Shale is a rock likely to cause problems in oil drilling and production. The main problems in shale and clay swelling studies published so far are: (1) there is little connection of swelling test results with possible engineering applications, especially to stress-strain analysis; (2) the advances in the microscopic swelling research are not applied in the macroscopic mechanical swelling research; and (3) most tests do not consider the effects of confining pressure on water content, swelling amount and Young's modulus.
Based on the published microscopic swelling data, this paper derives a general swelling model which includes the effects of water content and stress state (and transversely isotropic mechanical properties for the sedimentary formation, if needed). The parameters in the derived constitutive equations are determined by swelling tests under different confining pressure levels. The test apparatus and procedures are described. The test results have verified the proposed model to show the higher the confining pressure the lower the final water content and swelling amount. Based on rock mechanics theory and the experimental research, constitutive equations of shale from Daqing Oilfield, China, have been proposed. Discussions and suggestions for further research are given at the end of the paper.