Observation of Mobility Enhancement of Heavy Oils Flowing Through Sand Pack Under Solution Gas Drive

Abstract

Abstract In situ formation of foamy oils under solution gas drive has been proposed as one of the possible mechanisms responsible for high primary production rate from some heavy oil reservoirs. Maini et al.(7) reported some results from steady-state tests of two methane-saturated heavy oils flowing through a sand pack. They concluded that there was no evidence of the presence of nucleated gas bubbles enhancing the mobility of the heavy oil; rather the oil mobility being decreased due to gas entrapment. This paper is the re-interpretation of their test results through matching both the oil production and the pressure distribution data with a foamy oil viscosity model. It is found that entrapment of gas bubbles in oil would reduce the oil mobility; however, the nucleation of gas bubbles at the grain surface would enhance the mobility. The mobility of the heavy oil increases proportionally with the nucleation rate; but decreases with foam quality following Roscoe's viscosity model(13). A new mobility correlation with the nucleation rate and the foam quality is proposed. With the new correlation, we succeeded in matching both the production rates and the pressure profiles. The mobility enhancement due to the bubble nucleation can be explained by gas lubrication effects. Introduction Anomalously high primary production rates have been experienced by some solution gas drive heavy oil reservoirs in Alberta and Saskatchewan(1–4). The performances of these reservoirs could not be predicted by a Darcy flow model with measured reservoir and fluid properties. To match field performance, the mobilities of oil and gas had to be greatly increased and decreased, respectively, and trapped gas saturations as high as 35% needed(1,2). Based on the numerical simulations of single-phase radial flow to a well, the primary production rate behaviour resembles a dilatant non- Newtonian fluid flow(5). To explain the high primary production rates and recovery factors in heavy oil reservoirs, a number of potential mechanisms have been proposed(1,2, 6–9). One of the possible causes for the anomalous behaviour is the in situ formation of bubbly (or foamy) oil due to entrapment of a large number of microbubbles nucleated during pressure drawdown(10). Evidence of formation of foamy oils was observed in both well head samples and laboratory tests(1,7). The in situ formation of foamy oil can greatly alter the flow behaviours of oil and gas in a reservoir byenhancing oil mobility with gas bubble nucleation;preventing rapid depletion of reservoir energy through retardation of free gas flow; andchanging oil properties due to gas bubble dispersion. The Effect of Bubble Nucleation on Oil Mobility Results of Steady-state Tests To study the effect of gas bubble nucleation on oil mobility, Maini et al.(7) conducted a series of in situ generated foamy oil flow tests in a sand-pack under steady-state conditions. A two metre long sand-pack holder with five equally spaced intermediate pressure taps was used. Tests were done on heavy oils from the Lindbergh and Lloydminster reservoirs. Relevant test parameters and the properties of two oils were presented in their paper.