Borehole Pressure Measurement

The invention is related to applications bearing Ser. No. 414,060 entitled "PRESSURE MEASURING SYSTEM" which was filed November, 1973 now U.S. Pat. No. 3,895,527, and Ser. No. 414,062 entitled "METHOD AND APPARATUS FOR MEASURING PRESSURE RELATED PARAMETERS" which was filed November, 1973 now U.S. Pat. No. 3,898,877.

The invention relates to the measurement of fluid pressure at a remote location and is of particular application to measurement of pressures in a borehole.

The pressures of fluid in boreholes are particularly important in the production of oil and gas. Secondary recovery operations, for example, require pressure information to determine a number of factors necessary to the success of the operations.

Preliminary to the secondary recovery operations borehole pressures give an indication of well productivity potential and the amount of fluid that will be required to "fill up" the space in the formation before oil and gas will begin to be forced out. During operations the measurement of pressure changes in a number of boreholes in a formation indicate the location of injection fluid flood fronts as well as the efficiency with which the flood front is sweeping through the formation.

In addition to secondary recovery operations, borehole pressures are important in other areas of oil and gas production. For example, pressure measurements may be used to indicate wellbore damage or any number of other problems in pumping wells.

The invention provides substantial improvement over previous methods of borehole pressure measurement. Some previous methods provided only periodic measurements, which are not only inconvenient and time consuming due to the necessity of inserting instrumentation into the borehole at each measurement but also incomplete in the representation of borehole conditions. An example of this type of system is disclosed in U.S. Pat. No. 3,712,129 to Rhoades. Each time a pressure measurement is desired, Rhoades charges an open-ended tube with a gas until the gas bubbles from the bottom of the tube. The pressure in the tube at the surface at which bubbling begins is the pressure of the borehole fluids. In contrast the present invention is capable of providing continuous pressure information.

Other bubble tube systems using the principle of pressure equalization include U.S. Pat. No. 1,289,755 to Haynes, which relates to measuring the depth of water. The Haynes apparatus uses a tube having an enlarged sleeve on its lower end. The tube and sleeve containing air at atmospheric pressure are lowered into the water. Shortly after the tube enters the water, a pressure storage tank begins to automatically supply pressurized air to the tube through a spring-loaded valve. The spring tension is initially overcome by the sleeve, which collects a larger volume of air and has a larger cross-sectional area than the tube. Since force equals pressure times area, the larger area of the sleeve cross-section allows the air therein to be forced into the tube at a lower water pressure. This permits the valve to be opened at a shallower water depth measurement to thereby begin.

As the tube is lowered into the water, the air therein is compressed by the force of water pressure, which delays the opening of the valve. In order to make the valve opening closely follow movements of the lower end of the tube, the compression must be compensated. This can be done by increasing the volume of air in the tube. This extra volume of air is provided by the sleeve. The sleeve is used only when the tube is initially placed in the water and plays no part thereafter. It does not prevent water from entering the upper part of the tube, nor does it act to maintain the water level near the bottom of the tube in order that pressures measured closely reflect those at the bottom thereof. The pressures are measured at different depths at the pressure required to bubble the air from the bottom of the tube, not at a pressure at which the water level is higher in the tube than the bottom thereof.

A number of other pressure measuring devices have been devised to overcome the problems inherent in periodic measurement by providing for permanent installation in producing wells. One such device operates with a downhole pressure transducer having an electronic scanning system for converting downhole pressure into the data transmittable to the surface on a conductor cable. The cable is normally attached to the outside of the tubing and the transducer is mounted on the lower end of the tubing string. The electronics in such a system is expensive and produces maintenance problems stemming in part from high temperatures and corrosive fluids often present in boreholes. In addition, an electronic system using scanners and transmitting such data over conductors is subject to problems of maintaining a high resolution, and thus data may not be as accurate as needed to determine changes in reservoir conditions. In addition, downhole pressure transducers are often intricate in design and thus are subject to the hostile pressure, temperature and chemical fluids environment of wellbores.

It is therefore an object of the present invention to provide a new and improved apparatus for detecting a borehole pressure.

In accordance with the invention, a tube, preferably a microtube, having a chamber attached to the lower end thereof is lowered into the borehole into the area where fluid pressures larger than normal hydrostatic pressures are to be measured. The lower end of the chamber is open and the volume of the chamber is preferably substantially larger than that of the tube. The chamber is partially filled, preferably to about one-half its volume, through the tube from the surface with a test fluid by a pressurized fluid source, and the tube is sealed to prevent escape of the fluid. The entry of the test fluid partially displaces borehole fluids, from the chamber. The chamber is partially filled by observing the pressure of the fluid in the tube at the surface by observing the rate of pressure increase in the tube. Pressurization is ceased after a decrease occurs in the rate of pressure increase but before the rate of increase drops to zero. Thereafter pressure changes may be continuously measured over an extended period by observing the pressure of the test fluid in the tube at the surface.

The volume of the chamber is dictated by the maximum and minimum pressures that are to be measured. The preferable minimum ratio of the chamber volume to the tube volume is equal to the difference between the maximum pressure to be measured and the minimum pressure divided by the minimum pressure.

The invention may be more fully understood by considering a detailed description of the invention in conjunction with the following drawings:

FIG. 1 is a partial cross-sectional view of the wellbore and production equipment, including a pressure measuring system in accordance with the present invention;

FIG. 2 is a cross-sectional view of the downhole pressure measuring equipment shown in FIG. 1;

FIG. 3 is an alternative embodiment of the downhole pressure measuring equipment; and

FIG. 4 is a graph of the behavior of pressure with respect to time in the apparatus.

