74. It is of course possible to use pressure sensing means 32 and 74 in combination.

Referring now to FIG. 1, pressure sensing means 32 includes an outwardly protruding housing 35 connected through a small diameter tubing 88 to a pressure monitor device at the surface 14. The small diameter tubing 88 is attached to the exterior of tubing string 20 and encircles the exterior of the valve 30 at 86, so as to permit rotation of the upper tubing string 42 without damaging tubing 88. The small diameter tubing 88 may comprise, for example, stainless steel tubing having an inner diameter of from 0.05 inch up to approximately 0.75 inch.

The pressure monitor device includes a fluid source, S, providing a flow of a pressurized fluid according to the controlling effect of a pressure regulator 66. The fluid source S and the pressure regulator 66 may also be replaced by a suitable pump (not shown) characterized by a constant discharge pressure. The fluid may be a compressible or noncompressible fluid. An inert gas, such as nitrogen or helium, would work well as the fluid of the present invention.

Pressurized fluid flows by way of a fluid line 64 through a pressure sensor 62 and from there through a flowmeter 60 by way of an adjoining fluid line. The flowmeter 60 measures the flow rate of the pressurized fluid and communicates this flow data through an output signal line 70 to a suitable recording apparatus 68, such as a chart recorder, a digital record or a graphic plotter. The pressure sensor 62 provides pressure data to the recording apparatus 68 through signal line 72. The pressure regulator 66, the flowmeter 60, the pressure sensor 62, and the recording apparatus 68 are well known to those skilled in the art and are readily available from any of several manufacturers or easily constructed from well-known principles.

The open lower end of the small diameter tubing 88 is fixedly received within housing 35, the interior of which, as hereafter set forth, is in fluid communication with the flow bore of the medial tubing portion 44. In operation, the pressure regulator 66 is adjusted to obtain a desirable rate of fluid flow therethrough. The flow of fluid through the pressure tubing 88 is measured by the flowmeter 60 and compared by means of the recording apparatus 68 to data obtained by precise empirical calibration of the pressure measuring apparatus before its insertion into the borehole. The apparatus may be calibrated, for example, to relate flow through the pressure tubing 88, at a fixed source pressure, to a particular pressure at the downhole end of the tubing 88. In this arrangement, the fluid source S is a constant pressure source providing a flow of fluid at a rate which varies in response to changing downhole pressure. The apparatus is calibrated to correlate flow rate to downhole pressure. The recording apparatus 68 may, for example, include a microprocessor which relates the measured flow to the downhole pressure and plots that pressure as a function of time on a chart.

Pressure sensing means 32 alternatively may include the Pressure Transmission System (PTS) of NL Sperry Sun, a Division of NL Industries, Inc., as the pressure monitor device. The PTS device includes a downhole chamber, positioned within housing 35, connected by the small diameter pressure tubing 88 to a surface readout unit 68. The chamber and tubing 88 are charged with a single-phase gas, such as nitrogen or helium, from the surface. A diaphragm on the chamber compresses the enclosed gas in response to changes in the ambient borehole pressure. A pressure gauge 60 at the surface end of tubing 88 is calibrated to provide a readout of the downhole pressure. The PTS device is described generally in the 1982-1983 Catalog of NL Sperry Sun, reprinted in IV Composite Catalogs of Oilfield Equipment and Services 6425-26 (1982-83).

Referring now to FIG. 4, housing 35 defines an interior chamber 76 which is connected in fluid communication with the interior of the medial tubing string 42 through a port 82. The lower end 80 of small diameter tubing 88 is open to permit the efflux of fluids into the chamber 76. As may be noted from the above description, the open-ended embodiment of tubing 88 shown in FIG. 4 corresponds to the first-described embodiment of pressure monitor device. The second-described embodiment of pressure monitor device (PTS device) includes on the end 80 of tubing 88 an enclosed chamber (not shown) which includes a diaphragm. The enclosed chamber of the PTS device is affixed within the housing 35 on medial tubing portion 44, so as to communicate changes in downhole pressure to the surface.

Referring now to FIG. 1 and another embodiment of the pressure sensing means, pressure sensing means 74 includes a housing 75 protruding outwardly from the medial tubing portion 44 for supporting therein a self-contained temperature or pressure recording apparatus, such as an Amerada bomb, manufactured by Geophysical Research Corp. of Tulsa, Okla. The Amerada bomb is a gauge for measuring temperature or pressure, including a clock, a sensing element, such as a helically wound bourdon tube, and a recording element.

Referring now to FIG. 4, housing 75 supports the self-contained apparatus 92 between a pair of shock mounts 94, 96. A small pipe or port 90 connects the interior of housing 75 in fluid communication with the interior of the medial tubing portion 44 and thereby subjects the apparatus 92 to well temperature and pressure. Means (not shown) is provided on the exterior of housing 75 for removing therefrom and replacing the apparatus 92.

