- Copyright © 2002 Society of Exploration Geophysicists
Editor's note: This paper is excerpted from OTC13041 presented at the 2001 Offshore Technology Conference.
Predrill pressure prediction 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. The advent of the effective stress concept and pressure prediction methods that developed from that concept have 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. This technique has many variants including the Eaton method, the Bowers method, and the Sperry Sun method.
Weaknesses remain due to (1) limitations of the seismic velocities themselves, (2) lack of understanding of the basic causes of pressure, and (3) effects of pressure on physical properties (including velocity, density, and porosity) of rocks. The level of sophistication used in pressure prediction has improved steadily over the last few years, and the future looks even more promising. This paper will discuss some critical challenges facing pore-pressure prediction and some solutions 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 function of its depositional and burial history. Figure 1 shows a hypothetical loading path for a rock in a clastic basin in porosity-velocity-effective stress space. 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. After critical porosity is reached, the material follows the primary …