Evaluation of the stress sensitivity in tight reservoirs

The expressions of three types of stress sensitivity coefficients (S, alpha, beta) are obtained on the basis of analysis of the empirical models and theoretical models on the relationships between permeability and effective stress, and the stress sensitivity evaluation standard is proposed considering experiment data and rock micro-structural features. Then the empirical models and theoretical models were used to fit experiment data of low-permeability and tight sandstones and different types of granites, which promotes the understanding of the empirical models and the physical meanings of the stress sensitivity coefficients. According to the study of the three types of stress sensitivity coefficients and the relationship of effective stress and permeability, it is found that the stress sensitive coefficient S is suitable for evaluating the stress sensitivity (strong stress sensitivity: S>0.40; low stress sensitivity: S<0.25; medium stress sensitivity: 0.25 <= S <= 0.4). Meanwhile, it is also found that strong-stress-sensitivity cores are more suitable to be characterized by the logarithmic model, while the exponential model (or binomial model) occurred more frequently in low-stress-sensitivity cores, and medium-stress-sensitivity cores are more likely to be described with the power model. Finally, the evaluation standard is discussed based on the micro-structure in the low-permeability and tight sandstones. The results show that the crack-like pores in the strong-stress-sensitivity cores are obvious, but low-stress-sensitivity cores have the characteristics of porous rocks; moreover, the types of cemented clay minerals, the size of rock grain and lithology have impact on stress sensitivity.