A surface-charge model for mudstones and the application to Pore Pressure Prediction

Abstract

New geologic concepts have been developed that illuminate the critical role of bound water in the generation and prediction of overpressures in mudstones. The concept is based on new understanding of the surface-charge effects on water adsorbed on solid surfaces and comes in part from molecular modelling, atomic force measurements, and high-beam neutron diffraction studies reported in the literature. The picture that is emerging is as follows. Bound water on clay surfaces can support a lithostatic load. The bound water fraction increases with compaction, as free water is expelled, with a concomitant decrease in permeability. Overpressures commence at a threshold depth, the retention depth, where the rate of fluid loss is not sufficient to establish pressure equilibrium with the surface. With deeper burial there is a second threshold depth, a gating depth, where bound water condenses to a high-density phase. Below this gating depth, fluid expansion or other effects are responsible for secondary pressure anomalies. Knowledge of bound water effects accounts for discrepancies observed in laboratory measurements of mudstone properties. For instance, mercury intrusion and surface-area measurements depend strongly on how much (hygroscopic) bound water has-been absorbed or adsorbed on a sample before measurements. Surface charge effects tend to increase with compaction due, in part, to reduction of iron and beidellitization. A large data set for mudstones is used to show that the fraction of bound water tends to reach a maximum of almost 100 percent at a depth of 2 to 3 km. An important part of this research is the development of the empirical equation BW = 0.734 CEC (1-Փ)/Փ, where BW is bound water (fraction of total pore volume), Ф is porosity and CEC is cation-exchange capacity (expressed in meq/gm). Porosity and CEC are borehole derived using resistivity and acoustic methods described here. As an adjunct part of this research a software package (called P3) has been written that puts the concepts and relationships into practice.