Chunhe Yang

Influences of filling abandoned salt caverns with alkali wastes on surface subsidence

Abandoned salt caverns, particularly those that allow free draining of brine, are serious potential causes of subsidence above salt mines. Filling such caverns with alkali wastes may eliminate these troubles. Based on the filling project in Jingshen salt mine of China, the influences of filling abandoned free-draining salt caverns with alkali wastes on surface subsidence are discussed. The deposition characteristics of alkali waste slurry were studied by laboratory simulation tests. It is found that alkali waste slurry has good flowability during injection into a cavern. Based on the results of compression tests under drained conditions, a distribution model of void ratio of sediment is established. A method for determining the ultimate subsidence value is presented based on the distribution model. The relationship of average effective stress and void ratio of the sediment is also established. Further, a method for predicting development process of surface subsidence is presented. Finally, the specific application procedure of the prediction methods is discussed. According to the research results, filling free-draining salt caverns with alkali wastes can restrain surface subsidence successfully. The reasons can be summed up as: Volume convergence of salt caverns slows down after the caverns are filled owing to the supporting effect of alkali wastes on the rock surrounding caverns, and then, subsidence slows down and decreases, and finally, subsidence stops when the fill material is compacted to its compressed limit.

Effects of Cyclic Loading on the Mechanical Properties of Mature Bedding Shale

We investigated the mechanical properties of mature bedding shale under cyclic loading conditions, with an application to the design of hydraulic fracturing in shale gas wells. Laboratory experiments were conducted on shale samples under two principal loading orientations. Testing results showed that accumulated fatigue damage occurs in a three-stage process. Analysis of fatigue damage at different maximum stress levels shows that fatigue life increases as a power-law function with maximum stress decreasing. And the maximum stress significantly affects the fatigue life. Further, the elastic part of shale rock deformation was recovered in the unloading process, whereas the irreversible deformation remained. The irreversible deformation, growth trend, and accumulation of the total fatigue were directly related to the fatigue damage. This process can be divided into 3 stages: an initial damage stage, a constant velocity damage stage, and an accelerated damage stage, which accounted for about one-third of the fatigue damage. Shale rock is a nonhomogeneous material, and the bedding is well developed. Its fatigue life differs greatly in two principal loading orientations, even under the same loading conditions. All of these drawn conclusions are of great importance for design of hydraulic fracturing in shale gas wells.

Geological Feasibility of Underground Oil Storage in Jintan Salt Mine of China

A number of large underground oil storage spaces will be constructed in deep salt mines in China in the coming years. According to the general geological survey, the first salt cavern oil storage base of China is planned to be built in Jintan salt mine. In this research, the geological feasibility of the salt mine for oil storage is identified in detail as follows. The characteristics of regional structure, strata sediment, and impermeable layer distribution of Jintan salt mine were evaluated and analyzed. The tightness of cap rock was evaluated in reviews of macroscopic geology and microscopic measuring. According to the geological characteristics of Jintan salt mine, the specific targeted formation for building underground oil storage was chosen, and the sealing of nonsalt interlayers was evaluated. Based on the sonar measuring results of the salt caverns, the characteristics of solution mining salt caverns were analyzed. In addition, the preferred way of underground oil storage construction was determined. Finally, the results of closed well observation in solution mining salt caverns were assessed. The research results indicated that Jintan salt mine has the basic geological conditions for building large-scale underground oil storage.

Mathematical model of salt cavern leaching for gas storage in high-insoluble salt formations

A mathematical model is established to predict the salt cavern development during leaching in high-insoluble salt formations. The salt-brine mass transfer rate is introduced, and the effects of the insoluble sediments on the development of the cavern are included. Considering the salt mass conservation in the cavern, the couple equations of the cavern shape, brine concentration and brine velocity are derived. According to the falling and accumulating rules of the insoluble particles, the governing equations of the insoluble sediments are deduced. A computer program using VC++ language is developed to obtain the numerical solution of these equations. To verify the proposed model, the leaching processes of two salt caverns of Jintan underground gas storage are simulated by the program, using the actual geological and technological parameters. The same simulation is performed by the current mainstream leaching software in China. The simulation results of the two programs are compared with the available field data. It shows that the proposed software is more accurate on the shape prediction of the cavern bottom and roof, which demonstrates the reliability and applicability of the model.

