Reservoir Pressure Depletion and Water Flooding Influencing Hydraulic Fracture Orientation in Low-Permeability Oilfields

Abstract

Hydraulic fracture azimuthal orientation depends on stress distribution in the formation and is considered to coincide with the maximal horizontal stress azimuth. Knowledge of the hydraulic fracture orientation is of critical importance in field development planning, including well spacing and pattern, the location of water injectors that will lead to a desired line drive mechanism, optimized reservoir drainage, and maximized recovery factor. This information is not less critical for infill drilling, fracturing "old" wells, refracturing, fracturing of sidetracks, and the knowledge of hydraulic fracture orientation of the water injector wells that are fractured by the mere injection process. It is also known that refracturing and pore pressure re-distribution will reorient the stress field not only in the near wellbore area but also in the far field. Theoretical modeling and field experience suggest that hydraulic fractures do reorient under the influence of pore pressure changes because of reservoir fluid production and water injection. Field sector formation pressure distribution makes the fracture offset from maximal stress direction towards injection wells and this effect of local stress reorientation is more likely to occur in low permeable formations with low diffusivity and low horizontal stress anisotropy. A number of complex fracture geometry orientation investigations were performed on low permeability formations in Siberia to understand the phenomenon. This included acoustic measurements and micro-seismic monitoring. Though the main purpose was to optimize fracture geometry by comparing measured and modeled fracture parameters, the importance of fracture orientation for reservoir development was even more significant. The results undeniably indicate that a significant deviation of the field sector fracture azimuth from the maximal regional stress direction exists. It was discovered that the degree of deviation from the regional preferred fracture azimuth is affected by water injection and reservoir fluid production in the sector of the monitored well. Observations regarding the effect of fracture wing length asymmetry may also be explained by the disturbance of initial stress conditions. The results of this investigation will be used to further optimize hydraulic fracture design, reservoir pressure maintenance, including well spacing and well pattern, and water flooding strategies.