Abstract
In an unconventional reservoir exploration process, the horizontal stress profile across different rock types plays a critical role for drilling and frac design optimization. Present study provides insight about the challenge of estimating stress profile and breakdown pressure in tight sandstone gas reservoirs at 4900m-5500m TVD with varying petrophysical parameters across a complex geological setting. A geomechanical model is prepared prior to a hydraulic fracturing job based on open hole geophysical logs and history match borehole images features like breakouts and drilling induced fractures. Advanced sonic shear fast & slow radial profiles are inverted to directly estimate horizontal stress profile in applicable formations to constrain strain at different well locations. This helps to map changes in stress magnitude, contrast and direction in reservoir across different exploration areas. Algorithm has been successfully implemented earlier worldwide in deep water and has been tested successfully in tight shallow sandstone where dispersion data shows crossover of fast & slow shear near wellbore due to hoop stress. 3D anisotropic data from advanced sonic measurement has been utilized to predict true rock fabric based rock mechanical, elastic and stress profile in silty layers. It helps to understand containment of frac across different reservoirs and optimize stages. Rock types based on petrophysical results, image features and estimated breakdown pressure were used as reference for choosing frac stages. Post-frac closure and breakdown pressure values came close to estimated values using geomechanical results and provided confidence in strain values at different fields. Breakdown pressure seems to vary in range of 12500 psi to 19000 psi depending on stress and rock quality for same reservoir. Stress barrier at top of one reservoir vary at various locations as confirmed with tracer and pressure history match which was predicted using geomechanical model. This integrated petrophysical-geomechanics workflow has been utilized in two vertical wells for two reservoirs successfully to understand field level variation in closure and breakdown pressure and map degree of anisotropy with geological variation. Cost effective optimization of Frac design have been achieved through proper integrated approach saving rig time in decision making and avoiding extra stage as confirmed through Tracer logs.
