ABSTRACT

The goals of this work include reviewing the phenomena of stress rotation in the presence of faulting, salt bodies, and non-uniform distribution of physical rock properties, pore pressure, and tectonic deformation; numerically simulating the stress-field distribution for these cases at the field scale by using a 3D finite element method (the tectonic stress factor and gravity loads are included in the loading used in the analysis); and conducting a numerical analysis of the variation of stress orientation within formations caused by existence of salt body and/or pore pressure depletion. Examples from a field model which consists of a salt body of 7-km in diameter along with the Ekofisk field in the North Sea have been analyzed numerically. A 3D calculation of deformations of the formation matrix is combined with porous flow. Non-uniform initial stress field and non-uniform initial pore pressure field are constructed by means of user subroutines included in commercial finite element software. This paper also includes suggestions regarding stress orientation related topics, such as trajectory optimization and safe mud weight window design.

1. INTRODUCTION

The orientation of the principal horizontal stress has an important influence on completion design, i.e., casing direction and hydraulic fracturing. Several tectonic and depositional mechanisms influence the orientation of the principal stresses: 1) relaxation of stresses adjacent to faults; 2) accumulation of stresses adjacent to faults prior to slippage; 3) halo kinetics, i.e., movement of salt masses; 4) slumping; and 5) rapid deposition of sediments on top of a subsurface environment dominated by strike-slip or reverse faulting. Stress rotation has been reported by several authors in various drilling environments that share common complex geological structures, including active tectonic regions, complex fault and joint systems, salt bodies, and depleted reservoirs. Stress rotation can be observed within one well or from one well to another well. It causes extremely expensive and difficult wellbore stability problems during drilling, completion, or production, and represents a challenge for the oil industry in both operation and modeling. As a result, a large effort has been expended to study and fully understand this phenomenon. Martin and Chandler reported a maximum horizontal stress rotation near two major thrust faults that were intersected during the excavation of the Underground Research Laboratory (URL) in the Canadian Shield [1]. In this region, the fault system divides the rock mass into varying stress domains. Above the fault system, the rock mass contains regular joint sets, in which the maximum horizontal stress is oriented parallel to the major sub-vertical joint set. Below the fault system, the rock is massive with no jointing; the maximum horizontal stress has rotated approximately 90° and is aligned with the dip direction of fracture zone. Stress rotation is commonly observed where the block of rock above the fault has lost its original load because of displacement above the fault; this results in considerably less maximum horizontal stress magnitude than the magnitudes below the fracture zone, where the maximum horizontal stress magnitude is fairly constant [1].

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