Experimental Study On Seepage Characteristics Of Low Permeability Sandstone

Seepage in low permeability rock (LPR) has a significant influence on resource exploiting and project construction. Therefore, much attention has been paid to the research on seepage in LPR. However, due to the complexity and multiplicity of influential factors, this research is still in the stage of groping. Since low permeability sandstone (LPS) is a relatively homogenous and widely distributed medium, it is important to understand the seepage characteristics of LPS.In this thesis, seepage characteristics in LPS were investigated and analyzed by conducting comprehensive tests for physical properties, seepage experiments and threshold pressure gradient (TPG) experiments on the LPS cores from ChangQing oil field. These tests include physical property determination for rock core, microcosmic shape analysis on rock slices, mercury-intrusion tests, single-phase saturated seepage experiments using salt water and using oil with various viscosities, stress-coupling seepage experiments and TPG experiments.Experimental results show that seepage in the sandstone core exhibits a non-linear behavior: the seepage curve is not a straight line and the apparent permeability is not a constant value with the rise of pressure gradient. After analyzing the results of physical properties tests, mercury intrusion tests and single-phase saturated seepage experiments in rock using salt water, a new physical property parameter of rock, the ratio of gas permeability to porosity– , has been brought forward. Using this parameter in analysis of test results, the following findings can be reached: There is a high correlation between and the average radius of pore throat . With the decrease of value, the slippage coefficient of rock increases while the Klinkenberg permeability decreases. The influence of net confining pressure on rock permeability decreases as the rock's value increases. There is a close relationship between and the shape of seepage curve. In single-phase saturated seepage test, nine rock cores can be classified into three types according to the value and rock cores in each type have a similar seepage curve. These results show that is a representative parameter, which can reflect the characteristic of pore structure of the rock well. Based on the data of mercury intrusion tests and stress-coupling seepage experiments, reasonable interpretation for the behavior that similar seepage curves exist in rock cores with similar values was given. Based on tests results analysis, empirical formula considering the influence of viscosity on seepage in LPS was presented as well as the relational expression between permeability of LPS and net confining pressure. An experimental method has been designed to measure the TPG. The method was to measure the pressure attenuation in a transient seepage flow. At the same time, a numerical model was established to simulate the real process. Results of numerical computation consist well with the experimental results; therefore, the reliability of this method was verified. The boundary layer and the capillary pressure of tiny pore throat in low permeability rock may be the essential reason of TPG.