A feasibility study of thermal stimulation in unconventional shale reservoirs

Shale oil production from plays such as the Bakken and Eagle Ford, driven by advances in horizontal well drilling and hydraulic fracturing technology, has helped improve US domestic oil production. However, due to the low permeability of shale oil reservoirs, primary depletion only produces 5–10% of the original oil in place and there is a huge potential for improved recovery. Chemical and gas huff-n-puff have been piloted with varying degrees of success, but no breakthrough recovery method has been discovered. This study examines improved oil recovery from shale reservoirs by thermal stimulation, consisting of primary depletion (early in a well's life), followed by conversion of the well to a heat injector to elevate reservoir temperature, and finally followed by a secondary depletion. The proposed method was tested using a compositional, thermal reservoir simulator (CMG STARS) and key parameters affecting recovery during thermal stimulation were investigated. This study found that 1000 days of thermal stimulation with a 700 °F heater has the potential to economically increase oil recovery from about 7% to more than 11.5%, with potential for even greater recovery if heat injection time and temperature are optimized. The study also found that thermal pressurization of oil is the primary mechanism for the improved recovery. Kerogen decomposition into oil and gas results in a significant increase of hydrocarbons in place but is only a minor contribution to the additional recovery because production is limited by a lack of flow capacity. Furthermore, a two-fold increase in permeability is observed as a result of kerogen decomposition, but its contribution to recovery is also minor because the heated region is not well connected with the fractured region. The heating scheme may be improved in the future to better connect the heated region with the fractured region.