An Experimental Study of Cyclic CO2-Injection Process in Unconventional Tight Oil Reservoirs

Comparison of CO2 and Produced Gas Hydrocarbons to Recover Crude Oil from Williston Basin Shale and Mudrock Cores at 10.3, 17.2, and 34.5 MPa and 110 °C

“…These results show that recoveries of heavier hydrocarbons from both MB and LBS rock samples are enhanced both by richer gas hydrocarbons and by increasing pressure, with results similar to the ability of the same five gases to dissolve heavier hydrocarbons when in equilibrium with bulk crude oil . These results are also consistent with earlier reports showing that higher CO 2 pressures are more efficient at recovering hydrocarbons from tight rock samples. ,,− …”

“…Several studies have proposed that injected gas flow in tight fractured reservoirs will be dominated by the fracture flow and not by significant flow through the intact rock matrix. ,,,,− These observations have led to the proposed mechanism where concentration-gradient-driven diffusion may control oil recoveries from tight fractured reservoirs such as the Bakken, rather than oil recoveries being dependent on generating a “miscible” gas/oil front as for EOR floods in highly permeable conventional reservoirs. − ,,,− Thus, for EOR in unconventional reservoirs, the ability of an injected gas to efficiently dissolve oil hydrocarbons and enhance their concentration-gradient-driven transport from interstitial rock pores into the gas-dominated fractures may ultimately control EOR recoveries. Previous studies have also shown that oil recoveries using CO 2 from Bakken rock samples were more efficient with higher pressures, including whether the pressures were below, near, or above MMP. ,,− These results suggest that oil recoveries are controlled by the ability of injected gas to transport oil hydrocarbons from rock pores into the gas-dominated fractures. Recently, a successful field propane injection pilot study was conducted with a vertical injection well and a horizontal producer in the MB reservoir and showed the technical feasibility of enhancing oil recovery using high-pressure propane injection in the tight Bakken formation. , …”

Experimental Study on Spontaneous Imbibition of CO2-Rich Brine in Tight Oil Reservoirs

“…The growing concern about CO 2 emission promotes the development of CO 2 -EOR technologies with both miscible and immiscible. Many experimental and simulation studies − have shown that scCO 2 injection is one of the most effective and feasible methods to improve oil recovery in water-flooded formation. CO 2 -EOR mechanisms in the conventional reservoir for both CO 2 flooding and CO 2 huff-n-puff are well understood, including soluble in brine, oil viscosity reduction, oil swelling, light-hydrocarbons extraction, oil–gas interfacial tension (IFT) reduction, exerting an acid effect on rock, etc. , However, the nonwetted scCO 2 will flow along with fractures and is hard to flow into the tight matrix which is filled with fluids.…”

Volumetric Swelling of Bakken Crude Oil with Carbon Dioxide and Hydrocarbon Gases at 110 °C and Pressures of up to 34.5 Megapascals

“…13,14,21,22,31−36 Studies have also shown that oil recoveries using CO 2 and hydrocarbon gases from Bakken rock samples were more efficient with higher pressures, including whether the pressures were below, near, or above each gases' minimum miscibility pressure (MMP). 23,25,35,37,38 These results suggest that oil recoveries are controlled by the ability of the injected gas to transport oil hydrocarbons from rock pores into the gas-dominated fractures. Investigators have proposed that secondary oil recovery processes in tight fractured reservoirs are likely to be controlled by the ability of the injected gas to penetrate into the tight nano-pore rock matrices and dissolve into the captured crude oil, swell the oil, and dissolve the oil hydrocarbons into the rock fractures via concentrationgradient-driven diffusion, rather than oil recoveries being dependent on generating a "miscible" gas/oil front, as is observed for EOR floods in highly permeable conventional reservoirs.…”

“…However, experimental data that compare these gases’ abilities to mobilize crude oil at reservoir conditions relevant to unconventional plays are limited, especially considering the unique nature of tight, fractured shale oil plays such as the Bakken. In addition, downhole pressures in the Bakken during the initial production are generally reduced to a few megapascals, which is well below the bubble point, resulting in the remaining crude oil being depleted of its gas content. , Several recent studies have also proposed that injected gas in tight fractured reservoirs will primarily flow through the fractures, rather than through the rock matrix as occurring in EOR floods that are typical in highly permeable reservoirs. ,,,,− Studies have also shown that oil recoveries using CO 2 and hydrocarbon gases from Bakken rock samples were more efficient with higher pressures, including whether the pressures were below, near, or above each gases’ minimum miscibility pressure (MMP). ,,,, These results suggest that oil recoveries are controlled by the ability of the injected gas to transport oil hydrocarbons from rock pores into the gas-dominated fractures. Investigators have proposed that secondary oil recovery processes in tight fractured reservoirs are likely to be controlled by the ability of the injected gas to penetrate into the tight nano-pore rock matrices and dissolve into the captured crude oil, swell the oil, and dissolve the oil hydrocarbons into the rock fractures via concentration-gradient-driven diffusion, rather than oil recoveries being dependent on generating a “miscible” gas/oil front, as is observed for EOR floods in highly permeable conventional reservoirs. − ,,,− …”