ABSTRACT:

Knowledge of in situ stress and how stress changes with reservoir depletion and pore pressure drawdown is important in a multi-disciplinary approach to reservoir characterization, reservoir management, and enhanced oil recovery projects. Over 20 years of petroleum production from the Ekofisk field has resulted in a 21-24 MPa reduction in reservoir pore pressure. The effect of pore pressure drawdown on the minimum horizontal in situ stress in the Ekofisk field has been determined from shut-in pressure data of 32 hydraulic fractures. The effective stresses in the reservoir increase linearly with pore pressure drawdown, but at different rates. The ratio of the change in effective minimum horizontal stress to the change in effective vertical(overburdens) stress is approximately 0.20. Laboratory experiments, which simulate the stress path followed by reservoir rock during the production history of the Ekofisk field, clearly indicate that shear failure has occurred during compaction of high porosity chalk as the shear stress increased with pore pressure drawdown. It is suggested that shear failure during primary production has increased fracture density and reduced matrix block dimensions, and has therefore maintained reservoir permeability, which may account for the continued good producibility of the Ekofisk field, in spite of compaction.

1 INTRODUCTION

In situ stress affects nearly all physical properties of rock and hence the measurement and interpretation of (1) geophysical data, (2) petrophysical properties, such as porosity and permeability, (3) rock strength and ductility, and (4) mechanisms of rock deformation and failure. In naturally fractured reservoirs the influence of stress on reservoir behavior is even more pronounced, particularly with respect to fluid flow through fractures. Accordingly, knowledge of in situ stress and how stress changes with reservoir depletion is important in a multidisciplinary approach to reservoir characterization, reservoir management, and enhanced oil recovery projects. Over 20 years of petroleum production has resulted in a 21-24 MPa reduction in reservoir pore pressure. The decline in pore pressure has led to an increase in the fraction of the overburden load that must be supported by the structurally weak chalk matrix, which in turn has undergone significant compaction resulting in over four meters of seafloor subsidence. An important objective of this cooperative program is to determine the effect of production and pore pressure drawdown on the in situ stress state across the reservoir. As the pore pressure is reduced, the effective stresses in the reservoir will increase but at different rates, depending upon the loading path and boundary conditions on the reservoir. The purpose of this paper is to summarize the results of measurements of the minimum horizontal in situ stress determined from closure stresses derived from shut-in pressure data of hydraulic fractures conducted in 32 wells in the Ekofisk field during the past 15 years. These stress results provide an understanding of the stress path followed by reservoir rock during the production history of the field. Laboratory experiments have been conducted on reservoir chalk to simulate the observed stress path at Ekofisk.

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