Oil production from tight formations contributed about 49% of total crude oil production in U.S. in 2014 (EIA., 2015). Such amount is expected to grow significantly as the active development of low permeability reservoirs continues. Since primary recovery is becoming less effective and large amounts of oil are locked in matrix, various IOR techniques should be considered to face such challenge. Lots of recent studies evaluated the potential of gas injection in shale plays. However, few authors discussed the feasibility of water huff-n-puff process. This study aims to evaluate the potential of recovery shale oil by using cyclic water injection (CWI) method.
Eagle Ford outcrop core plugs were used in this study and re-saturated with crude oil. X-ray diffraction (XRD) technique was employed to analyze the mineral composition of the sample. To reduce clay swelling, we used 5% of potassium chloride (KCl) solution as the injection fluid. Two groups of experiments were performed to examine the effects of soaking time and injection pressure on recovery performance. During the test, core plug was soaked with water under a constant pressure for a certain time, then the surrounding pressure was released to enter the production period. Under the same test conditions, cyclic gas injection (CGI) was conducted for the purpose of comparing two IOR effects.
The mineral report shows that the clay content is 7.4 wt% in the sample. The use of clay stabilizer was a necessity when preparing the injection fluid. We present the results of cumulative recovery factor (RF) of each cycle for each test. It was found that soaking time impacts the RF within a certain range, which was similarly concluded from the gas huff-n-puff tests. Injection pressure can significantly affect the recovery performance. For example, after performing 12 cycles of water huff-n-puff, the RF for tests with injection pressures of 1,000 psi and 5,000 psi were 14% and 21%, respectively. When applying 5,000 psi, we observed that fractures were created and the width increased with further cycles operated. Comparing two processes of CWI and CGI, different recovery characters were observed. CWI showed the recovery potential at the first four cycles, and then the yielded oil was dramatically decreased with further cycle performed. By contrast, CGI had a continuous recovery performance that the incremental RF was contributed from each cycle, then gradually diminished after seven cycles of huff-n-puff process.