Huff-n-Puff gas injection is a method originally used in heavy oil reservoir to reduce oil viscosity, increase mobility and displacement efficiency to enhance oil recovery. Now this method has been applied to enhance unconventional oil recovery in shale or tight reservoirs in recent years and proved to be effective in experiment study. N2, C1, CO2 or other rich gases are used in shale oil EOR. The purpose of this paper is to compare the EOR potential of different gas and provide a guide to choose gas based on the Wolfcamp shale oil reservoir.
The composition of crude oil from Wolfcamp was analyzed by Gas Chromatography (GC). First, the core plugs from Wolfcamp with diameters of 1.5 inches were saturated with crude oil. Then gas huff-n-puff experiments using N2, C1, and CO2 were conducted in the laboratory with the same injection pressure of 2000 psi. Based on laboratory results, a compositional model is built and used to analyze the performance of gas huff-n-puff. The EOR capacity of gas mixture (N2, C1, CO2) and some solvents such as C3-CO2 mixture, the separator gases C1 to C4 from the field production were investigated using the simulation method.
From the experiment results of the three kinds of gas injections, the oil recovery in the first two injection cycles were large. The incremental oil recovery decreased as the increase in number of injection cycles. Comparing the three kinds of gas EOR effects on Wolfcamp core samples, CO2 EOR result was the best, followed by N2 and C1. Coupling the equation of state method with GC analysis, 24 components of crude oil were achieved and then lumped into 5 pseudo components for simulation. The EOR effects of other gas mixture and solvents are investigated using the field model. The results show that the EOR effect of mixture of C1, C2 and C4 is the most favorite, followed by CO2-C3 mixture, produced gas, C1-CO2 mixture, and N2-CO2 mixture. Combined all of these results with economic factors, a comparison of gas capacity is proposed.
This investigation is focused on injection gas selection for shale oil production and helps to provide us a screening criterion to choose effective and convenient gas when conducting huff-n-puff gas injection in shale oil development. Cheaper gas with higher EOR potential will reduce the production cost and bring huge economic benefits to oil company especially in this low oil price period.