| CPC G01N 24/081 (2013.01) [G01N 24/082 (2013.01); G01R 33/50 (2013.01)] | 6 Claims |
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1. A method for measuring critical condensate saturation pressure and critical condensate saturation of condensate gas by combining one-dimensional and two-dimensional nuclear magnetic resonance techniques, comprising the following steps:
step S1: performing calibration experiment on condensate signal intensity and condensate saturation to obtain a relationship curve between the signal intensity and the saturation;
step S2: performing depletion experiment on a rock core, and acquiring a one-dimensional nuclear magnetic resonance T2 spectrum and a two-dimensional nuclear magnetic resonance T1-T2 spectrum at each pressure point;
step S3: calculating the condensate saturation by using an improved difference spectroscopy method through original data of the two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained in the step S2; wherein the improved difference spectroscopy method comprises the following specific steps:
step S3.1: subtracting a two-dimensional nuclear magnetic resonance T1-T2 spectrum of a dry rock sample from all two-dimensional nuclear magnetic resonance T1-T2 spectra required to be used;
step S3.2: dividing signal intensity of a two-dimensional nuclear magnetic resonance T1-T2 spectrum scanned at a certain pressure point in the pressure building process by signal intensity of a two-dimensional nuclear magnetic resonance T1-T2 spectrum scanned at the highest experimental pressure point to obtain a series of ratios of pressure points to the highest experimental pressure point in the pressure building process, wherein the ratios are named as a condensate gas correction coefficient a, which is shown as follows;
the two-dimensional nuclear magnetic resonance T1-T2 spectrum at the certain pressure point:
 the two-dimensional nuclear magnetic resonance T1-T2 spectrum at the highest experimental pressure point:
 step S3.3: multiplying an integral spectrum of a two-dimensional nuclear magnetic resonance T1-T2 spectrum of saturated condensate gas at the highest experimental pressure point by the correction coefficient a in the step S3.2 to obtain a new corrected two-dimensional nuclear magnetic resonance T1-T2 spectrum, wherein a new corrected two-dimensional nuclear magnetic resonance T1-T2 spectrum is actually a two-dimensional nuclear magnetic resonance T1-T2 spectrum of pure condensate gas at the certain pressure point, which is shown as follows;
the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the saturated condensate gas at the highest experimental pressure point:
 the two-dimensional nuclear magnetic resonance T1-T2 spectrum of actual condensate gas at the certain pressure point:
 the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the pure condensate gas at the certain pressure point:
 step S3.4: subtracting the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the pure condensate gas at the certain pressure point obtained in the step S3.3 from the two-dimensional nuclear magnetic resonance T1-T2 spectrum of the actual condensate gas at the certain pressure point in the depletion process, calculating a difference between the two-dimensional nuclear magnetic resonance T1-T2 spectra of the actual condensate gas and the pure condensate gas to obtain a new spectrum, wherein a positive part represents precipitated condensate oil, and a sum of the positive parts is signal intensity occupied by the precipitated condensate oil; and substituting the signal intensity into the relationship curve between the signal intensity and the condensate saturation in the step S1 to obtain condensate saturation at the certain pressure point, which is shown as follows:
the two-dimensional nuclear magnetic resonance T1-T2 spectrum of actual condensate gas at the certain pressure point minus the two-dimensional nuclear magnetic resonance T1-T2 spectrum of pure condensate gas at the certain pressure point:
 wherein the signal intensity occupied by the precipitated condensate oil is a sum of all positive numbers in the matrix:
 wherein i represents a row index, j represents a column index, and the matrix is n×n; II(Dij−aCij>0) is an indication function, when Dij−aCij>0, the indication function is 1, and otherwise, the indication function is 0; and
step S3.5: repeating the step S3.2 to the step S3.4, and calculating signal intensity occupied by condensate oil precipitated at each pressure, thereby obtaining condensate saturation corresponding to each pressure point and finally obtaining a relationship curve between the pressure and the condensate saturation;
step S4: plotting a relationship curve between total signal intensity and pressure by total signal intensity of the one-dimensional nuclear magnetic resonance T2 spectrum and total signal intensity of the two-dimensional nuclear magnetic resonance T1-T2 spectrum obtained in the depletion experiment, and obtaining a pressure point corresponding to a sudden drop point of the relationship curve between the total signal intensity and pressure, wherein the pressure point is a critical condensate saturation pressure point; then calculating critical condensate saturation corresponding to the critical condensate saturation pressure according to a relationship curve between pressure and condensate saturation obtained by an improved difference spectroscopy method; and
step S5: verifying the reliability of the critical condensate saturation pressure and the critical condensate saturation by adopting a long rock core depletion experiment and numerical simulation.
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