Linear Model for Pore Pressure Predication in Gas Hydrate-bearing Sand Formation of Krishna-Godavari Basin (India) – A Case Study

Abstract

Pore pressure is a crucial geomechanical parameter, to decide the mud density while well drilling. The mud weight should be between formation pressure (pore pressure) and the fracture gradient of the reservoir rock, otherwise various kind of unwanted incidents like, kicks, blowout, breakout, well collapse, mud loss and lost circulation can happen; which can increase the cost of drilling and in worst case it may lead to a dangerous accident and consequently loss of life and assets at drilling site. In this study we are trying to find pore pressure within water bearing shale formations and gas hydrate (solid) bearing sand formation of four wells of Area B in the Krishna-Godavari Basin, namely, NGHP-02-17A, 19A, 22A, and 23A. Bower’s sonic, Eaton’s resistivity and Eaton’s sonic empirical equations have been used to estimate effective stress in the water-bearing shale formation. In gas hydrate-bearing sand formations these empirical equations cannot be useful for effective stress or pore pressure estimation and hence a linear model based on pressure-core data, relating effective stress and depth in meter below seafloor (mbsf) is generated .This model is applicable for the effective stress estimation in gas hydrate bearing sand formations of the Krishna-Godavari basin. The effective stress estimated from linear model and from core data are found in good agreement in all the wells. At site NGHP-02-23, in-situ pore-pressure was measured between depth of 270.38 to 271.38 mbsf with the help of modular dynamic formation tester (MDT) tool. The in-situ pore pressure (28.2 MPa) and estimated pore pressure (27.8 MPa) from the linear model are found in good agreement. Further, fracture pressures from Matthew-Kelly and Eaton’s method were estimated. The mud weight base on the reported mud density 1.3g/cc are found to higher than the pore pressures and lower than the fracture pressures and hence fulfil the criteria for safe drilling.