Pore Pressure and Fracture Gradient Prediction in Shale Gas Formations: Accounting for Complex Rock Properties and Anisotropies

Abstract

Abstract The prediction of abnormal pore pressure is necessary for avoiding, or at least mitigating, drilling risks but is even more critical in shale gas formations where pore pressure can increase from normal pressure to an abnormally high pressure over a comparatively short depth interval. Arguably, the pore pressure prediction in shale gas plays offers even greater challenges than conventional reservoirs. There are numerous reasons why shale gas presents additional challenges. First, anisotropic rock properties in shale formations thataffect log responses will also affect pore pressure prediction. Second, the presence of gas in the shale causes the compressional interval travel time to read higher (slower), which affects derived rock properties, negatively impacting the accuracy of conventional pressure prediction techniques. Third, the geologically-old formations have higher resistivities with complex compaction and uplift histories. The same properties that create challenges with pore pressure prediction also cause uncertainties in fracture gradient prediction. New pore pressure prediction methods have been developed to address the unique characteristics of unconventional gas plays. The new methods incorporate gas corrections in the calculation of pore pressure and fracture gradient. Becausethe shear wave velocity has a much smaller gas effect than the compressional wave, it is used to correct the compressional data. Based on the corrected compressional log data, a better pore pressure prediction can be achieved. For resistivity-based pore pressure prediction, a new depth-dependent compaction, which accounts for complex compaction and reduces the gas effect, is used for improved pore pressure prediction. Additional improvements in the prediction of pore pressures, fracture gradient, and minimum stress are achieved by characterizing the effects of shale property anisotropies (e.g., Poisson's Ratio). The improvements achieved with the new methods are demonstrated through comparative analyses with conventional methods in the Bossier and Haynesville shales.