基于核磁共振和高压压汞的孔喉半径确定方法OA

The conversion coefficient determines the accuracy of converting nuclear magnetic resonance(NMR)transverse relaxation time into pore-throat radius.To evaluate the applicability of different conversion methods,the linear correlation expression and power function expression for the conversion coefficient were compared,following the fundamental principles of NMR.Guided by the objective fact that the NMR pore-throat distribution encompasses the high-pressure mercury injection(HPMI)pore-throat distribution,three constraints were derived and then used to determine the upper and lower limits of the transverse relaxation time corresponding to the maximum and minimum HPMI pore-throat radii,respectively.Then,the range of conversion coefficient was defined,and a rationality evaluation chart for conversion results was constructed.Subsequently,a dual-eigenvalue method for calculating the conversion coefficient was proposed.Finally,the applicability of different conversion methods was evaluated,and corresponding correction methods were provided.The following results are obtained.First,the maximum pore-throat radius from HPMI and its corresponding upper limit of transverse relaxation time determine the lower limit of the conversion coefficient,while the minimum pore-throat radius from HPMI and its corresponding lower limit of the transverse relaxation time determine the upper limit of the conversion coefficient.Second,the dual-eigenvalue method takes the minimum and maximum HPMI pore-throat radii as eigenvalues,determines their corresponding approximate transverse relaxation times via the established constraints,and thereby calculates the conversion coefficient.Third,the dual-eigenvalue method yields a coincidence rate of 100%,and the cumulative saturation method also achieves 100%when curve shapes are similar.In contrast,the maximum eigenvalue method shows a coincidence rate of 80%when the maximum mercury injection saturation exceeds 40%.After correction,both the cumulative saturation method and the maximum eigenvalue method achieve 100%,without sensitivity to variations in maximum mercury injection saturation or curve shapes.The study concludes that the value range of the conversion coefficient,feasibility assessment templates,and dual-eigenvalue approach collectively address the challenges of inflexibility and inaccuracy in existing methods.The proposed correction technique,which enhances existing evaluation frameworks,provides reliable technical support and reference for the accurate evaluation of tight sandstone pore-throat structures.