Effect of compaction history on pore pressure prediction

Abstract

Summary Correct pore pressure prediction relies on the recognition of formation compaction history and good characterization of the velocity pressure relationships under various scenarios. Experiments were attempted to better characterize the normal compaction trend and unloading curves. Theoretical models were used to better understand the pore pressure’s effect on velocity. Introduction The relationship between velocity and pressure is currently the main resource used to predict pore pressure in formation. Eaton’s (Eaton 1975) equation remains the most popularly used method in the industry. While the overburden pressure follows a stable trend, the pore pressure experiences more variation during compaction. Furthermore, the effect of the pore pressure on velocity and other rock properties does not follow a one to one relationship. Figure 1 schematically depicts the pressure paths for three different scenarios. For a normal compaction trend, the overburden and pore pressures increase at approximately a 2:1 ratio during subsidence. If undercompaction occurs, the overburden and pore pressures may increase at approximately the same rate (dashed dotted line). In the third scenario, the formation first went through normal compaction, and then underwent a pore pressure increase which would most possibly be caused by the fluid expansion. Although the final overburden/pore pressure pair can reach the same value from scenario 2 and 3, the formation rocks have different compaction levels thus exhibit different properties including velocity, resistivity, and porosity. This non-uniqueness in velocity-pressure relationships imposes ambiguity into the pore pressure prediction. To reduce the ambiguity and improve the pore pressure prediction, we need attempts in the following four aspects: 1. Better understanding on the pore pressure mechanism and its relationship to velocity. 2. Ways to recognize the compaction history of the formation: mostly by geologic interpretation, maybe with help from multi-parameter analysis as suggested by Bowers. 3. Better characterization on the normal compaction trend by experiments, for different lithology, and different geologic settings. 4. Also to characterize the unloading trend by experiments. In this report, we present our recent work mainly on points 1, 3, and 4.