Abstract
Understanding mechanisms of oil mobilization of tight matrix during CO2 injection is crucial for CO2 enhanced oil recovery (EOR) and sequestration engineering design. In this study exposure behavior between CO2 and tight rock of the Ordos Basin has been studied experimentally by using nuclear magnetic resonance transverse relaxation time (NMR T2) spectrum and magnetic resonance imaging (MRI) under the reservoir pressure and temperature. Quantitative analysis of recovery at the pore scale and visualization of oil mobilization are achieved. Effects of CO2 injection, exposure times and pressure on recovery performance have been investigated. The experimental results indicate that oil in all pores can be gradually mobilized to the surface of rock by CO2 injection. Oil mobilization in tight rock is time-consuming while oil on the surface of tight rock can be mobilized easily. CO2 injection can effectively mobilize oil in all pores of tight rock, especially big size pores. This understanding of process of matrix exposed to CO2 could support the CO2 EOR in tight reservoirs.
Highlights
Data, as of 2013 tight oil is more than 42% of U.S domestic total crude oil production and it is expected to reach 59% in 2020
Hawthorne et al.[10] performed the exposure experiments between CO2 Bakken tight rock and proposed conceptual mechanisms of CO2 enhanced oil recovery (EOR) in a tight fractured system: (1) CO2 flows into and through the fractures, (2) rock matrix is exposed to CO2, (3) CO2 permeates the rock under pressure, swelling of crude oil extrudes some oil out of the pores, (4) oil migrates to the bulk CO2 via swelling and reduced viscosity, and (5) oil production is slowly driven by concentration gradient diffusion from pores into the bulk CO2 in the fractures
After tight matrix was exposed to CO2, NMR T2 test and magnetic resonance imaging (MRI) were continuously performed until the obtained T2 spectrum remained unchanged to investigate the effect of exposure time on oil mobilization
Summary
Data, as of 2013 tight oil is more than 42% of U.S domestic total crude oil production and it is expected to reach 59% in 2020. Understanding mechanisms of oil mobilization of tight matrix during CO2 injection is crucial for CO2 EOR engineering design in fractured tight oil reservoir. Oil mobilization characteristic of tight matrix in pore scale during CO2 injection is the key scientific question for effective production of fractured tight oil reservoir. We focused on the process of tight matrix exposed to CO2 under the temperature of 40 °C and pressure of 12 MPa and 22 MPa. NMR T2 and MRI were used to detect the tight matrix exposure process in situ to study mechanisms of oil mobilization during CO2 injection. After tight matrix was exposed to CO2, NMR T2 test and MRI were continuously performed until the obtained T2 spectrum remained unchanged to investigate the effect of exposure time on oil mobilization. Effect of immiscible and miscible pressure on oil mobilization was investigated
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