Permeability and elastic parameters under different pressures of tight gas sandstones in eastern Ordos basin, China

Abstract

The porosity, permeability and elastic parameters are important to the design and engineering of tight gas sandstone wells. Based on experimental tests of sandstones sampled continuously from newly drilled well cores, the porosity, permeability, and elastic parameters were characterized with a discussion of their controlling factors. The porosity lies in a range of 0.1%–13.8%, with 96.6% of all the samples being lower than 10%. The permeability is generally under 0.1 mD, with 43.4% clustered <0.01 mD. The permeability is much more sensitive to the confining stresses than the porosity is, which endured almost five times damage as the confining stresses increased from 500 to 5000 psi. The Young's modulus (E) and Poisson' Ratio (υ) values were tested by uniaxial (UA), triaxial (TA) and acoustic velocity measurement methods. Different methods show different scales and also different variation trend. The E values are similar between the UA and TA methods in a range of >2000 Psi, however, the TA results are generally higher than the UA results in E < 2000 MPa range. The υ values of UA are higher than the TA results in υ > 0.175, vice versa in υ < 0.175. The compressive strength values both from TA and UA methods show a linear relationship, with the TA results three times of the UA results. In general, the dynamic υ values increase sharply (>10%) as with the axial stress increases, while the E values show less of an increase (approximately 5%). The permeability is relatively high at a depth of 1700 m, and then decreases in deeper strata; this is likely caused by the higher stress conditions and complex clay compositions. The elastic parameters show no clear relationship with depth, as they are mainly affected by the combined influences of rock compositions and the diagenesis effect. The results improve the understanding of tight gas sandstone properties and will be useful in gas reservoir engineering.