A Rigorous Workflow for Evaluation of Huff and Puff Recovery Efficiency of Immiscible and Miscible Gases in Unconventional Reservoirs by Integrating Core Tests with NMR and GC Analysis

摘要

In the current study,we have evaluated the feasibility and efficiency of cyclic gas injection(also referred as Huff and Puff(HnP))as a potential Enhanced Oil Recovery(EOR)technique for unconventional shale reservoirs.Oil recovery through cyclic gas injection tests with a field produced gas(70% methane,20% C2-C4)was compared between two shale samples with varying permeability.Results showed that gas injection cycles performed on a lower permeability shale sample(<20 nano Darcy effective permeability to oil)recovered only 2.1% of original oil in place(OOIP)whereas in a higher permeability shale sample(180 nano Darcy as effective permeability to oil),gas injection cycles recovered nearly 7% OOIP.Each subsequent HnP cycle with gas produced a lighter color effluent.We integrated core tests with Gas Chromatography(GC)analysis of produced effluents and confirmed that effluents were lighter in composition than the in-situ oil.Oil recovery sensitivity to gas types with different level of intermediate(C2-C4)enrichment,Huff pressures and soaking times were carried out for the higher permeability sample.The sensitivity tests showed that behavior of gas HnP in shales differ significantly from conventional rocks.On increasing Huff pressure on the shale sample to MMP(Minimum Miscibility Pressure)and beyond,oil recovery continued to show a steady increase,a behavior not generally observed in conventional rocks.In conventional rocks oil recovery increases sharply at MMP and then increases only gradually for pressures above MMP.Cyclic gas injection performed with a heavier variant of the produced gas(enriched to 40% C2-C4)could not enhance effluent recoveries beyond that from the field produced gas,although the estimated MMP for the heavier gas was ~ 1000 psi lower than MMP for the produced gas.The results indicate that bulk phase miscibility may not be as important in shales as it is in conventional rocks.Nano-pore confinement effects can cause deviations in critical fluid properties and miscibility pressures,gas diffusion,etc;and thus,play a dominant role in determining efficiency of gas HnP process in shales.We performed scans of core samples before and after each set of tests in a 12 MHz NMR to better understand recovery behavior and more accurately determine oil recovery.In addition,we performed a set of tests where we imbibed frac fluid into the rock before starting gas HnP cycles to closely simulate a real field scenario.Our results show that frac fluid imbibition into the rock caused a reduction in oil recovery from HnP process when compared to the same test done on rock samples that were not subject to frac fluid imbibition.In another research conducted by Ali et.al.(2020),cyclic gas injection was observed to improve degradation to hydrocarbon effective permeability induced by frac fluid imbibition(Figure 1).We also present results of the simulation fit to our lab data.We found that the modeling results were highly sensitive on parameters such as permeability,MMP and connectivity between oil-wet and water-wet media;and these properties needed to be measured on the core to get the HnP simulation results to come in line with the core HnP testing.Hence,we recommend performing lab tests to effectively tune and calibrate the simulation model for better field predictions Overall,the tests have shown that there are unique mechanisms existing in shales that have not been adequately incorporated into the commercial simulators yet.Thus,to build a comprehensive understanding of oil recovery from shale formations,we recommend adopting a rigorous lab testing program with multiple shale samples of varying permeability and evaluate the oil recovery from different gases,huff pressures,soak times,etc.,while incorporating damage induced by frac fluid imbibition.Lack of proper benchmarking of an EOR pilot project on rock samples can result in large losses(compression equipment,wrong choice of formation to HnP,incompatible gas,soak and flowback durations,