Theoretical And Experimental Research On Oil Shale In-situ Underground Frozen Wall Temperature Field

For China, crude oil production is far from meeting the domestic demand at present. In2014,China imports3.08tons crude oil. The crude oil imports in2014is the highest over the year, andis9.4%growth than last year, so also is highest increase rate over the year. In fact, from thebeginning of1993, China has become a net importer of oil, and the dependence on externalresources is more and more high. In order to alleviate this situation, our country pays moreattention to the exploration and development of unconventional oil and gas resources andutilization. Reserves of oil shale resources is abundant in China, as conventional energy instead ofthe energy, potential is very great.In the background of government environmental requirements more and more demanding ofthe present, that underground in-situ mining method for the exploration and development of oilshale was adopted has become the inevitable choice. And the underground frozen wall technologyis a key technology in process of oil shale in-situ underground mining. It mainly has twofunctions: One is that it can prevent the groundwater outside of the mining area inflows to miningarea, this will ensure that oil shale in the mining area can be heated and produce oil and gas,second is that it can prevent the pyrolyzed products of oil shale inside mining area leaked to theoutside of mining area, this not only can reduce the loss of oil and gas, but also can prevent theoutside mining area contaminated.The oil shale mining area are generally very large, and considering there is a buffer areabetween the heating and freezing areas, so the diameter of the frozen wall should be very large.Coupled with the buried depth of oil shale is deep, so it is a very huge project that forming aunderground frozen wall which meeting the conditions of underground in-situ extraction. And thefreezing time is very long, the energy consumption is very huge, the cost of engineering is veryhigh. So the underground frozen wall project should must be solving the problem that how tooptimize each of parameter effectting the formation of frozen wall in the process of freezing, andhow to increase the speed of underground freezing wall get ring reasonable. That can short theconstruction period, and cut the cost. We try to seek a solution to solve the problem through themethod of experiment and theoretical study.Firstly, this paper briefly introduced the soil’s thermal physical properties which caninfluence the distribution of layer’s temperature field. On this basis, heat conduction differentialequation of temperature field of underground frozen wall was derived by using the law ofconservation of energy. There are two kinds of situations when buried tube heat exchanger andthe formation occurs heat exchange. One situation is that the load applied on the inner wall of buried tube heat exchanger is constant temperature, another case is that the load applied on theinner wall of buried tube heat exchanger is constant heat flux. This paper studies themathematical description and numerical solution of the wall temperature field of undergroundfrozen under the two conditions. The constant temperature heat conduction problem can beregarded as the steady state temperature field, and the condition of constant heat flux is regardedas a non steady state heat conduction problem of temperature field. Finally the basic theory aboutthis mathematical description was applied on the practical engineering of underground frozenwall. After discussed the ground freezing process briefly, the paper calculated heat capacity ofburied pipe heat exchanger and the freezing time respectively, and put forward the averagefreezing temperature of frozen wall, and finally got the mathematical model and the single valuecondition of underground frozen wall temperature field.Then this paper respectively calculated the time of frozen wall get circle and temperaturefield distribution when the cold liquid flow, freezing hole spacing, freezing hole diameter and thefreezing method take different values. And the optimal combination of each influence factor wascalculated. Then reached the following conclusions: increasing the cooling liquid flow can reducethe time of frozen wall get circle under the condition of keeping the freezing hole spacing,freezing hole diameter and freezing method constant, but when the flow rate increased to acertain extent, the effect which continue to increasing the flow rate on the time of frozen wall getcircle is not obvious; increasing the distance of the frozen holes can increase the time of frozenwall get circle under the condition of keeping the cooling liquid flow rate, freezing hole diameterand freezing method constant, and the greater the spacing, the time of increasing will be greater;increasing the diameter of the frozen holes can reduce the time of frozen wall get circle under thecondition of keeping the cooling liquid flow rate, freezing hole spacing and freezing methodconstant, and the function of time and freezing hole diameter is nearly linear; In keeping coolliquid flow, the freezing hole spacing and diameter unchanged, the method of partial freezing canreduce the time of frozen wall get circle; And the optimal combination of experiment: cold liquidflow rate is20m3/h, the freezing hole spacing is1m, the freezing hole diameter is90mm.Finally, the experimental platform of underground freezing wall temperature field wasestablished according to the requirement of the experiment, and the corresponding experimentwas carried in the experimental platform, and the results are consistent with the results obtainedby simulation calculation. However, the time of freezing wall get circle which got from theexperiment is shorter than that got from simulation calculation.To sum up, through theoretical analysis, simulation calculation and experimental research,this paper draws the following conclusion: the time of freezing wall get circle decreases with increasing cold fluid flow if keeping other influence factors under the condition of invariable, butthe time of freezing wall get circle decrease extent will be more and more unobvious along withthe increase of flow rate; in the influence of other factors remain unchanged, the time of freezingwall get circle increases with the increase of freezing hole spacing, and with the increase offreezing hole spacing, the time of freezing wall get circle increases will be more and moreobvious; the time of freezing wall get circle increases with the increase of freezing hole diameterif keeping other influence factors under the condition of invariable, the change of relationship isalmost linear; in addition, frozen partial freezing can reduce the time of freezing wall get circle,and more energy-saving, more environmental protection; and the most optimized combination ofexperiment: containing cold liquid flow rate is20m3/h, the freezing hole spacing is1m, thefreezing hole aperture90mm.