Earlier studies have shown that oil vaporization in the gas phase is one of the major mechanisms controlling oil recovery during gas injection Huff-n-Puff EOR in shale reservoirs. This vaporization process often leads to the selective production of hydrocarbon species. However, experimental studies documenting the hydrocarbon species vaporizable during Huff-n-Puff EOR are still scarce in the literature. The objective of the present study is to understand the vaporization mechanism during Huff-n-Puff EOR and what governs the production of the alkane fractions. This is done by assessing the hydrocarbon species producible by vaporization during CO2 Huff-n-Puff EOR in shales.
For the purpose of this study, we collected crude oil and wax preserved core samples from the Eagle Ford shale formation. The Eagle Ford shale sample was characterized by measurements of mineralogy, TOC, porosity, pore throat size and specific surface area. The hydrocarbon species contained in the crude oil and the residual oil in the Eagle Ford sample were determined by Gas Chromatography-Mass Spectrometer (GC-MS) measurements. To identify the hydrocarbon species vaporizable during CO2 Huff-n-Puff EOR, we conducted additional GC-MS measurements before and after CO2 Huff-n-Puff EOR laboratory experiments. During this experimental program, the GC-MS data were supplemented with Nuclear Magnetic Resonance (NMR) measurements.
Our results show a total oil recovery of 87% by vaporization in crude oil after 2 injection cycles and a 52% oil recovery by vaporization in the shale sample after 6 injection cycles. The GC-MS and NMR data show that hydrocarbon species up to C25 are vaporizable from the crude oil sample during CO2 Huff-n-Puff EOR. The experimental data also shows that the vaporization of C25+ is dependent on the amount of lighter species present in the crude oil, especially C14-. The crushed shale data shows that only hydrocarbon fractions up to C23 are vaporizable, regardless of the amount of lighter species. The lower recovery of heavier hydrocarbons indicates the complexity of the shale pore structure.
The significant differences between the hydrocarbon species vaporizable from crude oil and shale sample show that oil recovery during Huff-n-Puff EOR will largely depend on the composition and microstructure of the sample in addition to the residual oil properties. The data acquired during the present study also provide an opportunity for injection gas optimization as a function of injection cycles and crude oil composition.