Observations of Pore Pressure in Clay-rich Materials; Implications for the Concept of Effective Stress Applied to Unconventional Hydrocarbons

The concept of effective stress is a well-established relationship where the stress acting on a rock can be viewed as the total stress minus the pore water pressure. In clay-rich rocks this relationship has been seen to be imperfect and a Biot coefficient is added to account for the material properties of the clay matrix. Large, stable pressure differentials and gradients were observed in several argillaceous materials during water and gas injection testing for a number of experimental geometries, including triaxial (Callovo-Oxfordian claystone), shear (kaolinite and Opalinus Clay) and full-scale testing (bentonite). Pore-pressure during water injection appeared to be evenly distributed on the sample scale, whereas in full-scale demonstration a complex distribution was seen, which may partly be due to hydraulic disequilibrium. During gas injection testing all observations suggested that transport was predominantly by dilatancy flow and the formation of micro-fissures. This led to localized pore pressure variations and a complex temporally and spatially varying pore pressure distribution. Isolated pockets of increased gas pressure could be seen to be stable. The nature of pore-pressure distribution, both hydraulic and gaseous, and the stability of pore pressure differentials means that the description of a meaningful average pore pressure was difficult and thus the use of effective stress with a single χ value might misrepresent local stresses within the rock. Localized deformation in the formation of dilatant pathways was dominated by the local gas pressure and not the bulk pore pressure. Therefore the law of effective stress on the micro-scale will be valid, whereas on a bulk scale could lead to errors in model predictions. This also has implications on the release of gas from shale due to the localized influence of stress around a fracture. Flow along fractures was localized with only a proportion of the fracture surface playing a part in both water and gas flow.