Numerical Evaluation of a Novel Slot--Drill Enhanced Oil Recovery Technology for Tight Rocks

Enhanced oil recovery (EOR) is essential in shale/tight formations because primary recovery typically produces less than 10% of the original hydrocarbon in place. This work presents a novel "slot--drill" EOR (SDEOR) technology, which involves injecting gas through a horizontal fracture that is cut into the formation near the top of the reservoir (using a tensioned abrasive cable mounted to the drillstring) and producing oil from a second slot--drilled horizontal fracture near the bottom of the reservoir. A robust 3D projection--based embedded discrete fracture model (pEDFM; EDFM) is used to model the natural fractures in these slot--drilled unconventional oil reservoirs accurately and efficiently. Connectivity and uncertainty analyses are performed to determine the percolation threshold, where natural fractures influence hydrocarbon production appreciably. The results of this work indicate that the proposed technology can yield over a threefold increase in oil recovery relative to the cyclic gas EOR (CGEOR) method. This simulated recovery is high regardless of the presence of natural fractures or the type of gas/solvent injected (such as CH4, N2, CO2, and flue gas). The simulation results also indicate that the continuous gas injection, higher relative oil permeability, and the role of gravity--drainage are the main reasons why the oil recovery from the SDEOR is three times that from the CGEOR method. In conclusion, this is the first presentation and numerical simulation study of applying pairs of parallel slot--drilled fractures to enhance the recovery from challenging unconventional reservoirs (such as the Bakken shale) that have not been successfully enhanced using the CGEOR method. The dramatic increase in recovery from SDEOR, coupled with its applicability regardless of the stress state or formation brittleness, could change how unconventional reservoirs are completed and produced in the future.