GB2258923A - Method of determining compressibility of an offshore reservoir bed - Google Patents
Method of determining compressibility of an offshore reservoir bed Download PDFInfo
- Publication number
- GB2258923A GB2258923A GB9118252A GB9118252A GB2258923A GB 2258923 A GB2258923 A GB 2258923A GB 9118252 A GB9118252 A GB 9118252A GB 9118252 A GB9118252 A GB 9118252A GB 2258923 A GB2258923 A GB 2258923A
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- reservoir
- compressibility
- pressure
- measurement values
- tidal effect
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- 238000000034 method Methods 0.000 title claims abstract description 20
- 230000000694 effects Effects 0.000 claims abstract description 22
- 238000009530 blood pressure measurement Methods 0.000 claims abstract description 12
- 239000012530 fluid Substances 0.000 claims abstract description 12
- 125000004122 cyclic group Chemical group 0.000 claims abstract description 9
- 238000009795 derivation Methods 0.000 claims abstract description 5
- 238000005259 measurement Methods 0.000 claims description 12
- 238000012545 processing Methods 0.000 claims description 6
- 239000000463 material Substances 0.000 claims description 4
- 238000001914 filtration Methods 0.000 claims description 3
- 238000011156 evaluation Methods 0.000 claims description 2
- 239000013590 bulk material Substances 0.000 claims 1
- 230000002596 correlated effect Effects 0.000 abstract description 5
- 239000011343 solid material Substances 0.000 abstract description 5
- 239000011435 rock Substances 0.000 description 11
- 238000005056 compaction Methods 0.000 description 7
- 238000005070 sampling Methods 0.000 description 6
- 238000000605 extraction Methods 0.000 description 4
- 239000011148 porous material Substances 0.000 description 4
- 239000007787 solid Substances 0.000 description 4
- 230000003466 anti-cipated effect Effects 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 239000000470 constituent Substances 0.000 description 2
- 230000007423 decrease Effects 0.000 description 2
- 230000001419 dependent effect Effects 0.000 description 2
- 238000012544 monitoring process Methods 0.000 description 2
- 239000013535 sea water Substances 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
- 238000012937 correction Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000005553 drilling Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
Classifications
-
- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D1/00—Investigation of foundation soil in situ
- E02D1/02—Investigation of foundation soil in situ before construction work
- E02D1/027—Investigation of foundation soil in situ before construction work by investigating properties relating to fluids in the soil, e.g. pore-water pressure, permeability
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
- E21B49/006—Measuring wall stresses in the borehole
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B49/00—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells
- E21B49/008—Testing the nature of borehole walls; Formation testing; Methods or apparatus for obtaining samples of soil or well fluids, specially adapted to earth drilling or wells by injection test; by analysing pressure variations in an injection or production test, e.g. for estimating the skin factor
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
- G01N33/24—Earth materials
- G01N33/241—Earth materials for hydrocarbon content
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
- G01V1/38—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas
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- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Geology (AREA)
- Physics & Mathematics (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Mining & Mineral Resources (AREA)
- Environmental & Geological Engineering (AREA)
- Chemical & Material Sciences (AREA)
- Analytical Chemistry (AREA)
- General Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Geochemistry & Mineralogy (AREA)
- Remote Sensing (AREA)
- Fluid Mechanics (AREA)
- Oceanography (AREA)
- Acoustics & Sound (AREA)
- Pathology (AREA)
- General Health & Medical Sciences (AREA)
- Biochemistry (AREA)
- Food Science & Technology (AREA)
- Medicinal Chemistry (AREA)
- Immunology (AREA)
- Geophysics (AREA)
- Hydrology & Water Resources (AREA)
- Soil Sciences (AREA)
- Paleontology (AREA)
- Civil Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Structural Engineering (AREA)
- Geophysics And Detection Of Objects (AREA)
Abstract
A method of determining compressibility of an offshore reservoir bed comprises measuring pressure (Pz) within the reservoir and pressure (Pp) at the seabed above the reservoir at a sufficiently high frequency to detect cyclic change due to tidal effect. The reservoir and seabed pressure measurement values are correlated to allow derivation of a tidal effect ratio (dPp/dPz). This is used together with data specific to the reservoir, such as compressibility of the reservoir fluid and porosity of the reservoir solid material, to obtain a value for compressibility of the reservoir bed. The compressibility is used to calculate oil flow and predict possible subsidence of offshore platforms. <IMAGE>
Description
A METHOD OF DETERMINING COMPRESSIBILITY OF AN OFFSHORE RESERVOIR
The present invention relates to a method of determining compressibility of an offshore reservoir, especially an oil reservoir.
