Challenges and Surprises of Abnormal Pore Pressures in Shale Gas Formations

Drilling the gas-bearing shales in the Bossier and Haynesville formations in northwestern Louisiana presents many drilling challenges. Pore pressures increase from slightly elevated levels to a surprisingly high geopressures over short intervals. These pressures, if not correctly predicted, can lead to dangerous gas kicks and potentially blowouts. In the shale gas formations, conventional log-based pore pressure prediction techniques did not work well. The presence of gas in the shales adversely affected the compressional velocity. The impact of the Sabine uplift also reduced the ability of conventional pore pressure techniques to quantify pressures adequately in the objective formations. Instead we analyzed 54 kicks and other downhole measurements across the field to analyze the abnormal pressures. This approach yielded consistent results and provided a reliable basis for pore pressure prediction. The pore pressures in the lower Bossier and Haynesville shale gas formations show highly overpressured after a overpressure transition in upper Bossier shales, and unloading effect on pore pressure in those formation are also identified. Using the kick derived pore pressure model and improved log-based effective stress models, we developed a comprehensive pore pressure prediction method.