A new quantitative evaluation method for fluid constituents with NMR T1-T2 spectra in shale reservoirs

Abstract

Saturation estimation of different fluids is crucial for reserve calculation and perforation location selection in shale reservoirs. Due to the low resolution of different fluids on the inverted T1-T2 spectra, different fluid signatures overlap, making it difficult to quantitatively evaluate fluid constituents and accurately calculate the fluid saturations. In this paper, a new quantitative metric that integrates discrete wavelet principal component analysis (PCA) and two-dimensional (2D) PCA is proposed to determine the optimal number of fluid constituents, and a quantitative evaluation method consisting of geometry-based unmixing method and nonnegative matrix factorization (NMF) is proposed to obtain the T1-T2 signatures of different fluids. The geometry-based unmixing method is used to obtain approximate fluid signatures called pseudo-signatures, which are processed by NMF to obtain accurate fluid signatures at a low computational cost. Subsequently, the fluid saturations are obtained from the least-squares method with full constraints. Numerical simulations are used to process the T1-T2 data. The results show that the proposed quantitative metric effectively provides the optimal number of fluid constituents, and the quantitative evaluation method accurately predicts the T1-T2 signatures and saturations of different fluids. In addition, the quantitative evaluation method requires less time than comparable methods to obtain the T1-T2 signatures and saturations of different fluids.