| At present, main shallow geothermal energy utilization ways in China are ground water source heat pump and ground source buried pipe heat pump. Among them, the problem of heat exchange efficiency stability and surrounding geological environment are directly affected by underground energy acquisition system designed rationality and the project quality. This paper aimed the prominent problems during the process of shallow geothermal development, analysis the geological environment problems of heat transfixion caused by engineering problems, decline of groundwater level decline, ground subsidence, water quality deterioration by amount of investigation and data analysis, combined the engineering project and experiments, put forward the concrete solution measures and technical proposal from the point of well pipe, construction technology and material selection. Especially aim to the traditional model of "one well pump, multi wells reinjection" which has the disadvantages of large occupation area and reinjection difficult, put forward the "single well pumping and reinjection system" and "PVC-U plastic pipe well completion technology" design ideas, and carried on the laboratory and production test in combination with production project. Among them, "single well pumping and reinjection system" application in Xinyuan residential quarters in Xingyang Zhengzhou solved the problem of reinjection difficulties in fine particles and unconsolidated strata. During the four years running, the "single well pumping and reinjection system" in this project has been proved system stability, heating and refrigeration effect reliable, achieved remarkable benefit of economic, social and environmental.Shallow geothermal energy is a kind of renewable green energy, the energy equals to 9.4×1020KJ of thermal energy or 3.2×104 billion tons of standard coal, has the characters of shallow buried, wide distribution and low development cost. In the condition of present economic and technical, shallow geothermal energy can extract and exchange by heat pump, it can realized heating and cooling for buildings instead of coal, oil and natural gas.In China, the shallow geothermal energy development started late, but it has grow fast, in recent 10 years, a large area started shallow geothermal energy development and utilization in China gradually, such as north China plain, northeast, northwest and other regions. At the same time, a set of policies and preferential measures which encourage development shallow geothermal energy has been putforward. Look on present situation, lots of area has the problems of exploration evaluation suitability lag, management confusion, construction without plan, employees’ technical uneven. These problems caused by engineering lead to a serious problems of geological environment and the heat exchange efficiency stability, such as groundwater reinjection difficult, the system of energy exchange heat transfixion, these problems caused waste of groundwater, groundwater level declined, subsidence of land surface, water pollution and the destruction of the ecological environment. Aim to major problems, amount of survey and experiment has been conducted, the thesis research start from the point of geological environment and engineering problems, and give reasonable measures, it has the positive significance of shallow geothermal energy development and utilization in an science, reasonable, sustainable ways, national energy structure optimization and energy saving and emission reduction.Shallow geothermal energy development relates to hydrogeology, corrosion science, drilling, HVAC and mechanical and electrical installation engineering. Among them, the underground energy acquisition system is the key for whole heating and refrigeration system. Reasonable utilization directly affects the ecological environment and geological environment. According to the test for field soil temperature test, ground thermal balance analysis and ground source heat pump operation monitoring by TRT-9 vehicle type rock soil thermal testing instrument, in heating and cooling situation, the formatin can reach a thermal equilibrium in similar running time; in north or south area, the heating and cooling situation has different running time, the formation thermal balance will change over time, eventually led to the formation of hot and cold damage the balance. These can effect the soil microbial population, vegetation growth, underground N2O and CH4 release, the formation porosity changes, water quality changes. When recharge rate is lower, it may lead environment problems, such as waste of water, water level decline, land subsidence and collapse geological.Both the groundwater source heat pump and ground buried pipe heat pump system, the system corrosion and scaling may ocoured in the development and utilization process. Especially the ground source heat pump system is an open system, the corrosion and scaling is more serious in the whole network system and equipment. In many areas, because of the water source well pipe choosen, the corrosion and scaling is serious in groundwater source heat pump system, it lead to water levele drops year by year, recharge difficult, deterioration of water quality, ground pipeline blockage. The problems of corrosion and scaling effect the use time and running efficifent and geology environment of heat exchange system.At present, the main pipe is cement pipe, cast iron pipe and ordinary steel pipe in groundwater source heat pump, the most common used is ordinary steel pipe and bridge galvanized steel filter pipe. According to the experiment of corrosion and scaling well pipe material, all the metal pipe has the corrosion and scaling phenomenon, the fastest corrosion speed is galvanized bridge filter pipe, at the 7th day, it start corrosion, in the 150th day,all the water gap almost completely blocked. PVC-U plastic pipe has no corrosion fouling and dissolution in any time, the PVC-U pipe is the best material to well completion.Research