Pore Pressure Induced Stress Reorientation and its Impact on Hydraulic Fracturing

Abstract

This thesis presents an innovative methodology of describing propagation path of hydraulically induced fractures in the presence of pore pressure anomalies. Pore pressure anomalies can occur in different geological settings such as mud volcanoes, magmatic dykes, hydrothermal vents or fluid in faults. Also, many different physical processes may cause local non-uniform pore pressure distribution. The effect of non-uniform pore pressure (pressure anomaly) on propagation behaviour of single and multiple fractures is investigated by using a fully coupled poroelastic numerical model. A new technique, named mean rotation angle (MRA) is used to study the impact of evolved shear stresses on mixed-mode fracturing. MRA is a measure of potential deviation angle evaluated based on the average of normal and shear stresses at nodes located within a specified area near the fracture tip. The tempo-spatial variation of evolved shear stress which represents alteration of original orientation of principal stresses is correlated to the curvature characteristics of the pressure source. It is shown that minimum shear stress variation is consistently correlated to the minimum principal curvature of the pore pressure in semi-3D superficies. From the results of this thesis it is concluded that presence of induced or in-place pore pressure anomalies in a medium can change the stress state such that the dominant direction of propagation deviates from the straight course. In the case of single fracture, a closely spaced circular zone of anomalous pore pressure is considered. Non-uniform variation of pore pressure results in alteration of original stress state at the fracture tip and prompts fracture deviation. Finally, the concept of pore pressure induced stress reorientation is evaluated in the case of closely spaced multiple transverse fractures. Three distinct flow patterns and three associated shear stress patterns have been identified during simultaneous pressurization of a pair of parallel fractures. These patterns are clearly exhibited three distinct trends of the calculated mean rotation angle representing different fracture deviation tendency. The results show the significance of considering the spatial distribution pattern of pore pressure in evaluation of local stress reorientation. These implications can lead to development of next generation of hydraulic fracturing techniques which could control the propagation path of hydraulic fractures by inducing desirable local stress conditions.