Multistage hydraulic fracture stimulation of horizontal wells often results in asymmetric hydraulic fractures, which are suboptimal for draining reservoirs. Field observations suggest that hydraulic fractures grow further than expected toward, and sometimes up to and beyond, adjacent producing wells. Such interference from nearby wells is of increasing importance in densely drilled unconventional reservoirs, where hydraulic fracturing is common. Eventually, the underlying mechanism needs to be factored into well spacing plans and hydraulic fracture treatment designs, which can either mitigate the undesired consequences of the nonvirgin conditions, or take advantage of them.
An integrated approach has been developed to quantify depletion-induced stresses. In this approach, a three-dimensional (3D) Mechanical Earth Model (MEM) is constructed using the finite-element method to initialize the in-situ stresses. Coupled simulation between the dynamic reservoir model and the geomechanical model is used to quantify stress variation, induced by pressure depletion of the parent well. The altered stress profile is then used as input to a hydraulic fracture design.
The methodology has been applied to an unconventional resource play in the Williston Basin in North America. In this example, three adjacent horizontal wells were hydraulically fractured and put on production sequentially within four years. The analysis indicated that the minimum horizontal stress decreased in the area between a newly drilled well and the nearby producing well; whereas the area on the opposite side of the new well showed no obvious stress variations. Formations above and below the depletion zone experienced an increase of minimum horizontal stress as a balance effect of the finite-element calculation. Fracture modeling using the depletion-induced stresses showed that the effective half-length of the fracture directed toward the depleted well can extend up to two times the length of the hydaulic fracture in the opposite direction.