Permeability evaluation of tight sandstone based on dual T2 cutoff values measured by NMR

Based on the comparative analysis of the T-2 spectrum of tight sandstone in the fully saturated state and the bound water saturated state, the storage mechanism and the seepage law of fluid in tight sandstone are clarified, the limitations of conventional NMR methods for predicting permeability are pointed out and the concept of double cutoff values by NMR is proposed. According to this concept, the reservoir space is segregated into three parts, i.e. the absolute immovable pore, the absolute movable pore and partially movable pore. We perform univariate analysis to characterize the effects of different pore components on permeability, and establish a new model of NMR permeability. The results show that the permeability is negatively correlated with the absolute irreducible fluid saturation, while positively correlated with the absolute movable fluid saturation, NMR porosity and the geometrical T-2 value of partial movable fluid component. On this basis, a self-adaptive method of determining dual T-2 cutoff values is shaped by combining the second order difference of the T-2 spectrum, which permits to calculate the dual cutoff values continuously. These research results have been applied to production, greatly enhaning the accuracy of permeability calculation.