Horizontal completions have changed considerably in the last few years in an effort to substantially improve the drainage of shale gas reservoirs. The spacing of fracture stages and perforation clusters are among the most crucial completion decisions that impact well productivity and EUR. Yet, the decision regarding stage spacing is rarely guided by an engineering process, as it remains a challenge to tie production performance and completion design. In this paper, we offer some insight on the impact of fracture spacing on the propagation direction of multiple transverse fractures, and consequently the expected performance of the horizontal well.
Stress-shadow effects, related to the mechanical interference induced by a proppant-filled fracture, can cause fractures initiated from a horizontal well to deviate toward or away from previous fractures. A three-dimensional geomechanical model of the combined stress interference from multiple transverse fractures has been applied to typical wells in three shale gas reservoirs: Bakken, Barnett and Eagle Ford.
The existence of an optimum spacing is demonstrated, where fracture stages remain transverse even when subject to stress-shadow effects. Below the optimum spacing, induced fractures may intersect previous fractures, and re-stimulate previously fractured regions of the reservoir, while leaving undrained portions of the reservoir un-stimulated. Such behavior is highly dependent on the mechanical properties of the shale, in particular the Young’s modulus.
Our modeling results suggest that the net fracturing pressure data measured in the field reflects the propagation direction of the fractures induced from the horizontal wellbore. A monotonic increase in net pressure, going from one stage to another, would indicate transverse fracture propagation during all stages. On the other hand, an up-and-down trend in the net pressure data is an indication that the mechanical stress interference is causing the later stage fractures to intersect fractures from previous stages. The net pressure data can, therefore, be used to investigate fracture-to-fracture interference and can be used to optimize the spacing of fracture stages in horizontal completions.