Offshore oil reservoirs are frequently located in subterranean sedimentary rock such as chalkbeds, which undergo a significant degree of compaction as oil is extracted from the reservoir. It is important to assess the likely compaction, in particular by measuring the compressibility of the rock, so as to obtain an indication of the likely oil yield and also the anticipated subsidence of the drilling and production platform.
The association of likely yield and measured compressibility results from the fact that the compaction or lithic drive of the bed represents an expulsion force promoting extraction of the oil from the reservoir. The estimation of platform subsidence is of importance for determining the correct elevation of platforms. If insufficient allowance is made for subsidence, it may be necessary to carry out costly and complicated adjustment of platform deck heights, and even if adjustment is required it is desirable to obtain the earliest possible indication of a date when reconstruction measures should be initiated.
It is known to measure subterranean pressure at oil reservoirs by means of downhole pressure gauges permanently situated in well bores in the reservoir. Such gauges provide pressure measurements at a sampling period of, for example, once every 12 hours and the measurements indicate the decline of pressure over the life of the field. Since the field life may be 20 years or more, such a sampling period is more than adequate. Sampling at a shorter period may be carried out in some instances by appropriately sensitive gauges specifically in response to noteworthy short term changes in the reservoir pressure, for example when the rate of oil flow from the reservoir is intentionally reduced. When the gauges detect restoration of a predicted normal rate of change, the gauges revert to the longer sampling period.
Such reservoir pressure measurements provide a satisfactory indication of pressure changes occurring within the reservoir, but do not provide an accurate indication of the compressibility of the reservoir bed. Compressibility is dependent not only on the internal loading of the bed but also on external overburden loading, which includes the weight of seawater above the bed. This weight fluctuates with sea tide change, air tide change and barometric weather effect. Moreover, the compressibility of a bed often changes during the exploitation life of the reservoir, which means that it is highly desirable to be able to monitor the compressibility.
It is accordingly the principal object of the present invention to provide a method of determining compressibility of an offshore reservoir so as to obtain data for field management, in particular for calculation of yield and prediction of possible subsidence of an offshore platform at the field, as well as model data applicable to exploitation of geologically similar offshore reservoirs.
Other objects and advantages of the invention will be apparent from the following description.
According to the present invention there is provided a method of determining compressibility of an offshore reservoir, the method comprising the steps of measuring pressure within the reservoir and pressure at the seabed above the reservoir at such a frequency that the obtained measurement values are subject to cyclic change due to tidal effect, correlating the reservoir and seabed pressure measurement values to allow derivation of a tidal effect ratio of change in reservoir pressure to change in seabed pressure, and evaluating the tidal effect ratio by data specific to the reservoir to obtain a value for compressibility of the reservoir.
By means of such a method it may be possible to obtain a relatively accurate indication of the compressibility of the reservoir, and extended monitoring of the compressibility value will allow changes in the compressibility to be detected.
The obtained tidal effect ratio is preferably evaluated in data processing apparatus in conjunction with previously ascertained values for the compressibility of the fluid or fluids of the reservoir and the porosity of the constituent solid material, from which by appropriate mathematical analysis a value for the compressibility of the solid material can be derived.
The frequency of the pressure measurements is, for preference, several times per hour, for example at least 10 times per hour.
Pressure measurement can be carried out by subterranean and seabed gauges which sample at the preset frequency, the obtained measurement values then being suitably correlated so that tidal cyclic changes are reflected by the values from the two sources.
The correlation of the pressure measurement values and the derivation and evaluation of the tidal effect ratio can be carried out continuously by data processing apparatus with input means receiving signals indicative of the measurement values and with storage means storing the data specific to the reservoir.
A preferred example of the method of the invention will now be more particularly described with reference to the accompanying drawing, the single figure of which is a diagram illustrating measurement value curves obtained in the course of performance of such a method.
In the preferred example of the method, awell-bore is sunk into an offshore reservoir, for example a chalkbed reservoir from which oil extraction is undertaken by way of a platform supported on the seabed, and a high resolution pressure gauge is permanently installed in the well-bore to measure the internal pressure of the reservoir. The pressure measurement is carried out with a constant and relatively short sampling period of, for example, 6 minutes or less. The gauge can be a quartz-type electronic unit with resolution to three decimal places.