The use of the invention in connection with a borehole is shown in FIG. 1 in which a wellbore is shown extending into underground formations. Production equipment for producing fluids from the formation is shown schematically and includes a casing 11 in the wellbore having perforations 13 at its lower end to permit the entry of formation fluid. A tubing string 15 extends from a wellhead at the surface downwardly within the borehole to the lower end thereof. Spacers 17 are provided in the tubing string to maintain the tubing centered in the borehole. A mandrel 19 is shown attached to the lower end of the tubing to provide a seat for a downhole pressure probe 27. The downhole pressure measuring probe 27 is shown positioned in the mandrel at the lower end of the tubing. The small diameter hollow tube 29 extends from within the pressure measuring probe. Tube 29 as used in the present system if typically 0.026 inches and 0.054 inches. Tubing in this range is generally known in the art as microtubing. The tube is positioned on the outside of the tubing string and extends to the surface where it exits on the side of the wellhead on a fitting 31. Connected to the tube 29 at the surface is a fluid pressure source 33, which may be a bottle of pressurized gas, and a pressure indicating device 35. A rate meter 36 is shown in the system to measure the rate of pressure change when pressure is applied to the system. The rate may be determined by any sort of device that measures pressure as a function of time. Thus, as pressure is charted versus time or printed out in a timed sequence, this will serve to establish a rate of pressure change.

The particular embodiment of the downhole probe 27 is shown in more detail in FIG. 2. Probe 27 is an enclosed chamber having two ports through which fluid can communicate with its interior. At the upper end tubing 29 provides communication between the probe chamber and the surface. A filter 42 that may be made of a porous metallic material prevents foreign material from communicating between the probe 27 and tube 29. At the bottom of probe ports 44 allow borehole fluid to enter and exit the probe chamber.

An alternative embodiment of the downhole probe is shown in FIG. 3. A cylindrical shell 50 is disposed concentrically about tubing string 115 and attached thereto at its top and bottom to form an enclosed chamber 52 between the cylindrical shell 50 and the tubing string 115. Chamber 52 communicates with the surface at its upper end through porous filter 54 and tubing 29. Borehole fluids communicate with chamber 52 by means of ports 56 located at the bottom end of the probe.

The volume of the chamber in the probe must generally be much larger than that of the tube connecting the probe to the surface equipment. The larger size operates to scale down vertical fluid movement in the chamber in the tube resulting from pressure changes. If only a small tube were used, even a very small pressure change in the borehole would cause fluid to rise a considerable distance through the tubing toward the surface. Larger pressure changes would cause borehole fluids to be forced entirely through the borehole into the surface equipment. In a chamber of large volume, however, a change in pressure, although forcing upward the same volume of fluid as in the previous case, will cause a much smaller change in fluid level due to the much larger volume of the chamber. The volume of chamber required can be calculated from the following: ##EQU1## The ratio of the volume of chamber to the volume of the tube can thus be expressed as: ##EQU2## The test fluids supplied by the pressure source may be any of a various number of fluids of which nitrogen is one that has been found particularly suitable.

In the operation of the apparatus thus far described, the tube and chamber are filled with pressurized test fluid from pressure source 33. The point at which the tube or chamber are filled may be determined by monitoring the pressure of the test fluid with the pressure gauge 35. If the pressure of the test fluid is plotted versus time, a characteristic curve like that in FIG. 4 will be produced, if the tube and chamber are filled at a constant rate. The pressure will steadily increase from zero along portion 100 of the curve in FIG. 4 until it reaches a peak 102. At peak 102 the pressure of the test fluid will have become sufficiently large to begin displacing borehole fluid from the chamber 29 in probe 27 (FIG. 2). After the test fluid has begun to displace the borehole fluid, the pressure will begin dropping in portion 104 of the characteristic curve. The volume occupied by the test fluid increases as a borehole fluid is displaced. After the borehole fluid has been totally displaced test fluid will begin itself escaping from points 44 to prove 27, and the pressure of the test fluid will not be able to change further. This is shown as portion 106 as the characteristic curve. Point 108 of the curve is a point at which chamber 29 is filled with the test fluid. A portion of the test fluid may then be removed such that the chamber is approximately one-half filled. Alternatively, the tube and chamber may be pressurized until after a drop in the rate of pressure increase is measured, and the pressurization ceased before the rate drops to zero. The system is locked in at point 108 by sealing tube 29 at the surface, and thereafter changes in borehole pressure may be read directly from pressure gauge 35.

Continuous pressure measurement over a range of pressure variations without recharging the system are made possible by the chamber. The large volume minimizes the variation in fluid level in the chamber as borehole fluid pressure varies. This provides two advantages. First, corrosive borehole fluids and clogging debris are not blown up into the tube and surface equipment when borehole pressure increases. Second, the pressure measured at the surface, and thus the pressure at the level of the fluid in the chamber, is always very close to the pressure at the level of the fluid in the borehole outside the chamber.

The chamber is preferably filled to one-half its volume with the test fluid, leaving the remainder filled with borehole fluids in order that the borehole pressure can fluctuate over equal ranges upwardly and downwardly. Alternatively, the test fluid can be biased to any level in the chamber if general decreases or increases in pressure are anticipated.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from the true spirit and scope of the invention. It is therefore the intention in the appended claims to cover all such changes and modifications.