In general, hydraulic fractures propagate perpendicular to the horizontal well axis whenever the drilling direction is parallel to the principal minimum stress plane. However, operators frequently drill horizontal wells parallel to lease boundaries resulting in slanted hydraulic fracture planes at angles less than 90 degrees from the well axis.
This study provides a model for the inclined fracture case. It applies and further extends the unified fracture design approach for rectangular drainage areas, relating the dimensionless proppant number to the maximum productivity index in pseudo-steady state conditions. When simulating flow in shale reservoirs, the stimulated shale volume was represented as a rectangular drainage area that varies with changing angle, but preserves total area. Similarly, fracture length and width varies with changing angle, but total propped fracture volume stays constant.
Results show that for any given set of reservoir and proppant properties along with a given proppant mass, as long as the created fractures drain the same stimulated rock volume, there exists a well direction resulting in maximized well productivity that is not necessarily parallel to the minimum stress direction.
In addition, results yield two main correlations. The first one relates the optimal fracture angle to proppant number, for a given ratio of well spacing to primary-fracture spacing. In this way, operators can choose the drilling azimuth that would maximize production. The second correlation determines the optimal ratio of well spacing to primary-fracture spacing as a function of proppant number for a given fracture angle. This can be applied when selecting the optimum number of fracture stages given a well spacing plan and fracture angle. Two case studies show the application of these findings. In the end, this work provides a simple framework for well design incorporating slanted hydraulic fractures.