A further pressure gauge with comparable resolution and sampling periodicity is permanently installed at the seabed, for example on the bed itself, on the support structure for the platform or on the subsea oil outlet manifold, and serves to measure pressure at or substantially at the level of the seabed and above the reservoir.
The seabed pressure gauge detects an overburden pressure P represented by the weight of water above the reservoir, the actual weight being subject to a cyclic variation due to the sea tide coupled with variations in air tide and barometric weather effects.
The change dP in overburden pressure corresponds to the product of the seawater density and the tidally-induced variation in the height of the sea level.
The reservoir pressure gauge detects the reservoir internal pressure Pp which in the longer term undergoes an exponential decline due to extraction of the oil reserve, but which in the shorter term, i.e. every 12 hours, undergoes a cyclic change dPp due to compaction and relaxation of the bed caused by the cyclic change in overburden loading. The amplitude of the cyclic change in reservoir pressure is dependent not only on the height of the sea tide and prevailing atmospheric effects, but also on the compressibility Cbc of the rock bulk of the reservoir, i.e. the constituent chalk material in the case of a chalkbed reservoir, with respect to confining stress (stress induced by solids), the porosity of that material, and the compressibility Cf of the fluid content of the reservoir. The fluid content may consist of proportions of, for example, oil and water.
The obtained pressure measurements are correlated to extract cyclic components. This may be achieved by appropriate filtering or by mathematical techniques, in either case to eliminate any phase difference in the measurements from the two sources and thus ensure that correlated measurements pertain to the same tidal state. The correlated measurements are then processed in data processing apparatus to obtain the change values dP p and dP from which is formed a tidal effect ratio dP p/dP, In the drawing there are shown, by way of example, reservoir pressure Pp and overburden pressure Pz curves with pressure values in pounds per square inch plotted against time in hours, the tidal effect ratio being approximately 0.5.
In order to obtain from the tidal effect ratio an indication of the compressibility of the reservoir, and hence the anticipated rate of compaction of the bed solid material as oil extraction continues, the ratio is evaluated in the data processing apparatus in conjunction with data specific to the reservoir, in particular by the equation
in which dP p' dP Cbc, and Cf have the meanings previously attributed and s is Poisson's ratio. Values for the rock bulk compressibility Cbc and porosity lil as well as Poisson's ratio v are determined from core samples of the bed solid material and stored in the processing apparatus and the fluid compressibility Cf is similarly obtained from samples of the reservoir fluid or fluids and stored.
To avoid distortion of the compressibility value by short term pressure changes due to extraneous effects, such as partially shutting down of the oil flow, it is desirable to smooth the tidal effect ratio by filtering out pressure measurement values which exceed appropriately determined thresholds of change.
The equality of the obtained tidal effect ratio with the stated equation from which a useful compressibility value is derivable proceeds from recognition that the reservoir rock bulk compressibility Cbc under confining (solids) stress is related to rock bulk compressibility Cp under pore(fluids) stress and to pore compressibilities Cpc and C pp under confining and pore stresses, respectively, the relationship being as follows: Cp = Cbc Cr (1) Cp =( Cbc C Cr)/ (2) Cpp = [Cbc - (1 + ) C ]/0 (3) in which # is, as already stated, the solids porosity and Cr is the compressibility of the solid rock skeleton.
Similarly, it is recognised that pore strain due p can be expressed by reference either to rock compressibility, i.e.
dCp = -Cpc dP c + C pp dP (4) p or to fluid compressibility, i.e.
dEp = -Cf dPp , (5) in which dpc is hydrostatically applied confining pressure and dPp, Cf have the meanings already stated.
Equating equations (4) and (5) then gives
from which, by substituting for Cpc and Cpp from equations (2) and (3), can be obtained
If the compressibility of the rock skeleton Cr is much smaller than the fluid compressibility Cf and rock bulk compressibility Cbc, equation (8) can be simplified as
Taking into consideration the question of loading direction in the compaction of the reservoir bed, and assuming the the horizontal confining pressure dPh to be equal in the x and y horizontal directions, then
in which s is, as stated, Poisson's ratio.
