In this paper, we present a mechanistic study for understanding the major physics involved in shale oil production by integrating data collected from initial fracturing to two-year production history. Following the operations classification, we divided the process into fracturing, soaking, flow-back and production stages. Data from each stage contain unique information of specific reservoir parameters, which allows us to anchor some key reservoir parameters consistently and to quantify the major recovery mechanisms reliably.
We systematically assessed the impact of high capillary pressures on shale oil production, in addition to other major physics common in the literature. Our model demonstrated that capillary liquid hold-up at the fracture and matrix interfaces could explain the major observations of low GORs and low fracturing fluid return consistently. Strong capillarity slows down gas movement and maintains energy in the system. Water held-up at the fracture-matrix interfaces could enhance oil production by trapping gas in the matrix, which is contradictory to the conventional relative permeability arguments. Trapping gas in the matrix helps maintaining reservoir energy and enhancing oil production.
Our models could be used for assessing opportunities for enhanced oil recovery (EOR) and production optimization. The simulation results suggest the existence of EOR window after 3–5 years of primary depletion.
Over the past decade, technological advances in horizontal drilling and hydraulic fracturing have allowed the access to large volumes of shale oil that were previously uneconomic. The Energy Information Administration (EIA) estimated approximately 419 billion barrels of recoverable shale oil resources in 46 countries (EIA 2016). The United States (US) contributes more than 90 percent of the global shale oil production in 2016. Current production in the US relies heavily on drilling and fracturing large numbers of wells, which is capital intensive. The estimated recovery factors with current production practices ranges from 5 to 10% of OOIP. Production optimization to improve EUR is considered the next stage of technology breakthrough, for which understanding the major production mechanisms are critical. The property ranges of the unconventional reservoirs developed vary greatly from one basin to another, and the production characteristics are different as well. Three major observations have been reported in the literature related to unconventional oil production:
Much lower than expected gas-oil ratio (GOR) as compared to conventional reservoirs.
Significantly lower production of fracturing fluids during flow-back operations.
Rapid decline in initial potential (IP) of the wells.