Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2000-10-11
2002-11-12
Lefkowitz, Edward (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C702S016000
Reexamination Certificate
active
06480790
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of geologic modeling. More particularly, the invention is a process for constructing a three-dimensional (3-D) geologic model of a subsurface earth volume in which the positions of geologic interfaces within the model are adjusted as the model is being constructed in order to improve the consistency between the model and available geologic and geophysical information regarding the subsurface earth volume.
BACKGROUND OF THE INVENTION
Geologic Modeling in General
In the oil and gas industry, geologic models are often used to aid in activities such as determining the locations of wells, estimating hydrocarbon reserves, or planning reservoir-development strategies. A geologic model is a computer-based representation of a subsurface earth volume, such as a petroleum reservoir or a depositional basin.
Geologic models may take on many different forms. Most commonly, descriptive or static geologic models built for petroleum applications are in the form of a 3-D array of model blocks (cells), or less commonly discrete model points, to which geologic and/or geophysical properties such as lithology, porosity, acoustic impedance, permeability, or water saturation are assigned (such properties will be referred to collectively herein as “rock properties”). The entire set of model blocks constitutes the geologic model and represents the subsurface earth volume of interest. Each model block represents a unique portion of the subsurface, so the model blocks should not overlap each other. Dimensions of the model blocks should be chosen so that the rock properties within a model block are relatively homogeneous, yet without creating an excessive number of model blocks. Most commonly, model blocks are square or rectangular in plan view and have thickness that is either constant or variable, but any shape may be used.
A geologic model is generally constrained by stratigraphic or structural surfaces (e.g., flooding surfaces, sequence interfaces, fluid contacts, faults) and boundaries (e.g., facies changes). These surfaces and boundaries define regions within the model that possibly have different rock properties. The term “geologic interface” will be used herein to refer to any interface between two subsurface regions having potentially different rock properties, including but not limited to structural and/or stratigraphic surfaces, facies boundaries, and the like.
In the geologic-modeling process, geologic interfaces are generally interpreted and positioned with the aid of well and seismic data, and are integrated into the geologic model as surface grids, polygons, or in other forms. Typically, these geologic interfaces are fixed within the geologic model; therefore, negligible uncertainty in their position is assumed. If this assumption is wrong, i.e., if the positions of geologic interfaces are inaccurate, the resulting geologic model may be a poor representation of the subsurface earth volume of interest. Moreover, the fact that the model is inaccurate may not be apparent to the persons who constructed it. Use of an inaccurate model could be very costly, potentially resulting, for example, in inaccurate estimates of hydrocarbon reserves, missed hydrocarbon-reservoir targets, and inappropriate reservoir-development strategies.
To minimize the risks associated with inaccurate positioning of geologic interfaces, an effort should be made to ensure that the positions of the geologic interfaces within the model are consistent with all available information and target assumptions for the subsurface volume being modeled. For example, the positions of geologic interfaces in the model should be consistent with all available seismic and well data, and with target assumptions such as frequency distributions for rock properties within regions defined by the geologic interfaces. Such efforts to ensure consistency are rarely pursued, however, as they can be expensive, time consuming, and subjective.
The goal of the geologic-modeling process is to assign rock properties to each model block in the geologic model so that the resulting model is an accurate representation of the subsurface earth volume of interest. This process can use many different data types, including but not limited to rock-property data obtained from wells, seismic data, structural and stratigraphic surfaces in the form of 2-D computer grids or meshes, and polygons or polylines that define distinct regions within the model. The latter two data types are described in more detail below.
The geologic-modeling process uses these data to assign values of the rock properties of interest to all model blocks within the geologic model. The assignment of a rock-property value is a process known to persons skilled in the art of geologic modeling. The value that is to be assigned to each model block is calculated using one of many known estimation methods, though the most commonly used methods are geostatistical.
Geostatistical estimation methods (which may be either deterministic or probabilistic) take into account distance, direction, and spatial continuity of the rock property being modeled. Deterministic estimation methods calculate a minimum-variance estimate of the rock property at each block. Probabilistic estimation methods develop distributions of the rock-property values and produce a suite of geologic models for the rock property being modeled, with each model theoretically being equally probable. The spatial continuity of a rock property may be captured by a variogram, a well-known technique for quantifying the variability of a rock property as a function of separation distance and direction.
Model-Based Seismic Inversion in Geologic Modeling
There are many procedures for constructing geologic models. The preferred geologic-modeling procedure for use with the present invention is referred to herein as “model-based seismic inversion.” Model-based seismic inversion requires that numerous synthetic seismic traces be generated by perturbing model parameters (e.g., rock properties), until there is reasonable agreement between these synthetic seismic traces and actual seismic data traces for the subsurface volume being modeled. Obviously, the synthetic seismic traces should represent the type of actual seismic data being used (e.g., full stack, near-offset stack, far-offset stack, etc.).
Synthetic seismic modeling typically applies a convolutional modeling process. This process consists of using acoustic-impedance values (the product of acoustic velocity and density) to calculate reflection coefficients at the interfaces between layers in the model, and then constructing synthetic seismic traces by convolving the reflection coefficients with a specified seismic pulse. Model-based seismic inversion methods typically are constrained by various conditions that control the outcome. For example, the model may be constrained by measured acoustic-impedance data at wells and by stratigraphic surfaces interpreted in the seismic data. These constraints provide stabilization in the presence of seismic noise and reduce the number of possible solutions.
Some model-based seismic inversion approaches use geostatistical algorithms, such as sequential Gaussian simulation and sequential indicator simulation, to simulate reservoir properties (see e.g., Debeye et al., “Stochastic inversion”, The Strategic Importance of Oil and Gas Technology, Proceedings of the 5th European Union Hydrocarbon Symposium, Edinburgh, U.K., 1997, v. 1, p. 166-175 and Debeye et al., “Method for estimating or simulating parameters of a stratum structure”, European Patent Application No. EP 0 864 882 A2, published Sep. 9, 1998). The simulated values are systematically perturbed until a synthetic seismic trace calculated for a particular location within the model matches the observed seismic data trace for that location. This process is repeated until all traces are matched.
U.S. Pat. No. 5,838,634 describes a similar model-based inversion approach, except that geostatistical algorithms are not used in the geologic-modeling
Calvert Craig S.
Jones Thomas A.
Bell Keith A.
ExxonMobil Upstream Research Company
Gutierrez Anthony
Lefkowitz Edward
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