Method for calculating surface relaxation rate of shale

A method for calculating a surface relaxation rate of a shale, comprising:

• S1, selecting a rock sample from a target shale reservoir corresponding to the shale to perform a nuclear magnetic resonance (NMR) experiment under saturated formation water conditions to obtain a relaxation time T distribution curve;
• S2, selecting a target rock sample from the rock sample to perform a constant-rate high-pressure mercury injection experiment to obtain a pore throat radius r distribution curve;
• S3, standardizing abscissas of the relaxation time T distribution curve and the pore throat radius r distribution curve to obtain a standard relaxation time T distribution curve and a standard pore throat radius r distribution curve with a same abscissa starting value and a same abscissa ending value on a logarithmic scale;
• S4, determining an abscissa value corresponding to a maximum ordinate value on the standard relaxation time T distribution curve, denoted as a, determining an abscissa value corresponding to a maximum ordinate value on the standard pore throat radius r distribution curve, denoted as b, calculating a value of b/α, expanding, in response to the value of b/α>1, the abscissa of the standard relaxation time T distribution curve by b/αunits to obtain a transformed relaxation time T distribution curve, or shrinking, in response to the value of b/α<1, the abscissa of the standard relaxation time T distribution curve by a/b units to obtain a transformed relaxation time T distribution curve, to make an abscissa value corresponding to a maximum ordinate value in the transformed relaxation time T distribution curve be same as the abscissa value corresponding to the maximum ordinate value in the standard pore throat radius r distribution curve, and combining the transformed relaxation time T distribution curve and the standard pore throat radius r distribution curve in a same coordinate system to obtain a combined curve graph with both the transformed relaxation time T distribution curve and the standard pore throat radius r distribution curve;
• S5, drawing N straight lines, wherein N is a positive integer, parallel to a y-axis of the combined curve graph with each straight line passing through both the transformed relaxation time T distribution curve and the standard pore throat radius r distribution curve, determining abscissa values of the straight lines as high-pressure mercury injection pore throat radius r values and transformed NMR relaxation time calculated values, multiplying the transformed NMR relaxation time calculated values by b/α or α/b or dividing the transformed NMR relaxation time calculated values by b/α or α/b to obtain pre-transformation NMR relaxation time calculated values, and obtaining real relaxation time T values which are closest to the pre-transformation NMR relaxation time calculated values from original NMR experimental data obtained in the step S1;
• S6, calculating a surface relaxation rate ρ based on the high-pressure mercury injection pore throat radius r value and the real relaxation time T value for each straight line to thereby obtain N of surface relaxation rates ρ respectively corresponding to the N of straight lines, wherein a calculation formula of the surface relaxation rate ρ is as follows:
• ρ
• =
• R
• F
• S
• ⁢
• T
• ,
• where r represents the high-pressure mercury injection pore throat radius value, with a unit in nanometers (nm), T represents the real relaxation time value, and Fs represents a shape factor, determined by a pore throat structure shape of the target shale reservoir;
• and
• S7, performing data processing to the surface relaxation rates ρ to obtain a final surface relaxation rate ρ′ of the target rock sample, wherein the step S7 specifically comprises:
• S71, sorting N of the surface relaxation rates ρ in an ascending order to obtain sorted surface relaxation rates, wherein a minimum value of the sorted surface relaxation rates is ρmin and a maximum value of the sorted surface relaxation rates is ρmax;
• S72, calculating a difference between every two adjacent ρ values of the sorted surface relaxation rates to obtain difference values d, and calculating an average value D of the difference values d;
• S73, dividing an interval [ρmin, ρmax] formed by the minimum value ρmin and the maximum value ρmax into a plurality of secondary intervals with the average value D as an interval, counting ρ values and a number of the ρ values in each secondary interval, and calculating an average value ρ of the ρ values in each secondary interval;
• S74, calculating a weight coefficient ρ of the average value ρ in each secondary interval based on the number of the ρ values in each secondary interval;
• and
• S75, multiplying the average value ρ in each secondary interval with the corresponding weight coefficient ρ to thereby obtain products corresponding to the plurality of secondary intervals, and summing the products to obtain the final surface relaxation rate ρ′ of the target rock sample.