Effective stress coefficient in shales and its applicability to Eaton's equation

Abstract

Pore pressure within formations determines the mud weight required to build a balancing fluid pressure downhole. An improper understanding of the subsurface geology and the formation pressures may result in fracturing the formation if the mud weight is too high. In contrast, if mud weight is too low, then formation fluids can flow into the well, potentially leading to well blowouts if not controlled. Complex geological settings make pore-pressure prediction difficult and often inaccurate due to uncertainty in pressure-generating mechanisms. Estimation of proper pore pressures is necessary for designing stable holes and an optimized casing program. In exploration, knowledge of pore pressures can assist in assessing seal effectiveness and in high-grading reservoir sweet spots. It also provides useful calibration information for basin modeling. Hence, it becomes very important to accurately predict pore pressures.