14 small pressure changes is disclosed in the U.S. Patent

^5 application 374,193 entitled "Analyzing a Hydrocarbon _i g Reservoir by Determining the Response of that Reservoir to

.., Tidal Forces," which was filed concurrently with this appli- _i g cation and which is hereby incorporated by reference for all

^g purposes.

20

2i Currently, reservoir pressure measurements are made through

22 the use of various types of absolute pressure transducers 23 installed, either temporarily or permanently, in a well

2 ₄ communicating with the reservoir. These types of trans-

25 ducers do not have adequate precision to accurately measure ₂ g earth-tide induced pressure changes.

27 28 SUMMARY OF THE INVENTION 29 30 The present invention involves an apparatus and a method for

₃₁ the accurate measurement of small pressure changes. The 2 apparatus comprises a reference accumulator and differential

₃₃ pressure transducer. The reference accumulator stores a

34 reference pressure that is representative of the environment

0i being measured. The differential pressure transducer, which 02 is connected to the reference accumulator, measures the

03 difference between the reference pressure and the pressure 04 of the environment being measured. The measurement that is

05 made is the time variation of the reservoir pressure with

06 respect to an arbitrary reference (the pressure that is

07 representative of the environment being measured).

08

09 Connected to the reference accumulator is a valve that

10 allows the reference accumulator to be charged to the 1 pressure representative of the environment being measured.

12 The valve also protects the differential pressure transducer

13 from overpressuring. Preferably, the valve is controlled

14 remotely. More preferably, it is controlled remotely by electrical means.

16

17 The apparatus is protected from the environment being

13 measured by the use of an isolation means, such as an ig isolation bellows.

20

2i A data acquisition and control system is connected to the

22 differential pressure transducer to record measurements from

23 that transducer. The data acquisition and control system is

2 also connected to the valve to control the valve. A

25 barometer is connected to that data acquisition and control

26 system. A temperature transducer is physically located on

2 the reference accumulator such that the temperature trans-

23 ducer can be used to measure the temperature of the gas in

2g the accumulator. That temperature transducer is electri-

30 cally connected to the data acquisition and control system.

31

32 Accurate measurements of pressure changes are achieved by

33 storing a pressure representative of the environment being

34 measured, and measuring the difference between that stored

i pressure and the pressure of the environment being measured. 2 Preferably, the stored pressure (the pressure representative 3 of the environment being measured) is adjusted to remove 4 changes caused by temperature. 5 6 BRIEF DESCRIPTION OF THE DRAWINGS 7 3 In order to facilitate the understanding of this invention, 9 reference will now be made to the appended drawings of 0 preferred embodiments of the present invention. The 1 drawings are exemplary only, and should not be construed as 2 limiting the invention. 3 4 Figure 1 is a schematic drawing of one embodiment of the 5 present invention as used within a borehole. 6 7 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 8 g In its broadest aspect, the present invention is an 0 apparatus and method for the accurate measurement of i pressure changes. 2 3 The apparatus has a reference accumulator and a differential 4 pressure transducer. The reference accumulator stores a 5 reference pressure representative of the environment being measured. The differential pressure transducer, which is 7 connected to the reference accumulator, measures the 8 difference between the reference pressure and the pressure 9 of the environment being measured. 0 1 By "reference accumulator," we mean a pressure vessel that 2 has a compressible medium in it that can store hydraulic 3 energy. That compressible medium is usually a gas. The reference accumulator has a diaphragm or membrane which

0 separates the working fluid in the environment (which is an

02 incompressible liquid) from the compressible medium in the 03 accumulator. The reference accumulator is used to provide a

0 reference pressure.

05

06 ^Bv "Pressure representative of the environment being

07 measured," we mean a pressure in the environment that is U8 at one point in time and is used as a set point

09 over a longer period of time. That pressure is an arbitrary 0 pressure insofar as it is taken at an arbitrary point in

1 time. The purpose of the "pressure representative of the

12 environment being measured" is to serve as a reference (or 3 set point) from which pressure changes can be measured.

