An Investigation into Enhanced Recovery under Solution Gas Drive in Heavy Oil Reservoirs

Abstract

Abstract Some of the heavy oil reservoirs in Western Canada and Venezuela show anomalously high primary recovery under solution gas drive process. Pressure decline rate in these reservoirs is low compared to that expected under solution gas drive in conventional oil reservoirs. Several theories have been proposed for these anomalous behaviors. The objective of this study is to examine one of the proposed theories, which suggests that gas mobility in heavy oil is low. In this study, gas mobility under solution gas drive is measured by performing depletion experiments. The depletion was carried out at four different rates. The Eclipse-100 Black oil simulator was used to match the experimental data. The direct results from the experiments show that the critical gas saturation is not very high. Thus, critical gas saturation cannot explain high recovery. However, it was found that the gas mobility in heavy oil is quite low, when compared to that expected in conventional oil. Experimental results indicated that higher rates of pressure drop resulted in higher super-saturation, higher critical gas saturation and lower relative permeability to gas. Physical explanation of the observed behavior is offered in light of the anomalous behavior of heavy oil reservoirs. An empirical model was developed to predict gas mobility, critical gas saturation and supersaturation as a function of depletion rate for the system studied. Introduction When the pressure is lowered below the bubble point in an undersaturated reservoir, the gas phase is generated. The gas phase, being compressible, helps in maintaining the reservoir pressure and hence provides the driving force for primary production. Gas does not flow as an independent phase until it reaches certain saturation known as the Critical Gas Saturation. After this, free gas flows as an independent phase resulting in rapid decline in reservoir pressure. This mode of recovery is known as Solution Gas Drive. Critical Gas Saturation, is defined here, as the gas saturation at which a steady, although intermittent, gas flow can be sustained. Some of the heavy oil reservoirs in Western Canada and Venezuela show anomalous behavior under solution gas drive. Once below the bubble point pressure, producing GOR does not increase sharply and rate of pressure drop is low. Some wells have shown much higher oil rates than can be theoretically explained using conventional recovery performance criterion. The overall recovery under solution gas drive is higher than that expected from a similar conventional oil reservoir. Several theories1–8, 27–28 have been proposed for this anomalously high primary recovery under solution gas drive. One theory1,8 attributes this favorable behavior to low gas phase mobility in heavy oil. The low gas mobility implies lower gas velocity, which results in gas retention and pressure maintenance in the reservoir. Finally, this leads to higher primary recovery. Research was initiated to study solution gas drive in heavy oil, and investigate the reasons for this favorable behavior. Experiments are done on an unconsolidated sand-pack at various depletion rates and the effect of rate of depletion on critical gas saturation and supersaturation was studied. Further, the experimental results are used to determine mobility of gas phase at various depletion rates. Production and pressure data obtained from depletion are matched on a commercial reservoir simulator by adjusting critical gas saturation and gas relative permeability values.