The Application of Nuclear Magnetic Resonance 1D T2 and 2D T1–T2 Maps in the Research of Condensate Oil Saturation in Condensate Gas Reservoirs

Determination of condensate oil saturation in condensate gas reservoirs is the key parameter for estimating condensate oil blockage and gas well productivity. In PVT experiments, the constant volume depletion (CVD) method is usually adopted to obtain condensate oil saturation. However, in this method, the effect of porous media is not considered. This study applied a low field nuclear magnetic resonance (NMR) test instrument in which 1D T2 and 2D T1–T2 NMR techniques are carried out to investigate the condensate oil saturation of “condensate gas-saturated” core plugs in the depletion process of 29 to 5 MPa at 25 °C. It was observed here that the T1/T2 ratios of condensate oil and gas at the signal peak are approximately 20 and 60, respectively, and the condensate oil and gas could be distinguished by positions in 2D NMR T1–T2 maps. Additionally, a standard equation (the relationship between the signal intensity and the condensate oil saturation) with an excellent linear relationship was established for the calculation of the condensate oil saturation in core plugs. The results indicated that the condensate oil first appears in micropores, and the retrograde condensation phenomenon of the core mainly comes from the contribution of mesopores and macropores; furthermore, the maximum condensate oil saturation in the porous medium is 5.36% higher than that in the PVT cylinder, and the dew point pressure in the porous medium is 11.83% higher than that in the PVT cylinder. The T1–T2 NMR technique may become an efficient approach for assessing fluid distributions and saturations, simultaneously possessing potential applications for characterizing fluid occurrence states in the core.