Geomechanical Aspects of Fracture Placement in Horizontal Wells

For horizontal wells with cased-hole completion, multistage hydraulic fracturing is often performed to create multiple fractures through perforation clusters within each stage. The opening of propped fractures alters in-situ stress underground. The stress redistribution affects the propagation direction of the hydraulic fractures. The spacing between perforation clusters, the number of the clusters and the orientation of transverse fractures all have major impacts on well production. How to place these perforation clusters is a critical optimization issue. In this study, the spacing between perforation clusters is optimized based on stress analysis. A 3D numerical model is developed based on finite element method. The comparison between analytical and numerical solutions indicates that an excellent agreement has been achieved, which certificates the applicability of the numerical models to complex situations. Stress redistribution around hydraulic fractures can occur due to the opening of hydraulic fractures and poroelastic effects. Only the opening of hydraulic fractures is considered in the numerical model. Results indicate that the initial in-situ stress contrast in horizontal plane has a strong influence on the extent of stress-reversal region and reoriented-stress region. When the generated stress contrast is larger than the initial in-situ stress contrast, there will be stress-reversal region in the vicinity of fractures. As the distance from fractures increases, the generated stress contrasts become smaller. When the generated stress contrast is less than the initial in-situ stress contrast, the stress trajectories do not reverse. Usually the stress-reversal region is in the vicinity of fractures, the reoriented-stress region is beyond the stress-reversal region and around fracture tips.