14 5 By "differential pressure transducer," we mean a device that 6 measures the difference in pressure between two pressures.

17 In one embodiment, the differential pressure transducer

13 produces a voltage that is proportional to the difference in ig the two pressures.

20

2i Connected to the reference accumulator is a valve that

22 allows the reference accumulator to be charged to the refer-

23 ence pressure. This valve is left open while the device is

24 lowered into the borehole, then is closed when downhole.

25 The valve provides protection to the differential pressure

26 transducer from overpressuring that would otherwise result

27 from the increasing hydraulic pressure as the device is 28 lowered into the borehole. By "overpressuring," we mean a 29 situation where the measured difference in two pressures 30 exceeds the operating limits of the differential pressure 31 transducer. Preferably, the valve is controlled remotely so 32 that the controlling electronics are not exposes to the 3 rough environment downhole. More preferably, the valve is 4 controlled remotely by electrical means.

i Connected to the valve is an isolation means for protecting 2 the apparatus from the environment being measured. 3y

03 "isolation means," we mean a means that will prevent the environment from damaging the equipment but will not hamper

05 the normal functioning of that equipment. Preferably, that

06 isolation means is an isolation bellows.

07

08 Connected both to the valve and to the differential pressure

09 transducer is a data acquisition and control system. That

10 data acquisition and control system records measurements

11 from the differential pressure transducer and controls the •> 2 valve.

13

14 Preferably, a barometer is connected to the data acquisition

15 and control system. That barometer can be an electronic

16 barometer.

17 3 Preferably, a temperature transducer is connected

19 electrically to the data acquisition and control system and 20 physically to the reference accumulator. That temperature

2i transducer is used to measure the temperature of the

22 compressible medium in the reference accumulator to correct

23 for temperature-induced changes in pressure.

24

25 Accurate measurements of pressure changes can be made by

2 storing a pressure representative of the environment being

27 measuied, and measuring the difference between that stored 23 pressure and the pressure of the environment being measured.

29 In that method, the stored pressure is adjusted to remove 30 changes caused by temperature. 31 32 Referring to Figure 1, this invention can consist of two 33 parts. The first part is contained in a sonde which is

3 lowered into a well that communicates with the reservoir

The sonde contains a gas/fluid reference accumulator 10, a temperature transducer 20, an electrically controlled valve 30, an isolation bellows 40, and a differential pressure transducer 50. The second part consists of a data acqui¬ sition and control system 60 and a barometer 70. This second part stays on the surface.

During installation, the data acquisition and control system 60 causes the electrically controlled valve 30 to remain open. This allows the reservoir pressure to be transmitted to the reference accumulator 10 through the isolation bellows 40. It also protects the differential pressure transducer 50 from overpressuring by assuring that both transducer ports are connected to essentially the same pressure. Once installation is complete and the depth of the instrument is held constant, the data acquisition and control system 60 closes the electrically controlled valve 30. The reference accumulator 10 now holds a reference pressure which is the absolute pressure of the reservoir at 0 the time the electrically controlled valve 30 was closed. The data acquisition- and control system 60 now reads the 2 output of the differential pressure transducer 50 to record 3 variations in the reservoir pressure. The barometer 70 is also read to allow correction for the effects of atmospheric 5 pressure variations on the reservoir pressure. As the 6 reservoir pressure varies it will eventually increase or 7 decrease beyond the range of the differential pressure 8 transducer 50. When this occurs the data acquisition and g control system 60 will open the electrically controlled 0 valve 30 and allow the pressure in the reference accumulator 2 10 to match the then current reservoir pressure. The 2 electrically controlled valve 30 is then closed and data 3 acquisition is continued. 4

In another embodiment, the reference accumulator is pre-charged prior to being lowered into the borehole.

while the present invention has been described with reference to specific embodiments, this application is intended to cover those various changes and substitutions which may be made by those skilled in the art without. departing from, the spixit. and. scope.. o tiie. apend d, claims,