The hydrostatically applied confining pressure P c can thus be expressed as
By substituting from equation (9) there can then be obtained the afore-stated equivalency of the obtained tidal effect ratio, namely
The rock compressibility Cbc derivable from this equation can be used for calculation of oil yield from the reservoir, since the compaction capability of the bed material determines the lithic drive of oil from the reservoir, and for prediction of platform subsidence.
The data is of particular importance in oilfield management, both from the viewpoint of estimating the economic life of the field and the point at which, if platform subsidence is occurring, measures may need to taken to elevate the platform or possibly abandon the platform for another. The data may be able to be directly applied to the other sites in the same oilfield where further platforms are to be erected, or with appropriate correction to geologically similar reservoirs. Monitoring changes in the compressibility value over an extended period of time may also enable revision of reserves estimation for the field concerned.
Claims (7)
1. A method of determining compressibility of an offshore reservoir, the method comprising the steps of measuring pressure within the reservoir and pressure at the seabed above the reservoir at such a frequency that the obtained measurement values are subject to cyclic change due to tidal effect, correlating the reservoir and
seabed pressure measurement values to allow derivation of a tidal
effect ratio of change in reservoir pressure to change in seabed
pressure, and evaluating the tidal effect ratio by data specific te the reservoir to obtain a value for compressibility of the reservoir.
2. A method as claimed in claim 1, wherein the step of evaluating comprises determining the compressibility value from the equation tidal effect ratio =
wherein Cbc is the compressibility of the reservoir bulk material
with respect to confining stress,
is the porosity of that material,
Cf is the compressibility of the reservoir fluid, and
is Poisson's ratio.
3. A method as claimed in claim 1 or claim 2, comprising the step of filtering out measurement values exceeding a predetermined threshold of change.
4. A method as claimed in any one of the preceding claims, wherein the frequency of measurement is a plurality of times per hour.
5. A method as claimed in claim 4, wherein the frequency is at least 10 times per hour.
6. A method as claimed in any one of the preceding claims, wherein the correlation of the pressure measurement values and derivation and evaluation of the tidal effect ratio is performed continuously by data processing apparatus with input means receiving signals indicative of the measurement values and with storage means storing the data specific to the reservoir.
7. A method as claimed in claim 1 and substantially as hereinbefore described.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9118252A GB2258923B (en) | 1991-08-23 | 1991-08-23 | A method of determining compressibility of an offshore reservoir |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9118252A GB2258923B (en) | 1991-08-23 | 1991-08-23 | A method of determining compressibility of an offshore reservoir |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB9118252D0 GB9118252D0 (en) | 1991-10-09 |
| GB2258923A true GB2258923A (en) | 1993-02-24 |
| GB2258923B GB2258923B (en) | 1994-10-12 |
Family
ID=10700455
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9118252A Expired - Fee Related GB2258923B (en) | 1991-08-23 | 1991-08-23 | A method of determining compressibility of an offshore reservoir |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2258923B (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1994024538A1 (en) * | 1993-04-20 | 1994-10-27 | Gas Research Institute, Inc. | Monitoring soil compaction |
| WO2002075363A1 (en) * | 2001-03-15 | 2002-09-26 | Shell Internationale Research Maatschappij B.V. | Time-lapse seismic surveying at sea |
| CN105717552A (en) * | 2016-02-04 | 2016-06-29 | 中国石油大学(华东) | Method for predicting spatial distribution form of lacustrine beach-bar sand body based on logging curve high-frequency cycle |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110805419B (en) * | 2019-10-11 | 2022-08-02 | 长江大学 | Large-liquid-volume large-displacement large-pad-fluid low-sand-ratio slickwater volume fracturing method |
-
1991
- 1991-08-23 GB GB9118252A patent/GB2258923B/en not_active Expired - Fee Related
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1994024538A1 (en) * | 1993-04-20 | 1994-10-27 | Gas Research Institute, Inc. | Monitoring soil compaction |
| WO2002075363A1 (en) * | 2001-03-15 | 2002-09-26 | Shell Internationale Research Maatschappij B.V. | Time-lapse seismic surveying at sea |
| CN105717552A (en) * | 2016-02-04 | 2016-06-29 | 中国石油大学(华东) | Method for predicting spatial distribution form of lacustrine beach-bar sand body based on logging curve high-frequency cycle |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2258923B (en) | 1994-10-12 |
| GB9118252D0 (en) | 1991-10-09 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19970823 |