Abstract
Characterization of pore throat size distribution (PTSD) in tight sandstones is of substantial significance for tight sandstone reservoirs evaluation. High-pressure mercury intrusion (HPMI) and nuclear magnetic resonance (NMR) are the effective methods for characterizing PTSD of reservoirs. NMR T2 spectra is usually converted to mercury intrusion capillary pressure for PTSD characterization. However, the conversion is challenging in tight sandstones due to tiny pore throat sizes. In this paper, the linear conversion method and the nonlinear conversion method are investigated, and the error minimization method and the least square method are proposed to calculate the conversion coefficients of the linear conversion method and the nonlinear conversion method, respectively. Finally, the advantages and disadvantages of these two different conversion methods are discussed and compared with field case study. The research results show that the average linear conversion coefficients of the 20 tight sandstone core plugs collected from Yanchang Formation, Ordos Basin of China is 0.0133 μm/ms; the average nonlinear conversion coefficient is 0.0093 μm/ms and the average nonlinear conversion exponent is 0.725. Although PTSD converted from NMR spectra by the nonlinear conversion method is wider than that obtained from linear conversion method, the nonlinear conversion method can retain the characteristic of bi-modal distribution in PTSD.
Highlights
Tight oil is an unconventional resource that has attracted substantial attention in recent years [1,2].the effective exploration of unconventional oil and gas is difficult because of the ultra-low reservoir formation permeability [3,4]
As tight sandstone permeability is mainly dominated by pore throat size distribution (PTSD) [6], characterizing PTSD in tight sandstones is of significance for tight sandstone reservoirs evaluation
The principle of nonlinear conversion of nuclear magnetic resonance (NMR) T2 spectra to PTSD is the NMR T2 and the corresponding pore throat radius under the same saturation is fitted with least square method according to Equation (11)
Summary
Tight oil is an unconventional resource that has attracted substantial attention in recent years [1,2]. As tight sandstone permeability is mainly dominated by pore throat size distribution (PTSD) [6], characterizing PTSD in tight sandstones is of significance for tight sandstone reservoirs evaluation. The standard method of converting NMR T2 spectra to pore throat sizes is cross-correlation of the PTSD from MICP with. In addition to the linear relationship assumption, a nonlinear relationship between NMR T2 spectra and pore throat sizes has been assumed in the literature [5,20,21,22]. This paper used the linear relationship and the power function relationship between NMR T2 and pore throat sizes to obtain PTSD from NMR T2 spectra. Using tight sandstone core plugs collected from a tight oil reservoir located in China’s Ordos Basin, the linear and nonlinear conversion methods were tested
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