Measurement and evaluation of the stress sensitivity in tight reservoirs

The revised details of SY/T5358-2010 of National Energy Administration Standard from five aspects are put forward, it is aimed at existing flaws of the standard, including measurement procedures, evaluation method and discrimination criteria of the rock stress sensitivity. Terzaghi's effective stress was derived according to stress equation of Bane and Maximov, which indicates Terzaghi formula, is not only suitable for soil mechanics, but also for rock mechanics. An exponential model of Terzaghi effective stress and rock permeability was built from former researcher's formula of permeability modulus. The exponential model was used to fit experimental data, with a good accuracy under two different experiment conditions, constant confining pressure (constant-external pressure) and changing-pore-fluid pressure (changing-internal pressure), and constant-pore-fluid pressure (constant-internal pressure) and changing-confining pressure (changing-external pressure). Since constant-external pressure and changing-internal pressure accords with the actual oilfield development, it is recommended to be used in the laboratory operation. It is concluded through the study that the stress sensitivity of reservoir rock can be evaluated with stress sensitivity coefficient α of the exponential model, the stress sensitivity coefficient, 0.005 MPa−1<α≤0.025 MPa−1, denotes weak stress sensitivity; 0.025 MPa−1<α≤0.035 MPa−1 middle-weak stress sensitivity, 0.035 MPa−1<α≤ 0.045 MPa−1 middle-strong stress sensitivity, 0.045 MPa−1<α≤0.100 MPa−1 strong stress sensitivity, and α≥0.100 MPa−1 strongest stress sensitivity. Meanwhile, measurement data of stress sensitivity in several literatures proves that the lower the sandstone reservoir permeability, the stronger the stress sensitivity will be.