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In a CO 2 storage project, CO 2 may be able to escape from channels caused by the formation's uplift, fault reactivation, and wellbore failure owing to overpressure build-up and likewise, coal excavation during which equilibrium disruption of stress could possibly lead to fl oor heave, roof collapse, and surface subsidence. When the carbon capture and storage (CCS) project and coal mining engineering activity occur in the same location, coal mining may also induce wellbore failure if the stope border is too close to the well. This is a real and challenging problem for CO 2 geological storage in the Ordos Basin in China. We consequently investigated four key problems for such a combination system. The main conclusions obtained from numerical simulations are as follows. (i) Permeability is the most important factor that affects the fl ow rate at the injection point and the reservoir pore pressure. (ii) The interfaces between the coal layers and their adjacent rocks are easy to damage the casing. Our work suggests that the coal pillar should be set aside at least 90 m to maintain the tensile strength of the casing below the designed value. (iii) No impact occurs to the caprock from coal mining; thus, a coal seam 440 m in depth could be excavated if the original caprock is intact and no complex geological structure exists in the location. (iv) Permeability is the most crucial factor affecting surface displacement, and CO 2 injection permits a surface displacement reduction of 0.04136 m compared with individual coal excavation.

Throughout the several-hundred-year-long history of the concept of strength, inaccurate material strength as a result of the size effect and the inconsistency of strength theories have been two continuous and challenging issues, and have even been taken to be inherent attributes of material strength. Applying the structural stressing state theory and method, this study experimentally investigates the uniaxial load-bearing process of concrete specimens and reveals their stressing state mutation features at specific load levels. Exploration of this general feature resulted in the discovery of essential strength, which is basically without size effect. Then, biaxial and triaxial experiments with concrete specimens were conducted in order to obtain the results for various combinations of principal stresses on essential strength. Consequently, according to Yu’s unified strength theory, the formula for strength of concrete was determined by fitting the relation between the combined principal stresses and the essential strength, which was verified by experiments carried out using natural marble specimens. Essential strength could promote the accuracy of strength indices, and the formula for strength might replace the existing strength theories for brittle materials. The initial solution of these two classic issues could make a new contribution to Yu’s unified strength theory and its final goal, promoting related research on material strength and leading to a more rational use of material strength in practical engineering.

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The increase of fluid injection projects with large burial depths, such as CO2 geological storage, poses a new challenge for the change law of pressure in reservoirs. To obtain the pressure of anywhere at anytime conveniently and then evaluate the injectivity and safety of reservoirs, a Darcy formulation suited for two‐phase flow of displacement is put forward in this paper. A convenient and practical explicit integral (analytical) solution of pressure build‐up for two‐phase flow under a constant injection rate of CO2, based on an infinite reservoir with a constant pressure boundary whose location is a function of time is then derived. Subsequently, this work compared the results of the explicit integral solution with the results of Nordbotten's approximate solution and the simulated results of TOUGH2/ECO2N for an analysis case of CO2 injection, which demonstrated a good consistency, verifying the correctness and the reliability of the explicit integral solution. Furthermore, the sensitivity analysis of Slc (the saturation of brine in the CO2 domain) and Slw1 (the saturation of brine in the brine domain 1) showed that they both have a great impact on the pressure profiles in reservoirs, and the pressure is more sensitive to Slw1 than Slc. Therefore, the determination of Slc and Slw1 should be careful and based on the actual project in applications. Generally speaking, the explicit integral solution is simple, convenient, and practical compared with numerical simulators and other analytical solutions with similar assumptions. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd