Unusual behavior of produced gas oil ratio in low permeability fractured reservoirs

•Complex nature of GOR, oil rate and recovery from shale are investigated.•A single universal predictive factor, GOR Characteristic Index (GCI) is defined.•Little deviations from initial dissolved GOR are observed for GCI less than 0.07.•Positive deviations of produced GOR are observed for GCI greater than 0.2.•Well can be operated to produce desired GOR by using prescribed GCI value.

Leading unconventional plays in the US such as Eagle Ford, Bakken and Niobrara have average gas oil ratio (GOR) ranging from 500 SCF/STB to 4000 SCF/STB. The behavior of produced GOR is difficult to characterize for unconventional reservoirs. Initial reservoir pressure (Pi), well operating pressure and fluid properties directly related to Pressure Volume Temperature (PVT) such as bubble point pressure (Pb), initial GOR (Rsi), GOR at Pb (Rsb), GOR at flowing bottom hole pressure (Rsw) are the key factors affecting produced GOR from low permeability (10–5000 nD) reservoirs. Gas production may be controlled and kept in the desired production window by maintaining the flowing bottom hole pressure (Pwf   or BHP). A single characteristic factor affecting the produced GOR is found to be(1−RswRsb)(1−PwfPb)(1−PwfPi). The first part (1−RswRsb) considers the fluid PVT effect with operating condition, second part (1−PwfPb) accounts for the proximity of operating pressure with bubble point pressure and third part (1−PwfPi) is the drawdown effect. Production behavior in terms of produced GOR can be predicted using this single factor. Produced GOR increases with time when this factor exceeds a certain value, while, little or no deviations from the initial GOR are observed for lower values of the factor. It should be noted that the predictive factor does not depend on reservoir matrix permeability. Initially, a factor (Pb−Pwf)(Pi−Pwf) comprising only pressure terms was developed which failed to capture the behavior of produced GOR. Initial reservoir pressure and flowing bottom hole pressure (Pwf) are varied to study a wide range of reservoirs and production conditions. The oil rates, recovery factor and produced GOR are the key production parameters for this study. Suitability of this factor is validated by comparing simulation data with field data. The optimum well operating pressure can also be determined using this factor to maximize recovery. Deviation of GOR from its initial value is higher for low permeability reservoirs. Higher gas and oil are recovered from reservoirs with higher initial gas oil ratios (Rsi).