Fracture reorientation around injection and production wells

Abstract

The production or injection of fluids in reservoirs results in a redistribution of stresses. Field evidence based on fracture mapping in tight gas sands and micro-seismic monitoring in water-flooded reservoirs demonstrate this effect. In this study, the extent of stress reorientation has been calculated for fractured and un-fractured producers and injectors and the results have been analyzed for their impact on re-fracturing operations. It is shown that fractures tend to reorient themselves away from injection wells and toward production wells. For previously fractured wells, it is possible to create a secondary fracture that is perpendicular to the first fracture. The secondary orthogonal fracture can be created only within a certain time window that in turn depends on the reservoir properties and operating conditions. For of horizontal wells, the maximum stress trajectories tend to align along the injection well and across the production well. The extent of stress reorientation varies depending on the injection rates, reservoir pressure, production rates, in-situ stress contrast and distance between wells. One decisive factor in stress reorientation around horizontal wells is the orientation of the wells with respect to the in-situ maximum/minimum stress. Reorientation scenarios commonly faced in different well patterns such as line drive, staggered line drive, five spot, seven spot etc. have been analyzed for various possible field applications. A procedure for the selection of candidate wells for refracturing, that will improve the odds of a successful treatment is proposed. This study presents guidelines based on a poro-elastic model that allow an operator to (a) select candidate wells (b) choose the timing of the re-fracture operation in the life of the well, and (c) suggest a design for the re-fracture treatment that will result in the best chance of success.