Controls of Anisotropic In-situ Stress and Permeability in Optimization of Wells and Hydraulic Fractures for Unconventional Reservoirs: Examples from the Western Canada Sedimentary Basin

Exploitation of unconventional hydrocarbon resources relies heavily on drilling horizontal wells and pumping multi-stage hydraulic fractures, which is capital intensive and requires optimal well and fracture placements for economic success. The optimization process is a challenging task because it requires a wide spectrum of data on reservoir and mechanic properties. This study investigates the controlling effects of anisotropic in-situ stress and permeability on optimizing well and hydraulic fracture placements based on laboratory stress-dependent permeability measurements and field production data of several unconventional formations (Montney, Nordegg, Cardium, and Bakken) in the Western Canada Sedimentary Basin (WCSB). In-situ horizontal stresses in WCSB are highly anisotropic with the maximum horizontal stress (S_Hmax) approximately in NE direction. The strong stress anisotropy and pre-existing major NE-striking natural fractures likely result in strong anisotropy of horizontal permeability with the maximum permeability in NE direction. Optimization on horizontal well placements and fracturing treatments should be conducted with transformed coordinate based on the permeability anisotropy. SE or NW Horizontal wellbores drilled in the direction of the minimum horizontal stress (S_Hmin) are likely stimulated with transverse fractures as commonly expected, but high fracture tortuosity near-wellbore may occur due to the nearly 90 ° turning as fractures propagate away from wellbore. For NS or EW wells deviating from S_Hmin, fractures oblique to wellbore are likely initiated and propagate away from wellbore without turning, and thus high fracture conductivity near wellbore are expected with lower fracture initiation pressures. Consequently, better production can be expected for the NS or EW horizontal wells. This may explain partly that SE Montney wells at the studied location have much lower cumulative hydrocarbon productions than the NS or EW wells. However, the oblique fractures of NS or EW wells cover less reservoir area and smaller well spacing is required to cover the same reservoir area as for SE wells.