Simulation And Field Experience Of Steam Mining System For Permafrost Gas Hydrate

Gas hydrate is a new substitute for energy with great potential，green，large reserves,widely distributed, shallow burial and large energy density，which plays an important role inalleviating the energy crisis facing humanity. The only way, must be passed from endless R&D projects to commercial exploitation of gas hydrate, is pre-production research. Permafrostgas hydrate deposit in lower temperature and pressure conditions, what are easier to carry outtrial production test research in mining technology and construction operations.The general idea of gas hydrate mining is first breaking down the temperature andpressure equilibrium conditions to promote hydrate decomposition, and then pumping thenatural gas to the surface The mining methods of gas hydrate include heating mining,depressurization mining, chemical reagents mining, C02replacement method and so on. Eachmining method has its own superiority and limitations. Based on the research ondecomposition mechanism of gas hydrate, theory and practice of gas hydrate mining, andoccurrence of permafrost gas hydrate, steam mining method is proposed in this paper.The steam mining method combines the advantages of depressurization mining andheating mining. First, groundwater is pumped to the surface through the center channel of thedouble-wall drill pipe by the pump at the bottom of drilling hole, which is associated withfluid level descending. At the same time, pluse steam，which was produced by the heatingdevice and injected into the extraction well through the outward channel of the double-walldrill pipe, heats the hydrate stratum for further decomposition.The aim of steam heat calculation is providing the basis for the design of surface heatingdevice and the choice of field test parameters. According to the mining plan, the temperatureand the pressure of the steam are set at180℃and1MPa, while the mined thickness, minedradius and mined temperature are set at20m,0.5m and10℃. Pluse steam, produced by theheating device on the surface, has heat loss in pipeline in the process of mining. The heattransfer of steam flowing in the pipeline could be used as stable heat transfer. The heat loss inpipeline was21.35w/m. The process of steam heating gas hydrate stratum is dynamic, andthe equations for calculating are usually non-linear, which can’t be calculated in the normalway. Numerical simulation was used by means of FLUENT6.3in this paper. Select the bestpower is20KW through the results of the numerical simulation. The stratum needs to beheated for38hours to meet the mining demand. According to the mining plan, the designed mining system includes surface heatingdevice, gas collecting and firing device, and pressure releasing device. Steam generator is thecore components of heating device and the power is40KW according to the formula andsimulation results. Pluse steam is used in heating device, which is propitious to the hydratedecomposition and heat spread in stratum to expand the mining areas compared withcontinuous steam. The gas collecting and firing device plays a role in transporting the naturalgas to the surface, gas separation, gas filtration, traffic monitoring, flow calculation, gascollection and ignition test.Indoor test is carried out after the work and selection of the mining system according tothe design. The best opening and closing pressure are determined at1.5MPa and0.5MPathrough the comparative analysis of experimental data of the four groups of indoor test. Thebigger steam psid, higher temperature, longer steam injecting time, higher heat and less heatloss are better for the further decomposition of the hydrate. At the same time, numericalcalculation model with high precision of pipeline heat loss is build through the comparativeanalysis of heat transfer numerical simulation and indoor test data.The mining system was used in No.1test production wells of permafrost hydrate inQinghai. Ten hydrate layer was selected to carry out test mining in the mining well with floraltube into the well. First, depressurization mining was carried out for75hours and the gasproduction was72.6m~3. Next, joint exploration mining of depressurization method and solarheating or electromagnetic heating was carried out for10hours and the gas production was10.0m~3. The mining method was used when the hydrate dissociation rate is slower. Theopening and closing pressure of the steam were set at1.0MPa and0.5MPa. The steammining method lasted for2hours with12times steam injecting and the gas production was3.25m~3. Exploitation of field tests proved that the steam mining method could promotefurther decomposition of hydrate.The steam mining system has some inadequacies in the process of indoor debugging andfield tests. The energy efficiency of single well throughput method was very low for the heattransfer between heat-carrying agent and hydrate layer was slow. The application of dockingof horizontal wells and formation fracturing technology could expand the hydrate range andincrease gas production. New mining technologies, such as CO2displacement mining,required further study.