Abstract

Borehole instability, sand production, and completion failures during production and pore pressure depletion continue to be significant problems confronting the petroleum industry. This study develops quantitative models for predicting borehole shear failure in pressure-depleted and repressurized reservoirs by incorporating stress path into equations for shear of borehole wall rock. Critical reservoir pore pressure (CRPP) is defined as the pore pressure below which any drawdown or additional pore pressure depletion will result in the onset of borehole shear failure. Critical reservoir pore pressure and critical borehole pressure (CBHP) are defined in terms of initial stress state, initial reservoir pore pressure, the two horizontal principal stress paths, and commonly used failure criteria. The model includes both overbalanced (drilling) and underbalanced (production) scenarios. The model is applicable to both vertical and horizontal wells.

Introduction

Borehole instability and sand production are significant problems confronting the petroleum industry. A reliable model for predicting the onset of borehole shear failure during drilling and production in pressure depleted and repressurized reservoirs provides drilling and reservoir engineers with a useful tool for anticipating borehole instability, for designing successful completions, and for managing reservoirs to minimize problems associated with solids production.

According to Palmer and others (1999), "The allowable drawdown pressure, or critical drawdown pressure (CDP), is defined as the maximum drawdown pressure before sand production from the formation begins." The mechanism giving rise to sand production is quite different from that giving rise to formation of a tensile fracture on a borehole wall. Sand or solids production typically occurs when a portion of the borehole wall fails in response to high shear stress. In the general case, pore pressure depletion and drawdown combine to increase the effective tangential (or "hoop") stress and reduce the well bore support of the bore hole wall until the shear strength of the rock is exceeded, resulting in rock failure and fragmentation. (Fig. 1).

Drilling a cylindrical hole in a rock mass under stress removes load-bearing material and alters the stress field in the immediate vicinity of the bore hole. Drilling mud partially compensates for rock removed, but drilling mud cannot provide a stress difference, such as typically occurs when the principal stresses normal to the borehole axis are dissimilar. Hence, even high mud pressures cannot eliminate variations in tangential stress that typically develop around the circumference of a bore hole.

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