Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2002-01-29
2003-03-04
Lefkowitz, Edward (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C703S005000
Reexamination Certificate
active
06529833
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is related generally to the field of interpretation of reservoir monitoring using time-lapse seismic measurements. More specifically, the invention is related to methods for monitoring of reservoirs comprising laminated rocks.
2. Background of the Art
As is well known to geophysicists a sound source, at or near the surface of the earth, is caused periodically to inject an acoustic wavefield into the earth at each of a plurality of regularly-spaced survey stations. The wavefield radiates in all directions to insonify the subsurface earth formations whence it is reflected back to be received by seismic sensors located at designated stations at or near the surface of the earth. The seismic sensors convert the mechanical earth motions, due to the reflected wavefield, to electrical signals. The resulting electrical signals are transmitted over a signal-transmission link of any desired type, to instrumentation, usually digital, where the seismic data signals are archivally stored for later processing. The travel-time lapse between the emission of a wavefield by a source and its reception by a receiver after reflection, is a measure of the depths of the respective reflecting earth formations.
The seismic survey stations are preferably distributed in a regular grid over an area of interest with inter-station spacings on the order of 25 meters. The processed seismic data associated with a single receiver are customarily presented as a one-dimensional time scale recording displaying rock layer reflection amplitudes as a function of two-way wavefield travel time. A plurality of seismic traces from a plurality of receivers sequentially distributed along a line of survey may be formatted side-by-side to form an analog of a cross section of the earth. Seismic sections from a plurality of intersecting lines of survey distributed over an area of interest would provide data for three-dimensional representation of a subsurface volume of the earth.
Wavefield reflection from a subsurface interface depends on the acoustic characteristics of the rock layers that define that interface such as density and wavefield propagation velocity. In turn those characteristics depend inter alia on the rock type, rock permeability and porosity, fluid content and fluid composition. In a subsurface reservoir, a fluid transition or interface between gas and oil, or oil and water may act as a weak reflecting surface to generate the so-called bright spots sometimes seen on seismic cross sections. It is reasonable to expect that a change in the level or the characteristics of the reservoir fluids will create a change in the seismic signature associated with the reservoir. Thus, time-lapse or 4-D seismic data acquisition, that is, the act of monitoring the regional seismic signature of a reservoir over a long period of time would assist in tracking the depletion of the reservoir or the advance of thermal front in a steam-flooding operation.
Wason (U.S. Pat. No. 4,969,130) discloses a system of monitoring the fluid contents of a petroleum reservoir, wherein a reservoir model is employed to predict the fluid flow in the reservoir, includes a check on the reservoir model by comparison of synthetic seismograms with the observed seismic data. If the synthetic output predicted by the model agrees with the observed seismic data, then it can be assumed that the reservoir model correctly represents the reservoir. If not then the reservoir model, in particular its reservoir description, is updated until it predicts the observed seismic response. The seismic survey may be periodically repeated during the productive life of the reservoir and the technique used to update the reservoir model so as to ensure that the revised reservoir description predicts the observed changes in the seismic data and hence reflects the current status of fluid saturations.
Laurent (U.S. Pat. No. 5,724,311) discloses a method of monitoring of underground reservoirs. Seismic sources and receivers are installed in a fixed position on the production site, so as to have time stable operating conditions of identical source and receiver characteristics. A plurality of seismic sources are positioned at the surface or buried beneath the surface, on either side of a production well, and at least one array of receivers are positioned at the surface or in at least one well. Explosive sources, hydraulic sources, or electromechanical sources, etc, can be used. The seismic reflection from the underground reservoirs changes with time due to changes in the reservoir conditions such as fluid saturation.
Reimers et al. (U.S. Pat. No. 6,253,848) having the same assignee as the present application teaches the use of permanently installed sensors in a plurality of wellbores for reservoir monitoring. The source(s) may be at the surface or in a wellbore, and both seismic reflection as well as seismic transmission tomographic methods may be used for monitoring reservoir changes.
In reservoir monitoring, two properties that are of considerable interest are the fluid saturation and the pressure of the reservoir. Fluid saturation affects seismic data because of changes in the impedance of the reservoir when one fluid is partially or fully replaced by another fluid. This could be the replacement of heavy oil by steam in a secondary recovery operation, replacement of gas by water or oil in a gas reservoir, replacement of oil by water in an oil reservoir, etc.
When a rock is loaded under an increment of compression, such as from a passing seismic wave, an increment in pore pressure occurs which resists the compression and therefore stiffens the rock. In a classic paper, Gassman predicts the increase in effective modulus of a saturated rock by the following relations:
K
sat
K
0
-
K
sat
=
K
dry
K
0
-
K
dry
+
K
fl
φ
(
K
0
-
K
fl
)
,
μ
sat
=
μ
dry
(
1
)
where
K
dry
=effective bulk modulus of dry rock,
K
sat
=effective bulk modulus of the rock with pore fluid
K
fl
=effective bulk modulus of the fluid
K
0
=bulk modulus of mineral material making up rock
&phgr;=porosity
&mgr;
dry
=effective shear modulus of dry rock
&mgr;
sat
=effective shear modulus of rock with pore fluid.
The relationships given be Gassman are valid in the low frequency limit.
Biot used a model incorporating mechanisms of viscous and inertial interactions between the fluid and the rock matrix and came up with a similar result in the low frequency limit. The results derived by Biot are frequency dependent and include a coupling coefficient between the fluid and the rock as well as a term related to the tortuosity of the fluid paths within the rock matrix.
Prior art methods for interpretation of seismic reflection amplitude changes in a reservoir have generally relied on eq. (1). If the matrix and fluid properties are known, then from a knowledge of the compressional and shear velocities for a first fluid saturation, all the terms in eq. (1) can be determined. A change in seismic reflectivity over time is used to determine a change in velocities and hence the fluid saturation. A commonly used method relies on the Amplitude-versus-offset (AVO] variation of reflection seismic amplitudes for compressional and/or shear wave data, generally described by Zoeppritz's equations. The AVO effects are measured and based on an initial knowledge of the elastic modulii of the rock and its constituent fluids. Modeling results using the Gassman and Zoeppritz equations are used to derive the fluid saturation.
In addition to the fluid saturation effects, it is well known that elastic modulii of rocks are dependent upon the effective stress. The effective stress is defined as the difference between the overburden stress and the formation fluid pressure. As the reservoir is depleted, the formation fluid pressure drops so that the effective stress and the elastic modulii of the rocks increases. Landro (Geophysics, vol. 66, No. 3, pp 836-844), the contents of which are incorporated herein b
Fanini Otto N.
Treitel Sven
Withjack Eric
Baker Hughes Incorporated
Lefkowitz Edward
Madan Mossman & Sriram P.C.
Taylor Victor J.
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