Study on Sealing Failure of Wellbore in Bedded Salt Cavern Gas Storage

The wellbore tightness of a salt cavern gas storage must be tested before solution mining. According to the test results, it will be evaluated whether the wellbore can meet the cementing requirements of gas storage. However, there are many complex reasons that may cause wellbore leakage; hence, how to comprehensively analyze the test results and accurately expose the causes, locations, and scale of the leakage pose many challenges. These mainly include the incomplete test method and lack of theoretical analysis model. A nitrogen leak test was carried out for five wellbores that have been completed in Jintan (Jiangsu, China). The results show that two of them had leakage risk. To clarify the leakage causes and leakage types, we carried out an investigation of engineering geological data of the wellbores and further conducted laboratorial tests and theoretical analysis. The studies of drilling design and engineering geology show that the wellbores have good integrity and initially reveal that a mudstone interlayer intersecting the open hole between the casing shoe and the top of the salt cavern is a potential leaking layer. Furthermore, the permeability experiments and CT scans confirm that this mudstone interlayer is a leaking stratum and that the internal cracks develop severely. They are the key reasons leading to wellbore tightness failure. The proposed seepage theoretical model determines that the leakage type is horizontal flow in the mudstone interlayer. Comparing the theoretical results with the field test data, we find that the leakage rate curves of the two are in good agreement, which completely confirms that the mudstone interlayer is the fundamental cause of the wellbores tightness failure. This research not only identifies the leakage causes and leakage types of wellbores, but also enriches the leakage rate analysis method of gas storage and provides a theoretical and experimental analysis method for tightness evaluation of bedded salt cavern storage.

A Damage Constitutive Model of Rock Salt Based on Acoustic Emission Characteristics

The acoustic emission (AE) signal observation is a technique to observe the generation of microcracks in rock salt. By monitoring acoustic emission in whole process of stress - strain curve under uniaxial compression tests, the damage characteristic of rock salt is obtained. Damage to the rock salt appears to be mainly a shear failure under the condition of a lower loading strain rate. After shear failure, a lot of small crushed particles spread on the surface of the failure surface. The AE rate-strain curve is able to reflect the damage development process with better consistency which is evident from the stress-strain curve. A damage constitutive model of rock salt is suggested on the basis of acoustic emission characteristics.

Study and Verification of a Physical Simulation System for Formation Pressure Testing while Drilling

Based on a brief overview on the determination methods of formation pressure and their features, the major principle of formation pressure testing while drilling (FPTWD) and existing physical simulation systems was introduced, and the deficiency of the existing physical simulation systems was also discussed. A laboratory high-precision physical simulation system was therefore developed to simulate the downhole testing environment and testing process of FPTWD. The present experimental system was designed to endure pressures up to 20,000 psi, and the relative control accuracy of pressure is approximately 0.02% FS. Two kinds of man-made specimens with the permeability of 10–110 mD were used to test the pressure response and to verify the present physical simulation system. The debugging results indicated that the variation amplitude under the stable condition is approximately 0.07 psi, 0.08 psi, 0.11 psi, and 0.11 kN for the annular pressure, pore pressure, confining pressure, and thrust force, respectively. Thus, the high control accuracy is approximately ±1.0 psi under the stable conditions. The experimental results indicated that the pressure drawdown declines rapidly in the stage of withdrawing formation fluids and then recovers slowly. The pressure drop amplitude also decreases with permeability, while the pressure buildup amplitude increases with permeability. The tendency of pressure change is nearly the same for both the present and the previous systems, and the pressure curve of the present system is much smoother and better than that of the previous system. The relative error of explaining formation pressure is less than 1% and 4% for the present and the previous systems, respectively. In addition, this physical simulation system has important applications to verify the interpretation model, to analyze pressure response recorded by FPTWD tools, to test the capability and design of FPTWD tools, and to calibrate the formation pressure, formation parameters, and instrument factors.