Changes in elastic properties of artificial shales due to compaction

Abstract

SUMMARY The effects of compaction on elastic properties of shales and their anisotropy are important for seismic imaging, seismic to well tie and borehole stability issues. Compaction trends in shales remain poorly studied, but it is well known that porosity of shales rapidly decreases with the increase of burial depth due to mechanical compaction and chemical transformation of clays in particular. These processes affect all the physical properties of shales including their elastic moduli, electrical conductivity and permeability. In this experimental work we study changes in the anisotropic elastic properties of artificial shales caused by mechanical compaction. Investigation of anisotropy is performed on two specimens made of kaolinite and quartz powder mixtures: (1) 75% / 25 % and (2) 60% / 40%, respectively. An uniaxial stress is applied progressively to achieve distinct levels of porosity. Ultrasonic P- and S-wave velocities in the specimens are measured at every stage of the compaction. Thomsen’s anisotropy parameters are calculated from these velocities. Qualitative characteristic of microfabric anisotropy is performed using micro-CT image analysis. The results allow to conclude that at a given level of porosity of the specimen 2, with 40 % of quartz, has higher compressional and shear velocities than the specimen 1 with only 25% of quartz. However, the specimen 1 shows higher degree of elastic anisotropy than the specimen 2 due to higher fraction of anisotropic clay.