There is an uncertainty over the production contribution from the Upper and Lower (U&L) Bakken Shale to the Middle Bakken reservoir. For the Bakken system, reservoir studies involving the fluid flow and recovery mechanism cannot be fully understood without resolving this uncertainty. Performance-anomalies in the GOR trends of the production-history of the Middle Bakken wells in the Reunion Bay, Sanish, Parshall and the Elkhorn-Ranch fields indicate the possibility of the anticipated contribution.
Quantifying the U&L Shale contribution requires knowledge of the mechanism of fluid storage and flow in the liquid rich shale systems. For the U&L Shale, adsorption is considered as the significant mode of fluid storage, and the process of diffusion is considered crucial for the matrix-to-fracture fluid transfer. The governing mathematical equations for desorption and diffusion was adopted from the shale gas systems. These equations are utilized in CMG™'s compositional simulator GEM™ to propose a reservoir simulation-based quantification scheme for the U&L Shale contribution.
Through the sensitivity analyses, the effect of variation in the parameters of the U&L Shale, the Middle Bakken layer and the hydraulic fracture is investigated. Utilizing the ranges of these parameters, the U&L Shale layers are found to contribute from 12% to 52% of the cumulative production from a Middle Bakken well, whereas, the mean contribution is 40%. Relative sensitivity study suggested that the U&L Shale production contribution is the most sensitive to the U&L Shale matrix parameters, such as total organic carbon (TOC, wt.%) and molecular diffusion coefficients. The TOC controls the desorption-parameters; therefore, the findings suggest that the phenomena of desorption and diffusion are expected to play a crucial role in the anticipated production-contribution.
The Bakken Formation lies within the oil-window of the vast Williston Basin, which extends over the regions of North Dakota, Montana and the Canadian province of Saskatchewan (Figure 1). The production history of the Bakken wells in Figure 2 suggests that before year 2000, most Bakken wells provided marginal economic success, partially because of extremely low matrix permeability (0.0001-.01 mD) and meager chances of exploiting the many localized natural fractures with a vertical well. Commercial production and development activities have become increasingly economically viable in recent years with the advances in horizontal drilling and the use of multi-stage fracture stimulation. In the month of March, in 2013, the Bakken play in North Dakota alone produced with an average daily rate of 0.71 Million BOPD and 681 MMSCF of gas per day.