and practical detection shows:the choose of well pipe and well completion technology determines the service life and the operation quality for pumping wells and recharge wells in ground source heat pump system. Among them, the well drilling technology, drilling method and drilling speed, drilling fluid selection, selection of well washing methods and quality of well has direct relationship with the effect of water well pumping and recharge and effect. At present, the service life of well pipe is 2-8 years; the mian reason is corrosion lead to pump and recharge amount decreasing year by year, pipe corrosion rupture lead to muddy water, sanding and borehole collapse.During the design and construction for ground-coupled heat pump system, analysis and research by thermal resistance, the Reynolds number, Fourier formula, it shows the ground-coupled pipe’s heat exchange efficiency has close relationship with pipe material, diameter, wall thickness and borehole diameter; different backfill material has different thermal conductivites, so the heat exchange efficiency is different. Sand and clay has good thermal conductivity, is the suitable backfill material, these materials has the characteristic of convenient to obtain, take the mud+cuttings as backfill material can reduce the cost of waste residue and wast disposal. In the mixture of mud+cuttings, take sand or rock fragments as aggregate, its pratical size and unigormity will influence the properties of backfill material, backfill aggregate should choose coarse sand which the size is 0.5～2 mm. when mud+cuttings as backfill material, as far as possible to choose the uniform sand (cuttings) and 8% of bentonite in order to achieve the best heat exchange effect.Many problems and factors may affect the development of shallow geothermal energy, espically in the groundwater source heat pump, the quanty of energy collect system can effect the whole system’ running and heat exchange, it is the key technology of system. The research shows:the traditional positive circulating mud drilling technology has the problems of low efficiency, mud pollution formation serious, well washing difficult, poor quality of well completion and effect the well pump and recharge quantity; the unbalanced drilling technology, such as suction pump reverse circulation drilling and air lift reverse circulation drilling, these methods has the characteris of clean bottom, non-repeat fragenmentation, high drilling efficiency, no pollution in fragement, easy to wash well, reflect the water pump and recharge quantity for single well. Therefore, in the design and construction for well drilling, in loose formation, recommended the suction pump reverse circulation drilling or air lift reverse circulation drilling, in rock formation, recommended DTH hammer drilling technology.During the process of well completion for PVC-U pipe, the pipe sinking difficult and pipe burst effect the appilication of PVC-U pipe. Take PVC-U pipe as pumping and recharge pipe, compared with metal pipe, it has the advantage of non-corrosion and non-scaling, fast well completion speed, long service life ect. Aim to the characteristics of physical and mechanical performance for PVC-U plastic pipe, use the "the borehole pressure balance" method, solved the problem of pipe sinking difficult and pipe burst. The maxium well completion depth is reach to 437m, fill the gaps at home and abroad. According to analysis, the main reason for plastic pipe burst is the difference between well pipe inside and outside and dynamic load(impact load). The main reason for pipe sinking difficult is the large mud density and filter gap pluged. Therefore, in the actual project must take necessary technical measures to reduce the pressure difference between inside and outside of the well pipe and the dynamic load (impact load).At present, "recharging in the same well" technology in domestic and international has been used in the area which has good hydrogeology condition, and get better effect. However, limitations and different degrees of heat penetrating questions have been existed. On the basic of this thesis, the author puts forward "single well pumping and reinjection system", it use a single well (parallel pumping and recharge tube wells) realized the energy exchange and extraction by the underground water slowly rise up in the way of vertical distance. Three design schemes have been determined according to the different hydrogeological conditions, central pumped-upper and lower recharge, lower pumped-upper recharge, upper pumped-lower recharge. Conducted two well experiments with the project in Xinyuan residential quarters in Xingyang in Zhengzhou city. In the final, the design and construction scheme of No.2 well is successful, the recharge amount rate reached 100%, and realized the building heating and cooling for 16500m2. After four years operation, the system is stable and reliable, water pumping and recharging is normal (70m3/h), Significant economic benefits, social benefits and environmental benefits achieved.In conclusion, based on the actual project test and a large number of survey data, use the theory system of environmental geology, groundwater science and corrosion science analyzed the geological environment problem caused by shallow geothermal energy development, research the key technology in groundwater source heat pump by drilling and corrosion science theory system, that is well quality and timely recharge of groundwater, and puts forward the concrete measures. Through experiment and actual engineering, put forward "single well pumping and recharging technical system" and well completion technology by independent innovation, and achieved significant benefits in practical application. The research result plays a promoting role in shallow geothermal energy development and utilization in a reasonable and scientific way in future, energy-saving emission reduction and sustainable development. |
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