Conventional reservoir simulators calculate the effect of rock compaction on pore volume change through the concept of rock compressibility under a defined loading condition (hydrostatic or uniaxial strain). This approach usually is appropriate for reservoirs with competent rock. For weaker formations and complicated rock compaction behavior, however, a coupled analysis of geomechanics and multiphase fluid flow may be required for obtaining more rigorous and accurate solutions from reservoir simulation. In general, computational efficiency and convergence of numerical solutions are two critical factors in order to make coupled analysis economically and numerically feasible for practical field applications. In this paper, an iterative procedure for coupled analysis of geomechanics and multi-phase flow in reservoir simulation is proposed for large-scale, full-field, 3D problems. The proposed procedure isgeneral and effective for handling reservoir rock with complicated constitutive behavior of rockcompaction and permeability change as well as for simulating various reservoir production scenarios. Descriptions of model formulations, constitutive equations, solution procedures, and strategies for enhancement of computational efficiency are presented in the paper. To demonstrate the capability of the developed procedure for iterative coupled analysis, several problems including a large-scale field example were studied and are presented.

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Iterative Coupled Analysis of Geomechanics and Fluid Flow for Rock Compaction in Reservoir Simulation

This paper focuses on the aspects of fully coupled continuum modeling of multiphase poroelasticity applied to the three-dimensional numerical simulations of the Ekofisk oil reservoir in the North Sea (56°29′–34′N, 03°10′–14′E). A systematic presentation is chosen to present the methodology behind fully coupled, continuum modeling. First, a historical review of the subsidence phenomena above an oil and gas reservoir is given. This will serve as a background against which the relevance of the present approach to compaction and subsidence modeling will be demonstrated. Following this, the governing equations for a multiphase poroelasticity model are briefly presented. Particular attention is paid to the analysis of the pore-compressibility term usually used in an uncoupled approach for characterising the host-rock deformation. A comparative numerical analysis is carried out to contrast and highlight the difference between coupled and uncoupled reservoir simulators. Finally, a finite-element numerical model of the Ekofisk field is presented and a significant result is a contour map of seabed subsidence which is in general agreement with the shape of the subsidence contours based on past bathymetric surveys. Analysis of the simulation reveals that, due to the downward movement of the overburden, oil migration occurs from the crest of the anticline in which the field is situated, towards the flank. The pore-pressure depletion in the reservoir is significantly delayed due to the replenishment of the reservoir energy via the formational compaction. Horizontal movement in the reservoir, which is neglected in traditional modeling, can be significant and comparable in magnitude to the vertical subsidence.

Fully coupled modeling of seabed subsidence and reservoir compaction of North Sea oil fields

Numerical simulation of shear-induced compaction in the ekofisk reservoir

The Ekofisk field, Norwegian North sea,is an example of compacting chalk reservoir with considerable subsequent seafloor subsidence due to petroleum production. Previously, a number of models were created to predict the compaction using different phenomenological approaches. Here we present a different approach, we use a new creep model based on microscopic mechanisms with no fitting parameters to predict strain rate at core scale and at reservoir scale. The model is able to reproduce creep experiments and the magnitude of the observed subsidence making it the first microstructural model which can explain the Ekofisk compaction.

Compaction of North-Sea Chalk by Pore-Failure and Pressure Solution in a Producing Reservoir

In this paper the authors present the results of a coupled nonlinear finite element geomechanics model for reservoir compaction and well-to-well interactions for the high-porosity, low strength diatomite reservoirs of the Belridge field near Bakersfield, California. They show that well damage and failures can occur under the action of two distinct mechanisms: shear deformations induced by pore compaction, and subsidence, and shear deformations due to well-to-well interactions during production or water injection. They show such casting damage or failure can be localized to weak layers that slide or slip under shear due to subsidence. The magnitude of shear displacements and surface subsidence agree with field observations.

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Two-dimensional nonlinear finite element analysis of well damage due to reservoir compaction, well-to-well interactions, and localization on weak layers

To evaluate alternative reservoir management plans and to assess needs for modifying offshore facilities, a computational procedure has been developed to simulate the reservoir compaction and subsidence processes observed at the Ekofisk field in the Norwegian North Sea. The procedure involves combining the results of two axially symmetric, 2-D finite element calculations so that in plan view the Ekofisk reservoir is treated as an elliptically-shaped body. Reservoir compaction is assumed to be driven by reductions in pore pressure, with input information deduced from reservoir simulators used for reservoir management. Porosity distributions and geometrical features of the reservoir are based on porosity logs from some sixty wells drilled prior to the onset of significant compaction. Mechanical properties for the reservoir rock (chalk) are based on laboratory compaction data; for the overburden they are based on data from vertical seismic profiles and compaction tests. The simulation procedure has been applied to several reservoir management scenarios, three of which are considered in the paper. One is a pressure maintenance scenario involving aggressive gas injection to mitigate reservoir compaction and subsidence while the others are depletion scenarios which involve a waterflood that was planned prior to discovery of subsidence to enhance production, but also will serve to maintain reservoir pressure. The good agreement between calculations and recent measurements provides a basis for confidence in future projections. Hence, the prediction of about 6m of subsidence by the year 2011 for depletion reservoir management scenarios suggests that the recently completed operation to elevate the central platforms of the Ekofisk field by 6m represents a long-term solution to the subsidence problem.

W.T. Siemers

Papers

Forecasting Of Ekofisk Reservoir Compaction And Subsidence By Numerical Simulation