Anisotropic Stress Models Improve Completion Design in the Baxter Shale

Abstract

Abstract In unconventional reservoirs, stress models that account for anisotropy yield a stress profile which better represents in-situ conditions than the profile suggested by an isotropic stress model. Completion designs based on an accurate petrophysical model and stress profile which quantifies containment, influences perforating and staging strategies. This can help improve stimulation coverage from discreet shale intervals and lead to more economic completion decisions. This paper shows a comparison of stress magnitude estimated with a traditional linear poroelastic model from sonic data, with stress magnitude estimated from a model which accounts for transverse isotropy. A case study from the Baxter Shale play will show static and dynamic elastic moduli measured from core and acoustical logging, which vary significantly when measured in both the vertical and horizontal directions. The resultant stress profile estimated with a stress equation which accounts for this anisotropy better characterizes subtle stress changes that are significant for staging and perforating design in unconventional gas plays such as the Baxter Shale. Introduction As the demand for energy increases and conventional resources decrease, there is a growing need to develop and understand unconventional resources. Unconventional shale gas plays have become a key exploration target for the petroleum industry. Since intrinsic permeability and primary porosity values are lower than conventional reservoirs, producing hydrocarbons from tight shale reservoirs depends on successful hydraulic fracturing. With the demand of completion services and increased treatment volumes required to effectively stimulate these reservoirs, current completion costs are often 40% or more of the total well costs. By identifying and quantifying stress anisotropy, completion design in plays with thick perspective reservoir intervals can be improved. When incorporated with an accurate petrophysical model and integrated with rigorous post-completion analysis, accurate in-situ stress models can be used to help focus completion capital on discreet intervals within the overall reservoir package. With continuous refinement, the ultimate product is a more economic overall completion design that focuses on improving production within high-impact intervals while managing costs allocated to less perspective layers. It is well understood that shales have an anisotropic microstructure. Therefore, in thick shale plays, it is essential to use a stress model that considers this anisotropy. Estimating in-situ stress assuming isotropy has been the standard in the industry for more than 30 years; not because isotropy was a good assumption, but because anisotropic logging measurements were unavailable. Isotropic stress models applied to anisotropic formations generally predict inaccurate stress magnitudes (Thiercelin and Plumb 1994). Today, anisotropic measurements from acoustical logging are available (Pistre et al. 2005; Walsh et al. 2006). A calibrated anisotropic stress model provides a stress profile which better defines zone containment and often changes the perforating and staging strategy from that suggested by an isotropic model.