Unconventional shale oil reservoir exploitation has increased exponentially in recent years in reservoirs such as the Bakken, Eagle Ford, and Niobrara formations. As with shale gas plays, the production optimization of shale oil plays must be addressed routinely by the industry to help maximize recovery. Fracture spacing in horizontal wells as a function of reservoir and fracture properties and well spacing are two of the most critical issues that should be addressed.
Because the reservoir simulation process can take significant time and effort, and analytical solutions are sometimes very complex, a simple ?back of the envelope? methodology to estimate the optimum fracture spacing can be advantageous for everyday application and used as a starting point for completion design.
This paper uses a numerical reservoir simulation study to develop simple correlations that quantify the required fracture spacing necessary to optimize recovery factors in unconventional shale oil reservoirs and how various hydraulic fracture parameters and non-ideal reservoir behavior affect the horizontal well completion design. The detailed hydraulic fracture parameters discussed in this paper are spatial distribution of fracture conductivity, effective fracture length and effective fracture height, and fracture conductivity degradation. The non-ideal reservoir behaviors discussed include stress-sensitive reservoir permeability and overpressure.
Finally, shale oil spacing optimization is compared with previously published work for unconventional shale gas to provide insight into how liquid-rich shale reservoirs might be affected.
US oil and gas production has reversed the historic decline trend since 2008 because of rapid production from shale play development. As a result of the success of shale gas production, the gas market is depressed because of over-supply or under-demand. While oil demand, and hence the price, is still strong, operators are diligently exploring and developing liquid-rich shale plays, such as the Bakken formation in Montana and North Dakota (LaFollette et al. 2012; Rasdi and Chun 2012), the Eagle Ford formation in South Texas (Centurion et al. 2012; Orangi et al. 2011), and the Niobrara formation in DJ basin (Gonzales et al. 2012).
Because of the high costs associated with horizontal completions, operators are forced to optimize the development plan regarding well/stage spacing and completion efficiency issues. Many different approaches for this are discussed in the literature (Vincent 2011; Quirk et al. 2012).
Sierra et al. (2013) present a novel and simple correlation to determine the optimum fracture spacing and provide guidelines for selecting well spacing for shale gas reservoirs. Using the same approach, while accounting for different fluid properties between dry gas and live oil, a simple correlation was developed to determine the fracture spacing or the number of perforation clusters or entry points needed for a horizontal well.