Experimental Techniques and Rock Physics Modeling to Study the Effects of Mineral Content and Thermal Maturation on Elastic Properties of the Organic-Rich Shale: Case Study of the Longmaxi Formation

Summary To evaluate favorable fracturing locations and layers of shale gas reservoirs, it is essential to comprehend how thermal maturity and mineral composition affect geomechanical characteristics. The effects of kerogen thermal maturation on elastic properties of organic-rich shale under consideration of the vitrinite reflectance and how it relates to kerogen density and organic porosity demand in-depth investigation. We propose rock physics modeling of heterogeneous shale rock samples to model kerogen as a function of thermal maturity and elastic properties as a function of shale composition. Nine Longmaxi shale samples from different depths of Well A were collected and tested by using X-ray diffraction (XRD), light microscope, open system pyrolysis gas chromatography, nanoindentation, and uniaxial-triaxial compression techniques to examine mineralogy, geochemical, and geomechanical characteristics. Using the Voigt-Reuss-Hill average and differential effective medium approach, shale matrix was modeled first and incorporated kerogen matter in terms of incremental thermal maturation. The acoustic characteristics of Longmaxi laminated shale affected by the thermal maturation were also studied using the Hudson model. The results indicate that the Longmaxi shale is part of highly brittle hard rocks; more brittle minerals increase their strength. As clay minerals increase, we observe that at 10-MPa confining pressure, triaxial compression strength (TCS) decreases from 72 MPa to 43 MPa in the Longmaxi upper member sample (SUM), 63 MPa to 39 MPa in the Longmaxi middle member sample (SMM), and 55 MPa to 36 MPa in the Longmaxi lower member sample (SLM). However, with the rise of the quartz + carbonate + feldspar mineral group, we observe an increase in TCS, which goes from 58 MPa to 75 MPa in the SUM, 52 MPa to 68 MPa in the SMM, and 48 MPa to 66 MPa in the SLM. Research indicates that the effective modulus and P-wave velocity of Longmaxi shale falls with increasing of Ro from 0 to 3. In the SUM, Young’s modulus decreased from 11.4 GPa to 1.3 GPa. SMM shows a reduction of Young’s modulus from 11.1 GPa to 0.5 GPa, while SLM shows a decrease from 10.9 GPa to 1.14 GPa. Also, the Vp falls from 5.63 km/s to 4.2 km/s, while the Vs decreases from 3.4 km/s to 2.16 km/s in the SUM. However, in the SMM, Vp decreases from 5.58 km/s to 4.22 km/s, and Vs falls from 2.9 km/s to 1.7 km/s, while Vp and Vs are reduced in the SLM. The results of our modeling approach are confirmed by the variation of elastic properties measured in the laboratory and interpreted in well logs, which show a satisfying validity and usefulness. However, given the extreme variety of reservoir characteristics and basin evolution conditions, we also recommend that it be locally calibrated when applied to particular shale reservoirs.