Lost circulation is a major challenge in well construction operations, especially where drilling margins are narrow or in pressure-depleted reservoirs. Wellbore strengthening techniques (e.g., StressCage) have successfully been used to increase formation fracture resistance and reduce mud losses during drilling operations. The increase of fracture resistance in sands has been used to improve wellbore stability (through the use of higher mud weights), reduce casing requirements, and access resources that may have been undrillable using conventional drilling methods.
The StressCage technique requires the estimation of the width of an induced fracture at a target fracture length for a given wellbore pressure. This estimation involves either a finite element calculation or a closed form line crack analytical solution. Populating and running the finite element solution requires specialized software and expertise, which has limited its use to larger operators and service companies that are staffed with geomechanics experts. The closed form line crack analytical solution is both simple to implement and easy to use, but it assumes transverse isotropic in-situ stress conditions relative to the borehole axis, which is almost never the case in the presence of a deviated well. This assumption results in either the underestimation of the calculation of the fracture width in the presence of deviation or abnormal in-situ stresses, which can result in a failed implementation of a StressCage formulation.
We have developed a new semi-analytical line crack solution that accounts for stress anisotropy from either borehole inclination or abnormal in-situ stresses. This new solution is simple to implement. The calculation of the fracture width has been verified against finite element calculations through a range of stress anisotropies, borehole sizes, Young’s moduli, Poisson’s ratios, and target increases in wellbore strength.