Microseismic examples, Bakken Formation, Williston Basin

Abstract

The Bakken and Three Forks unconventional reservoirs of the Williston Basin are prolific producers of oil and gas. The reservoir properties for these formations are similar, having low porosity and permeability. The combined technologies of horizontal drilling and multistage hydraulic fracture stimulation are used to obtain economic production from these tight reservoirs. Hydraulic fracture stimulation is used to open existing fractures or create new fractures to help produce tight oil and gas resources. Passive seismic (microseismic) monitoring of fracture stimulations provides information about induced fracture azimuth, width, length, height, and stimulated reservoir volume. This information is critical in determining sizes of drilling units and spacing of wells, what is contributing to production from a wellbore, hydraulic fracture orientation and other properties. Microseismic information is a very important tool for reservoir characterisation and management. In situ horizontal stresses in the Williston Basin are anisotropic with the maximum horizontal stress being in the northeast direction. Microseismic and other data from the Williston basin suggest that the regional maximum horizontal stress direction is NE SW (N55° to N70°E). Regional fractures with a similar orientation result in strong anisotropy of horizontal permeability with the maximum horizontal permeability being in the same direction. This orientation is confirmed by lineament analysis, fractures observed from oriented core data, borehole breakout data, and regional stress field published data. Operators generally orient their horizontal wells in a north-south or northwest-southeast direction to encounter maximum numbers of fractures and to be somewhat perpendicular to the maximum horizontal stress and permeability (kmax) direction.