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In order to quantitatively characterize the porosity of tight cores, the porosity, permeability and pore size distribution of tight cores can be measured by nuclear magnetic resonance (NMR) using the proportional relationship between TT2 relaxation time and pore radius, and the free fluid and bound fluid in pores can be distinguished by TT2 cut-off value. Based on the principle of core porosity characterization by NMR, the relationship between TT2 and core porosity, permeability, pore size distribution and movable fluid in pore is discussed. Through above principles and examples, the accuracy and applicability of the NMR in the quantitative characterization of porosity and permeability of tight cores are verified. The cut-off values and pore size distribution tests of the experimental cores show that the cut-off value of core 1 is 21.85 ms and that of core 2 is 40.11 ms. Core 1 has a large small pore fraction and core 2 has a more uniform pore distribution. The results show that the cut-off values of T2 obtained by regional method are not fully suitable to the case of tight core, and it is necessary to establish a regional version of the cut-off variation range chart. [ABSTRACT FROM AUTHOR]

The surface relaxivity for tight cores is commonly determined by the methods of average pore radius (ARS) or surface-to-volume ratio (SVR). The ARS method is time-consuming, and permanent damage to core samples will occur due to the injection of mercury. In this study, a non-destructive method is proposed, instead of the ARS method, to calculate the surface relaxivity of tight core samples based on T2 cut-off value. Firstly, surface relaxivity is calculated using the new method (pseudo T2 cut-off, PTC). Secondly, the calculated surface relaxivity of ARS and SVR methods are compared. Thirdly, the T2 spectrum is converted into pore diameter distribution by choosing appropriate surface relaxivity. Finally, residual oil distribution can be obtained. The ultimate surface relaxivity values for tight sandstone core samples are 5.85, 2.98, 4.66, and 2.17 μm/s. Moreover, we obtained the pore diameter distribution in mesopores and macropores by jointly using these three methods. Residual oil is mainly distributed in micropores and mesopores. The new method proposed in this study is nondestructive and is helpful to quickly and efficiently determine the surface relaxivity of tight cores.

Although CO2 huff-n-puff process has been widely employed as an efficient oil recovery and carbon storage technique in unconventional reservoirs, it suffers from the disadvantages of lack of availability near a large field and formation leakage due to the high CO2 mobility. Carbonated water injection (CWI) has been extensively studied as the alternative of CO2 injection process, but the laboratory studies concerning on CW huff-n-puff process is scarcely reported. In this paper, the oil recovery characteristics of CW huff-n-puff process has been scrutinized under reservoir conditions of 28 MPa and 65 °C together with NMR tests to reveal the oil mobilization rules in the tight cores. Comparative studies on CO2 huff-n-puff and water huff-n-puff process were also performed. It is found 4 rounds of CW huff-n-puff processes could recover considerable amount of 36.2% of the oil from the tight core. Although the recovery rate is lower than 55.4% for CO2 process but it is much higher than 14.4% of the water process. NMR T2 results reveal the oil in the large size pores of the tight cores are mobilized and recovered in early huff-n-puff cycles, whereas the oil from small size pores are collected in the later stage of the CW and CO2 huff-n-puff processes. The research results indicate the CW huff-n-puff process is promising for tight reservoir development.

Although CO 2 huff-n-puff process has been widely employed as an efficient oil recovery and carbon storage technique in unconventional reservoirs, it suffers from the disadvantages of lack of availability near a large field and formation leakage due to the high CO 2 mobility. Carbonated water injection (CWI) has been extensively studied as the alternative of CO 2 injection process, but the laboratory studies concerning on CW huff-n-puff process is scarcely reported. In this paper, the oil recovery characteristics of CW huff-n-puff process has been scrutinized under reservoir conditions of 28 MPa and 65 °C together with NMR tests to reveal the oil mobilization rules in the tight cores. Comparative studies on CO 2 huff-n-puff and water huff-n-puff process were also performed. It is found 4 rounds of CW huff-n-puff processes could recover considerable amount of 36.2% of the oil from the tight core. Although the recovery rate is lower than 55.4% for CO 2 process but it is much higher than 14.4% of the water process. NMR T2 results reveal the oil in the large size pores of the tight cores are mobilized and recovered in early huff-n-puff cycles, whereas the oil from small size pores are collected in the later stage of the CW and CO 2 huff-n-puff processes. The research results indicate the CW huff-n-puff process is promising for tight reservoir development. [ABSTRACT FROM AUTHOR]