Measuring the Biot Stress Coefficient and is Implications on the Effective Stress Estimate

Reservoir compaction is an effective mechanism for enhanced oil recovery (EOR) and a threat to well bore stability and installations resting on the sea floor. It is of great importance to understand the underlying physical and chemical processes in compacting formations in order to precisely manage the reservoir development. It is a common procedure to reduce the set of state variables by introducing an equivalent effective stress variable calculated from the overburden weight and a fraction of the pore pressure. The fraction of the pore pressure is termed the Biot effective stress coefficient. A precise determination of the effective stress is needed to predict compaction, since it is the effective stress that imparts deformation on the matrix. Within a reservoir, the overburden and pore pressure are measurable quantities, while the Biot stress coefficient and therefore the exact value of the effective stress itself remain unknown without the use of tailored experimental techniques. Here, we present the results from four independent testing methods for estimating the Biot stress coefficient of high porosity chalk. We discuss the microscopic interpretation of the Biot coefficient and show how the Biot coefficient may be pore pressure sensitive. As such, the attempt to reduce the number of variables by introducing an equivalent effective stress state with the usage of a constant coefficient is potentially flawed for wider ranges of the pore pressure.