Pore pressure prediction : from vertical stress to mean stress to the full stress tensor

Lockhart, Landon Paul
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My thesis focuses on evaluating the relative contribution of both mean stress and deviatoric (shear) stress and understanding how to incorporate their role in order to better predict pore pressure. In Chapter 1, I introduce my thesis by providing a brief background of pore pressure prediction, discussing the importance of using the full stress tensor (mean and shear stress) to predict stress and pressure, and summarizing the agenda of the following two Chapters. In Chapter 2, I predict pore pressure in the deepwater Gulf of Mexico Mad Dog Field, using three different methods that are based on (i) the vertical effective stress (VES), (ii) the mean effective stress (MES), and (iii) the full stress tensor (FES). The VES and MES methods are traditional workflows, whereas the FES method is a new technique. I use ultra-high resolution sonic velocity data, geomechanical modeling, and the Modified Cam Clay soil model. I compare the predicted pore pressures against those that were measured while drilling. I also evaluate the fraction of pore pressure induced by the mean stress and deviatoric (shear) stress. I show that the MES method can account for the mean stress-induced pressure, but neither VES nor MES can account for the deviatoric (shear) stress-induced pressure. In Chapter 3, I present the new University of Texas Full Application of Stress Tensor to Predict Pore Pressure (UT-FAST-P3) online software that I developed to predict pore pressure. I created the software to be a learning tool to illustrate how pore pressure and stress interact in non-uniaxial settings. I wrote the program to predict pore pressure based on the VES, MES, and FES methods. I communicate the results in a velocity vs. mean effective stress plot, and a mean effective stress vs. deviatoric (shear) stress plot. This allows for a side-by-side comparison of each method, thus providing physical insight into the relative contribution of mean stress and deviatoric (shear) stress to compression and pore pressure development. Overall, my thesis contributes to our understanding of the interaction of pressure and stress in the subsurface, demonstrates the importance of using the full stress tensor to predict pore pressure, and explores a new technique (FES approach) that is applicable to a wide range of complex geological environments where the traditional VES and MES methods underperform.