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This paper presents underground measurements (e.g., mini-frac tests) for in situ stress, hydraulic fractures, strata failures and hydraulic conductivity enhancement induced by coal extraction to study mechanisms of coal mine water inrushes. Mining perturbs the original state of in situ stress and results in stress redistribution. This causes rock failures and creates a water-conducting failure zone around a mining panel. If a confined aquifer is situated in the coal seam floor, the mining-induced stresses near the aquifer also generate hydraulic fractures in the rocks near to the aquifer. The mining-induced water-conducting failures and hydraulic fractures by the water pressure of the aquifer are the major reasons to cause water inrushes. The mining-induced water-conducting zone is obtained from theoretical studies and field measurements. The generating condition of the hydraulic fractures by the water pressure is also studied. Methods for predicting water inrushes are given based on the study of water inrush mechanisms. The mechanisms of water inrushes induced by faults are also analyzed from the case studies of water inrushes. It is of crucial importance to detect and map faults, paleo-sinkhole and other geological structures before mining. New mining methods are proposed for mining near confined aquifers to reduce the risks of water inrushes.

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The fracture gradient is a critical parameter for drilling mud weight design in the energy industry. A new method in fracture gradient prediction is proposed based on analyzing worldwide leak-off test (LOT) data in offshore drilling. Current fracture gradient prediction methods are also reviewed and compared to the proposed method. We analyze more than 200 LOT data in several offshore petroleum basins and find that the fracture gradient depends not only on the overburden stress and pore pressure, but also on the depth. The data indicate that the effective stress coefficient is higher at a shallower depth than that at a deeper depth in the shale formations. Based on this finding, a depth-dependent effective stress coefficient is proposed and applied for fracture gradient prediction. In some petroleum basins, many wells need to be drilled through long sections of salt formations to reach hydrocarbon reservoirs. The fracture gradient in salt formations is very different from that in other sedimentary rocks. Leak-off test data in the salt formations are investigated, and a fracture gradient prediction method is proposed. Case applications are examined to compare different fracture gradient methods and validate the proposed methods. The reasons why the LOT value is higher than its overburden gradient are also explained.