In operation, tubing string 20 of FIG. 1 is lowered into the cased borehole 10 substantially dry. Packer 22 is actuated to effect a seal between upper annulus 24 and lower annulus 26 and vent assembly 38 is actuated to open the ports therein, so as to equalize the pressure of lower annulus 26 with that of the interior of tubing string 20.

When the apparatus is ready, perforating gun 28 is fired, creating a plurality of perforations and channels through casing 16 and into the surrounding formation 18. The pressure differential between the formation pressure and tubing pressure induces an immediate backsurge of well fluids into lower annulus 26, into the interior of tubing string 20 and up to the surface. The backsurge cleans the perforations and channels of debris, mud filtrate, and other contaminants and thereby reduces permanent damage to the formation. The well is permitted to produce until thoroughly clean and then is shut-in by closing valve 30, such as by rotating upper tubing portion 42 180.degree..

The relatively negligible length l.sub.2 of medial tubing portion 44 as compared to the length l.sub.1 of upper portion 42 significantly reduces the period of time required to build up to the shut-in pressure as FIG. 5 graphically illustrates. If the well is shut-in by means of a valve 55 at the surface 14, the length of time required to reach the ultimate shut-in pressure might correspond roughly to point 100 on the solid curve. When the volume of tubing string 20, which must be filled, is substantially reduced by shutting in the well at valve 30, the length of time required to attain the shut-in pressure is reduced to point 101 on the dashed curve. Hence, use of valve 130 rather than valve 55 to shut-in the well substantially reduces the period of time required to measure the shut-in pressure.

The actual pressure measurements are obtained using one or both of the aforedescribed pressure sensing means 32, 74. Sensing means 32, for which two possible embodiments were described, includes means at surface 14 for providing an indication of the downhole pressure. The surface indication of pressure may include a plot of pressure as a function of time, in addition to an indication of absolute pressure at any point in time.

Pressure sensing means 74, which includes the self-contained apparatus 92, (FIG. 4) records pressure (or temperature if such is more desirable) as a function of time over an extended period of time. The record generated by apparatus 92, however, is available for examination only after the tool string (which includes the upper tubing portion 42 and the medial tubing portion 44) is pulled from the well, as hereafter described.

Referring now to FIG. 2, casing 16 has been perforated to form perforations and channels 99 within the surrounding formation 18, the well has been shut-in, and all initial testing and measuring has been completed. If necessary or desirable, the tool string may now be removed. Such may be required where, as mentioned above, it is is necessary to access self-contained apparatus 92 (FIG. 4) or where test apparatus in the tool string makes it cumbersome to produce through the string or where it is desired to complete upper formations before producing the formation just completed. It should be noted, however, that production may proceed immediately without removing or replacing the tool string.

Before removing the tool string, a blanking plug (not shown) is positioned within the corresponding profile of blanking plug sub 36 so as to seal off the flow of well fluids through lower tubing portion 46. The tool string (upper and medial tubing portions 42,44) may then be disengaged by use of an on/off tool (not shown) at on/off sub 34 from lower tubing portion 46.

Referring now to FIG. 3, formation 18 subsequently may be produced by stabbing a string of production tubing 98 onto blanking plug sub 36, removing the blanking plug, and opening valve 55 to permit the flow of well fluids therethrough. Where desirable, the perforating gun 28 may be disengaged at releasable coupling 40 to drop gun 28 down into the well.

Referring generally to FIGS. 6-8, there is shown an embodiment of the invention wherein another type of pressure sensing means 74, 174 includes a pair of outwardly protruding housings 75, 175 which open to the interior of tubing string 20, so as to facilitate removal and replacement of apparatus 92 without pulling tubing string 20 from the well.

Referring particularly to FIG. 6, a string of tubing 20 is concentrically disposed within a cased borehole in a manner substantially the same as in FIG. 1, described above. Tubing string 20 of FIG. 6, however, does not include a housing 35, but includes an additional lower housing 175. Housing 175 is identical in structure to housing 75, but is positioned on tubing string 20 between releasable coupling 40 and perforating gun 28.

Referring now to FIGS. 7 and 8, housing 175 (which is identical to housing 75) is open to the interior of tubing string 20. A slickline or wireline 103, having a fishing tool 104 affixed to the lower end thereof, is used to remove self-contained pressure measuring apparatus 92 from, and replace the same within, housing 175 in a manner similar to that used to remove and replace a gas lift valve. As shown in FIG. 8, apparatus 92 includes a fishing neck 105 whereby fishing tool 104 can engage the apparatus 92, in a manner well known to those skilled in the art.

In summary, the present invention provides a method and apparatus for measuring the pressure and temperature of a well perforated by means of a perforating gun suspended on a tubing string. Prior to the development of the present invention, detailed well evaluation was a cumbersome or impossible task for a well completed by use of such method.

While a preferred embodiment of the invention has been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit of the invention.