Research on stress shadows between stages and wells has provided new elements for understanding how the initial stress state in the subsurface is altered by the creation of hydraulic fractures. Furthermore, variations in pore pressure and the presence of faults and natural fractures are key factors contributing to heterogeneous initial stress conditions that ultimately influence fracture propagation patterns. In this case study, time-based poroelastic and viscoelastic approaches are used to quantify lateral stress gradients between wells and analyze stress shadow interactions at the stage-to-stage scale observed during completion of two Upper Montney siltstone wells.
Utilizing interpreted microseismic data and offset well pressure monitoring data recorded during continuous-pumping completions of the two wells, a fault and pore pressure constrained poroelastic stress and relaxation model is derived. This model is used to map local modification of the initial stress state along parallel Upper Montney laterals. Initial stress conditions and stress shadow influences on observed fracture propagation patterns in the second well at multiple points during the completion is then used to determine dynamic, reservoir specific stress state modification characteristics as a function of time.
Microseismic data from the two wells showed preferential fracture growth toward a fault identified in the seismic volume and an existing production Well 1. Well 2 situated closest to the fault and the existing producer Well 1, showed a modal fracture growth bias to the southwest, towards Well 1. Well 3, situated furthest from the fault and existing producer exhibited a bimodal fracture propagation direction. The initial southwesterly propagation bias repeatedly alternated to the northeast, away from the low stress envelope caused by the fault and existing producer. Based on the initial conditions and the unique fracture propagation patterns in the two wells, lateral stress variation is mapped and modeled using poroelasticity. To better understand the short-term stress relaxation effects on fracture propagation and the phenomenon of repeated asymmetric growth on either side of Well 3 (NE-SW-NE-SW…), a viscoelastic model is used to quantify the impact of stress shadowing and various relaxation times on fracture propagation.
These observations led to the conclusions that the initial stress state is locally modified during hydraulic fracturing operations and that the stress disturbance caused by the depletion in one compartment around the existing production well continues to affect the stress conditions over significant distances, even in hydraulically disconnected reservoir compartments. This in turn affects the stimulation patterns of recently completed wells.