Intermittent Pumping Optimization for Ultra-Low Permeability Reservoirs Sampling

Abstract

At present, the low-permeability reservoir fluid identification and formation dynamic evaluation mainly relies on the wireline formation test (WFT) technology in China Offshore exploration (Yang 2014 and Zhang 2018). However, because of low permeability, formation fluids cannot be continuously supplied while sampling, which makes it impossible to achieve continuous pumping during sampling operations. Therefore, there are problems such as low success rate and poor timeliness in the sampling operation of low to ultra-low permeability reservoirs. In some extreme cases, formation fluids samples cannot be obtained, which increases operating costs and exploration risks. As a spare and passive sampling method, intermittent pumping is often used for fluid identification and sampling in ultra-low permeability reservoirs. However, at present, there is no systematic analysis and demonstration of the application limit and related feasibility of intermittent pumping in the industry, nor the scientific optimization of intermittent pumping. The rationality of the intermittent pumping scheme affects the efficiency and sampling success rate. This paper aims to find out the influencing factors of intermittent pumping, and to form the optimal intermittent scheme to guide the operation. This paper uses the Modular Formation Dynamic Tester Tool (MDT) formation testing tool as the research object. First, the lower limit of MDT was analyzed. Second, through numerical simulation, the pressure recovery ratios (flowing pressure at the end of pump stop/formation pressure) of 40%, 50%, 60%, 70%, 80%, 90% were respectively defined; 8 in-, 15 in-, 20 in- invasion depths were simulated, which represent shallow, middle, and deep-mud filtrate invasion scenarios, respectively. Three samples of purity at 20%, 50%, and 80% were set as achievement goals; a total of 54 kinds of "intermittent" pumping operations were simulate, and the total operation time in the corresponding situations were obtained as well. During intermittent pumping, the continuous invasion of mud filtrate is the key factor affecting the pumping time and samples assurance. In this study, the mud filtrate invasion rate of different formations and mud systems was confirmed through laboratory and previous WFT data analysis. Moreover, this paper realized the world's first numerical simulation about intermittent pumping while there was mud filtrate continuous invasion. The intermittent pumping performance under different original invasion depths and different continuous invasion rates were also simulated and analyzed to provide guidance for the optimization of subsequent operations. Through the above studies, the optimized intermittent pumping method has been successfully applied to ultra-low to low permeability reservoir WFT sampling in multiple wells, which in the past could only be abandoned or it had to rely on advanced tools.