Optimizing Fracture Spacing and Sequencing in Horizontal-Well Fracturing

Abstract

Summary Horizontal wells with multiple fractures are now commonly used in unconventional (low-permeability) gas reservoirs. The spacing between perforations and the number and orientation of transverse fractures all have a major impact on well production. The opening of propped fractures results in the redistribution of local Earth stresses. In this paper, the extent of stress reversal and reorientation has been calculated for fractured horizontal wells using a 3D numerical model of the stress interference induced by the creation of one or more propped fractures. The results have been analyzed for their impact on simultaneous and sequential fracturing of horizontal wells. Our results demonstrate that a transverse fracture initiated from a horizontal well may deviate away from the previous fracture. The effect of the reservoir's mechanical properties on the spatial extent of stress reorientation caused by an opened crack has been quantified. The paper takes into account the presence of layers that bound the pay zone but have mechanical properties different from those of the pay zone. The fracture vertical growth into the bounding layers is also examined. It is shown that stress interference, or reorientation, increases with the number of fractures created and depends on the sequence of fracturing. Three fracturing sequences are investigated for a typical field case in the Barnett shale: (a) consecutive fracturing, (b) alternative fracturing, and (c) simultaneous fracturing of adjacent wells. The numerical calculation of the fracture spacing required to avoid fracture deviation during propagation, for all three fracturing techniques, demonstrates the potential advantages of alternate fracture sequencing and zipper fracs to improve the performance of stimulation treatments in horizontal wells.