Feasibility of an EOR MicroPilot for Low-Salinity Water Flooding

Abstract

Abstract Over the past decade low-salinity water flooding has emerged as a viable enhanced oil recovery (EOR) method. Both laboratory tests and field trials have shown that injecting chemically modified water instead of seawater can lead to incremental oil recoveries. Although much research has been conducted, the governing physical and chemical mechanisms for this increase in recovery are not yet agreed upon, but are generally believed to involve some form of interaction between the rock, oil, and brine leading to changes in wettability, oil/water interfacial tension, or both. The relative uncertainty in the physics of the recovery process calls for a carefully staged screening and pilot program before committing to full-field implementation. The EOR MicroPilot* is a single-well piloting technique that enables rapid and inexpensive testing of EOR methods under in-situ downhole conditions. It is a log-inject-log technique conducted with a wireline formation tester (WFT). A small quantity of EOR fluid is injected and the resulting change in oil saturation is then determined based on a set of openhole logs, which are run both before and after the injection. In this paper we investigate the feasibility of a MicroPilot for low-salinity water flooding. Through simulation we show that the changes in oil saturation and salinity are measurable by the openhole logs. We further quantify the conditions under which a low-salinity MicroPilot is feasible in terms of the minimum measurable saturation changes and the contrast in salinity between the injected and resident brines. To assess the accuracy of the numerical solutions, we have conducted grid sensitivity studies as well as comparisons with analytical solutions to simplified 1D problems. The results of this paper are directly applicable to the planning of low-salinity waterflood pilots. The methodology proposed, based on the MicroPilot concept, can reduce piloting costs as well as reduce the time required between laboratory testing and field implementation. Introduction The MicroPilot (Arora et al. 2010) was introduced as an intermediate step in the enhanced oil recovery (EOR) screening process between laboratory testing and traditional multiwell field pilots. The technique works like a downhole laboratory by injecting a small quantity of the EOR fluid and measuring the resulting decrease in residual oil saturation. In this way laboratory results can be validated at downhole conditions, thus reducing the uncertainty associated with subsequent field pilots. The MicroPilot is a log-inject-log technique which can be executed in a matter of hours to days, making it a relatively inexpensive step in the EOR screening process. After the target well is drilled for application of the technique, a suite of wireline openhole logs is run to determine the initial saturations. A formation tester string is then used to inject the EOR fluid, typically 10 to 20 L, through a pencil-sized hole drilled into the side of the wellbore at the target depth. Following injection, the suite of openhole logs is run again to evalutate the effectiveness of the flood by measuring the change in oil saturation and the dimensions of the flood. Injection stations can be selected at multiple depths, which provides the ability to test an EOR method in different zones of the reservoir or test different EOR fluids within a single zone, all within a single job. A schematic of the MicroPilot is shown in Figure 1. Arora et al. (2010) reported on the first EOR MicroPilot which was conducted in a medium heavy-oil sandstone reservoir using a mixture of alkaline, surfactant, and polymer chemicals (ASP) as the EOR fluid. They covered aspects of the job design, execution, and log interpretation. Cherukupalli et al. (2010) presented flow modeling of the first MicroPilot showing that the saturation changes measured by the logs could be successfully matched by a relatively simple model of the ASP process. Finally, Edwards et al. (2011) recently reported on the second application of the technique, which was conducted in a light-oil carbonate reservoir using an alkaline-surfactant mixture as EOR fluid.