Study About The Impacts Of Oil Shale In-situ Mining On Groundwater Environment

As a rich resource with concentrated distribution, oil shale is taken much more attentionat domestic and international. The total amount of Chinese oil shale resources rank fourth inthe world, mainly distributes at the SongLiao, Ordos, Junggar Basin. The mining methods ofoil shale contain ectopic mining and in-situ mining. Taking into account the limitations ofectopic mining and great harm to the environment, in-situ mining of oil shale is becoming ahot topic.In-situ mining is directly heating for underground oil shale layers for obtaining oil andgas, which can develop deep, high thickness of shale deposits and has high recoveryefficiency, good product quality, less land and environmental protection, etc. However, in-situmining is done entirely in underground, which will not only change the structure, physical andchemical, and mechanical properties of underground rock strata, also will influence thegroundwater environment, causing the change of the flow field and water quality ofgroundwater. Taking Shell ICP electric heating technology of in-situ mining as an example,the article analyzed the effects of oil shale in-situ mining on groundwater environment. Thesisresearch results are as follows:1. Based on mass of relative data collected on oil-shale in-situ mining demonstration areain Fuyu county of Jilin province, this paper analyzes geological and hydrogeologicalconditions in the demonstration area for determining the oil shale layer and its overlyingaquifer, which include loose rock pore water and clastic rock pore fissure water. Pumpingtests are done respectively for determining the aquifer hydrogeological parameters.Hydrogeological parameters of the aquifer are calculated using the formula method, lineargraphical method, the entire curve fitting. For loose rock pore water, the pumping flow rate Qis243.29m3/d, permeability coefficient K is30.09m/d, hydraulic conductivity T is198.58m2/d, specific yield μ is0.18, influence radius is94.61m. For clastic rock pore fissure water,the pumping flow rate Q is36.94m3/d, permeability coefficient K is0.07m/d, hydraulicconductivity T is0.35m2/d, storage coefficient μ*is0.00009. In the range of around situ oil shale extraction demonstration area169km2we conducts two field surveys, The watersamples were got in23pumping wells and17indicators of them were measured. Therefore,the environmental background of groundwater were determined. Groundwater quality in thearea were evaluated by the groundwater quality comprehensive evaluation method. The resultshows the groundwater quality which is near the surface water is poor, and it is almost Ⅴ.Excessive ion mainly are the ammonia nitrogen, manganese and iron.2. According to the relationship between gravity and bulk density, the porosity of the oilshale before and after its being heated can be determined through the experiment, the resultsshowed: the oil shale is heated to the higher the temperature, the porosity is larger. At roomtemperature, the oil shale porosity is2.552%, heated to300℃, oil shale porosity is10.120%,and heated to500℃, oil shale porosity is19.048%.3. Free immersion method is used to determine rock water absorption, the results showed:the oil shale is heated to the higher the temperature, the bibulous rate is greater. At roomtemperature, oil shale water absorption is0.06%, heated to300℃, oil shale water absorptionis0.34%, and heated to500℃, oil shale water absorption is0.77%. Thus, it can be seen thatthe change of bibulous rate and rock porosity is synchronized. From room temperature to300℃, the porosity and water absorption of oil shale is less than300℃~500℃.4. By soaking test we determined the content of inorganic and organic matter and metalelements in different shale soaking water samples, then analyzed the impact of In-situ miningon the groundwater quality: the pH of water rises with the time extension, the hydrocarbons(C10~C20) and metal elements (Pb, Cd) will increase accordingly. Oil shale powdery heatedto300℃release most hydrocarbon, especially C16, a maximum content is25.08g/L,followed by C15. Affected by the heating temperature of the oil shale, the release process ofCd, Cu and Pb are different. Oil shale heated to low temperature, immersed water samplesreleased more Cd, Cu, and oil shale heated to low temperature, the immersed water samplesreleased more Pb.5. Using fluid dynamics software Fluent to simulated the frozen wall temperature field ofgroundwater, the influence of the flow field before in-situ mining. Temperature fieldsimulation mainly includes six kinds of circumstances: temperature in the process of coolantcirculation reached a steady state respectively set to-3℃、-6℃、-9℃, convective heat transfercoefficient is200、600W/（m2K）. The simulation results show: The factors affecting groundwater temperature field distribution is heat conduction effect of frozen wall. Thethickness of frozen wall is determined by the temperature of the frozen well under steady state,the lower the temperature, the thicker the formation of frozen wall. The relationship of thefreeze wells and formed the wall thickness is the linear, the relationship is h=8.6k-2241.7.6. Distribution of groundwater flow velocity around the frozen wall is uneven. Beforefreezing wall, from the3840.02m, the groundwater flow rate decreased slowly, until4290.67m, the flow rate decreased rapidly, reaching the freezing wall, the flow rate is reducedto zero. After flowing through the frozen wall, velocity goes through lower, rise, lowersuccessively (small scale), and then gradually increased until revert to the actual average flowvelocity of groundwater. From4560.00m to4754.01m, flow rate first increases and thendecreases; from4754.01m, the flow rate rising, until8430.17m the flow velocity just return tothe groundwater actual average speeds. From which can see that in the horizontal direction theeffect scope of freeze wall is3840.02m~8430.17m on groundwater velocity. In the verticaldirection, the vertical cross-sectional in development center line, at105m from freeze wells,the maximum flow rate is2.86×10-6m/s, which is2.86times as much as groundwater actualmean velocity.7. The uniaxial compressive strength and tensile strength of oil shale and overlying rockand the internal friction angle and cohesive strength of rock were measured by point load testbefore and after heating. The uniaxial compressive, tensile strength, internal friction angle andcohesion of mudstone are14.32MPa,0.57MPa,33°and140kPa. Those of pelitic siltstoneare23.0MPa,0.92MPa,42°and113kPa. The uniaxial compressive, tensile strength, internalfriction angle and cohesion of oil shale are14.85MPa,0.59MPa,29°and51kPa. Afterheating to500℃, The uniaxial compressive, tensile strength, internal friction angle andcohesion are3.60MPa,0.14MPa,26°and40kPa. These mechanical parameters shows themechanical properties of the oil shale is significantly worse after heated.8. We adopted SIGMA/W module of the Geostudio geotechnical software to simulatestress, strain and displacement of any point in the demonstration area, finally determined thatthe maximum subsidence displacement in the mining center is0.58m. Destroy parts of therock mass distribute in the overlying shale vicinity developed oil shale, the destructionprocess is the first tension, resulting in a large number of cracks, and then slide along thesection where the shear stress is maximum. In shaort, In-situ mining oil shale would inevitably influence surrounding groundwatertemperature and flow velocity. More importantly, in situ heating results in change in physicaland chemical, mechanical and water physical properties of oil shale, causing a change ingroundwater storage environment, resulting in change in groundwater flow and water quality.