(706a) Hybrid Gas and Chemical Stimulation for Shale Enhanced Oil Recovery

Background:Production of oil from tight oil shales has more than doubled the US oil production in the last decade. Oil is produced by depressurization using horizontal wells with multiple fractures. However, extremely low permeability (~0.1 micro Darcy) and microfracture closing reduce the oil productivity by more than 70% in the first year. Depressurization produces less than 10% of the oil present in these shale oil formations. Novel technology is needed to extract more oil out of these shale formations. Treating these formations with gases and surfactant solutions are being investigated. Injection of CO2 or hydrocarbon gases can swell the oil and reduces oil viscosity; the decrease in pressure following the injection can nucleate gas bubbles in situ, push oil into the fractures and prolong the oil recovery. Such processes are called huff-n-puff processes. Injection of surfactants can change the wettability, lower interfacial tension and improve oil flow. The goal of this work is to study mechanisms of improved oil recovery due to combined gas and chemical injection and compare the effectiveness hydrocarbon gases with CO₂in huff-n-puff EOR processes for a shale oil.

Methods:Gas and chemical huff-n-puff experiments were conducted at the laboratory scale using outcrop shale core plugs. Core plugs were saturated with a light oil at a high temperature and high pressure (7000 psi) for two weeks. Initial liquid volumes and added oil volumes were evaluated using NMR T2 relaxation times. The saturated shale cores were placed in a high-pressure reaction cell to conduct experiments. All huff-n-puff experiments were conducted at the reservoir temperature (75 ºC) and elevated pressure (huff: 2800~5200 psi, puff: ~2000 psi). The oil/water volume change was evaluated by NMR after the experiment to calculate the oil recovery factor. Three sets of huff-n-puff experiments were conducted, one using CO₂, another using hydrocarbon gases (C1, C2, C3 mixtures) and the third using CO2 and surfactant solutions at several pressures. The composition of hydrocarbon gas varies with reservoirs. For this work, a mixture of 60% methane, 35% ethane, and 5% propane was used. The cores were pressurized with the gas to certain huff pressure, soaked for 2 days, then depressurized to 2000 psi. In hybrid experiments, a surfactant solution was used in some of the pressure cycles along with the gas; D2O was used in place of water for NMR detection. Three huff-n-puff cycles were conducted for each experiment.

Results:NMR experiments showed that the injected oil responds at the 10 ms to 100 ms T2 relaxation time region. For a 1.5 in x 1.5 in shale outcrop, around 0.8 ml ~ 1 ml oil was injected. CO₂huff-n-puff is an efficient technique to improve recovery from oil shales. Most of the added oil was recovered in all the cases. The liquid recovery was about 40% of the fluid in place. The pressure to which the cores were pressurized with CO₂ did not affect the oil recovery significantly as long as the pressure was high enough (2800 psi). The “huff” pressures used in this study were higher than the CO₂MMP. In the hydrocarbon gas huff-n-puff experiment, the total recovered oil depended on the huff pressures. At high pressures (4000 psi and 5200 psi), the oil recovery factor was similar to the CO₂cases. But at low huff pressure (2800 psi), the oil recovery was substantially lower. Chemical blend plus CO₂was more effective than pure CO₂huff-n-puff (40% vs. 64%) mainly by increasing the oil recovery from the original liquid inside the shale. The surfactant can lower the interfacial tension and oil-wettability while CO₂dissolves in oil and swells the oil. The sulfate ions can delay the acid reaction allowing acid molecules to reach far into the matrix and dissolve carbonate minerals. This process can improve the stimulated volume around fractures. Pumping chemical blend to pressurize previously injected CO₂gave equally high recovery as injecting CO₂after injection of the chemical blend. This novel technique would avoid the use of large compressors.

Implications: This work evaluates using of CO2 and produced gas in addition to aqueous chemicals in Huff-n-Puff process. The use of produced gas reduces the issue of gas availability. The use of produced gas can also reduce the need for gas flare and reduce the associated environmental impact. The use of aqueous chemicals improves gas huff-n-puff. These processes are effective in the laboratory scale, but their scale-up to field scale needs to be studied.