Theoretical Analysis Of Stress, Pressure, And Formation Damage During Production
Authors: Vaziri H.H. 
Published: March 2010

The geomechanics of formation failure around deep wellbores are described. Using coupled flow and deformation formulations, a numerical model has been developed/or predicting the exent of formation failure, both in tensile and shear modes, due to the stress relief caused by well drilling and subsequent fluid production. Application of the numerical model to simulate the recovery processes conventionally employed in oil production has revealed that, for specific geometry and formation characteristics, there exists a critical fluid pressure gradient which, if exceeded, would result in formation collapse around the well face. The increase in permeability that occurs in the collapsed zone causes a corresponding reduction in fluid pressure gradient to subcritical levels. This results in a temporary stabilization of the formation until the fluid pressure gradient exceeds a new critical level commensurate with the modified wellbore geometry and formation properties. The analyses also demonstrate that evolution of gases in an occluded form can result in an appreciable increase in fluid production.

Introduction

The stability of subsurface formations in the vicinity of a borehole is a problem of continuing interest to the petroleum industry. From the earliest days of exploration and production, petroleum engineers have been confronted with the difficult task of maintaining the stability of deep boreholes. Greater production depths, exploitation of marginal reserves, horizontal drilling for production from thin oil zones, increased use of strongly deviated wells from expensive deep-sea gravity platforms, and the use of enhanced recovery techniques such as steam injection and hydro fracturing in heavy oil sand deposits, have been responsible for the recent sharp increase of interest in reexamining the mechanics of borehole stability. The consequences of wellbore instability, including increased drilling time, stuck pipe, side tracking, shearing of well casings, severe channelling in waterfloods, direct communication between offset wells, and significant loss of heat during secondary recovery processes, form an appreciable component of drilling and production capital expenditure. In the case of sand production accompanying the exploitation of a hydrocarbon reservoir accurate prediction of the response of a formation to the local pore pressure and stress environment is a prerequisite to proper anticipation and prevention of costly downhole problems. Reliable recovery predictions must take account of geometric modifications that develop as a result of sand production, especially the possible enlargement of the cavity around the borehole, and the enhancement of permeability in the zone affected by the removal of sand.

As a vertical borehole is drilled, radial stress in the formation is relieved and the load is transferred around the borehole circumference as a hoop stress. The radial component decreases from the far-field horizontal stress level and becomes equal to the pressure drop across the mud cake, or to zero if there is no fluid overpressure. Wellface failure results from the mechanical inability of the wall material to sustain the loss of radial support and the redistributed stresses. In uncemented formations, the potential for sand production develops once fluid starts flowing toward the wellface; however, sand production will only occur once the fluid-pressure gradient exceeds a certain critical value.