The Future of Pore-Pressure Prediction Using Geophysical Methods

Abstract

Abstract The art and science of pore pressure prediction has improved dramatically over the last 10 years with major advances in the areas of imaging and velocity analysis and in the theory of pore pressure in clastic basins. This paper discusses the current state of pore pressure theory for clastic basins and also addresses some of the developments that are occurring and will occur in the next ten years that will impact this critical technology area. Introduction Pre-drill pressure prediction using geophysical data and methods has historically been done using very simple models and overly simplistic estimates of the Earth's velocity field. The methods usually incorporate a locally calibrated set of curves for pressure that contained imbedded assumptions about the cause of pressure in the geological section sampled by the control wells. The advent of the effective stress concept and the pressure prediction methods that developed from that concept led to a much-needed inclusion of fundamental physics into the art of pressure prediction. The use of effective stress methods has become the standard for pressure prediction with many variants including the Eaton method, the Bowers method1, and the Sperry Sun method to name a few. The range of software available for pressure prediction has grown significantly in recent years, along with the sophistication of the parameters used. Still, weaknesses remain due to (1) the limitations of the seismic velocities themselves, (2) the lack of understanding of the basic causes of pressure and (3) the effects of pressure on physical properties, including velocity, density and porosity, of the rocks. The level of sophistication that is used in pressure prediction has improved steadily over the last few years, and the future looks even more promising. This paper will discuss some of critical challenges facing pore pressure prediction and some of the solutions that are on the horizon. Effective Stress And Loading Path Dependency Of Pressure One way to think about abnormal pressure is to recognize that the velocity of any rock in the subsurface is a direct function of its depositional and burial history. Fig. 1 shows a hypothetical loading path for a rock in a clastic basin in porosityvelocity-effective stress space. This diagram demonstrates the interplay between velocity, porosity and effective stress during the burial and loading process. The loading path starts at an effective stress of zero, and the velocity increases and porosity decreases until the material changes over from a Wood's Equation material to a frame-bearing clastic rock that can support an effective stress on the grains. The Wood's Equation portion of the loading path occurs as the material is initially deposited and compacted near the surface. Once the critical porosity is reached, the material follows the primary compaction curve, achieving either a compacted or undercompacted state. If allowed to compact normally with fluid draining out of the pore spaces, a rock will continue up the normal loading path and velocity will increase and porosity will decrease.