A Comprehensive and Comparative Experimental Study of the Effect of Wettability on the Performance of Near Miscible WAG Injection in Sandstone Rock

Abstract

Accurate predictions of the performance of near-miscible water-alternating-gas (WAG) injection under different wettability conditions using the current commercial reservoir simulators are difficult and associated with significant uncertainties. Reliable experimental data are needed to tune reservoir simulators and optimize the performance of WAG injection in the field. In this study, a series of coreflood experiments has been carried out, under ultra-low gas/oil interfacial tension (IFT) level, on the same sandstone core with three different wettability conditions; water-wet, weakly-water-wet and mixed-wet. We present the performance of WAG injection, in terms of oil recovery and differential pressure, under near miscible and weakly water-wet conditions in a homogenous sandstone rock and the comparisons of these results with those in water-wet and mixed-wet systems. Each WAG injection experiment started with a water flood period, followed by three cycles of gas and water injections. To minimize experimental artefacts, the same long and large sandstone core (2in × 2ft) was used in all of the coreflood experiments presented in this paper as well as the same core preparation and experimental procedure were repeated. Analysis of the average oil recovery profile, for the weakly water-wet core, shows that about 62% (IOIP%) of recovered oil was achieved by the initial water flood, whereas 81.5 % (Sorw%) of the remaining oil was recovered by the alternation of water and gas injection cycles post-waterflooding. Our experimental results revealed that the performance of secondary water flood increased as the direction of wettability changed from water-wet to mixed-wet, passing through weakly water-wet conditions. As a result, the remaining oil saturation in the water-wet system is about a factor of two larger than that in its mixed-wet counterpart, whereas its value in the weakly water-wet system is lying on between that of the other two wettability systems. The oil recovery efficiency by tertiary gas injection cycles, under near miscible and the three different wettability conditions, increased as further WAG cycles were carried out. However, their oil production rates decreased when wettability changed from water-wet towards mixed-wet system. Although the ultimate oil recoveries were 96.7 %, 93% and 88.5 % (IOIP%) in mixed-wet, weakly-water-wet and water-wet respectively, the overall oil recovery performance (post-waterflooding) was slightly higher in water-wet, then less in weakly water-wet and much lower in mixed-wet system. Comparison of the water and gas injectivity behaviors based on their DP values during near miscible WAG injection revealed that water and gas injectivity increased when core wettability changes from water-wet towards mixed-wet, passing through weakly water-wet conditions. Most of the existing three-phase relative permeability correlations were developed for water-wet systems; however, they are widely used for non-water-wet reservoirs. This has increased the uncertainties associated with those three relative permeability values, especially for the data obtained for mixed-wet systems. Hence, representative and accurate experimental data, for different wettability regimes, are needed, firstly to obtain a reliable three-phase relative permeability and its hysteresis values, and secondly to optimize the WAG process using the existing reservoir simulators.