Depth Limit of Velocity-Effective Stress Relationships for Pore Pressure Prediction, Implications for Wellbore Stability Analysis

摘要

Effective stress-P-wave velocity relationships are generally used to estimate pore pressure particularly where conventional porosity-based methods are of limited use. The rock physics basis of velocity-effective stress relationships are often ignored and the equations may have been used beyond their range of applicability, which would result in erroneous pore pressure estimates. A rock physics-based model is presented to approximate the depth range applicability of the velocity-effective stress relationship for pore pressure prediction. An empirical relationship between the velocity-stress plateau and the porosity is derived using a worldwide dataset of laboratory acoustic velocity measurements as a function of effective stress on siliciclastic rocks with a range of porosities and rock fabrics. A graphical illustration of the corresponding depth of effective stress-velocity plateau versus formation porosity for a range of vertical stresses and pore pressure gradients can be used in pore pressure prediction algorithms to determine the depth range applicability of velocity-effective stress relationships. Extrapolation of velocity-effective stress relationships to deeper areas while ignoring the depth dependent nature of stress sensitivity of acoustic velocities in pore pressure prediction algorithms may lead to inaccurate pore pressure estimates which in turn could lead to erroneous mud weight recommendations and hole stability analyses. A field example is shown where correction to the pore pressure model are applied using geomechanics principles and the stress-velocity-depth relationship proposed in this paper.