The Operating Mechanism And Optimization Research On Carbon Dioxide Plume Geothermal System In Quantou Formation Of Songliao Basin

The problems of environment and energy are the two topics to the human society.In recent years, the excess emission of carbon dioxide is one of the main factorscausing global warming and serious environmental problems. CO2geologicalsequestration (CGS) is an effective technology recognized by the internationalcommunity to reduce the greenhouse gas emission and cope with the climate change.The study on large-scale engineering application indicates that single CGS is tooexpensive if there are no other ancillary benefits. Therefore, the research on how touse natural and stored CO2reservoirs to extract the deep geothermal resource isnecessary. In this paper, a concept of CO2plume geothermal system (CPGS) whichinvolves pumping CO2into deep, naturally porous and permeable geologicalformation where CO2displaces native formation fluid is presented. It can be apractical way to improve the efficiency and economics of CGS, and to realize thereasonable development of clean energy and CO2recycling.This paper is supported by Ministry of science and technology project863(No.2012AA052801) and the Natural Science Foundation of China (NSFC)“TheFundamental Theoretical Research on Carbon dioxide Plume Geothermal (CPG)System in Sedimentary Basin”(No.41272254). It developed the study of themechanism and optimization of CPGS in the Songliao basin, China. The goal ofproject is to (1) explore the flow and heat transmission mechanism of CO2in wellboreand deep reservoir;(2) identify the influence of water-rock-gas interaction on flowfield;(3) determine the condition of stable operation and influence factors forQuantou formation, and optimize the scheme;(4) evaluate the performance of CO2orwater as the heat medium under complex geological conditions.In the theory of flow and heat transmission of CO2, according to the conservationof mass and energy, the wellbore model that coupled flow and heat transmission wasdeveloped on the basis of drift flux model (DFM). Based on the Darcy’s law areservoir model was set up. We coupled the wellbore with the reservoir flow anddeveloped a complete system of underground heat extraction cycle model. The model is helpful to precisely describe the flow and heat transmission process of CO2indifferent stages and phases in the sandstone reservoir. The results show that the energytransmission and transformation of CO2in wellbore are completed in the form ofconvection and diffusion. Pressure and temperature increase during the injectionprocess. It is note-worthy that when CO2breakthrough in production well, thepressure, temperature and density profile will plunge sharp.With the comprehensive function of gravity, buoyancy and the circulatedpressure, CO2distributes as a plume shape during migrating in the reservoir. Threedistinct zones appear,(1) the central zone where fluid is a single supercritical CO2phase;(2) a two-phase water-CO2mixture in surrounding intermediate zone; and (3) a singleaqueous phase with dissolved CO2in peripheral zone. Three zones in the reservoir transformand evolve with time. The distribution of temperature also shows a significant plumeprofile. After being fully heated, CO2flows into production wellhole. In the initialperiod, what we get from the production well is water. The pressure of productionwellhole decreases rapidly after CO2breakthrough. Due to CO2expansion, the lowerthe production wellhead pressure is, the more drastically temperature drops frombottom to wellhead. The velocity and kinetic energy of CO2increase in the process ofextracting from the production wellhole to the surface, whereas the density andtemperature decrease. Because of CO2breakthrough, water mass flow of productionwell and net heat extraction rate decline dramatically. When the mass flow of twophases gets nearly the same, the net heat extraction rate reachess the lowest level. Andthen, as the displacement going on, the effect of two phases gets weak, the net heatextraction gets to stabilize at10MW.Based on the target of reservoir of Quantou formation in the Bongliao basin, theexperiment and the numerical simulation were employed to reveal the change ofminerals in the heat reservoir due to the water-rock-gas interaction, supplementedXRD and SEM observation method. The significance of effect of differenttemperature, pressure and salinity to water-rock-gas interaction was analyzed throughthe experiment and numerical calculation. Results show that abundances of quartz,illite and kaolinite increase to a certain extent. The amounts of feldspar decreases, andcalcite disappears completely in the reaction. The dissolved minerals are mainly feldspar and calcite, the precipitation minerals are mainly quartz, clay minerals. Theexperimental process is more sensitive to temperature change, and less affected bypressure. The numerical simulation of the target reservoir results show that calcite isthe mainly mineral for fixing carbon. Feldspar dissolves, witch is agreement with theexperimental results. Porosity and permeability of the whole region decreasegradually from the injection well to the outside. The total production flow rate ischanged weakly by the influence of change of flow field; the average heat extractionrate will decrease by4.1%in40years.Identify the optimization goal, based on3-D “five-spot” coupledreservoir-wellbore model, multiple sets of analysis model are established for eachfactors such as production mass flow, production pressure, injection pressure,injection temperature, parameters of wellbore properties of reservoir and well pattern.We analyzed the influence of each factor on production capability and changes oftemperature and pressure, identified the range of stable operation for each factor, andthen made optimization. It shows that: production mass flow rate, pressure differenceand injection temperature should be within a certain range in order to guarantee thestable and efficient operation. The circulation pressure difference could even benegative at a given mining range; innate buoyancy of CO2also can enable theautomatic closed circle by a thermosiphon. Appropriate diameter of wellbore (0.2m)and relative small roughness is favorable for operation. Relative low-permeability isconductive to be sufficient to heat the transmission fluid and the stable operation forCPGS. High temperature of reservoir is helpful to improve net heat extraction rate.Different wellbore distribution pattern has an effect on distribution of two-phase fluidand CO2breakthrough time. Comparing with the five-spot wellbore pattern, theproduction fluids of two-spot and three-spot contain amount of water during therunning period. This phenomenon could make the heat extraction rate higher, butmake the drying process for later more difficult.As a novel heat transmission fluid, the thermophysical property of CO2is quitedifferent from that of water. It has many advantages, such as larger mobility andbuoyancy resulted from the lower density and viscosity. This could reduce theconsumption of driving pressure of the circulation, and save the energy consumption of external equipment. The cycle even could be achieved by thermosiphon under thenegative circulating pressure difference. However, there are still some disadvantagesfor CO2as heat transmission fluid, such as small heat capacity, whitch leads tocarrying less heat at the same mass flow rate. At the same time, if temperature andpressure change, it will cause a more complex thermodynamic processe during heattransmission because of the lager expansion and compression coefficient for CO2.Lager compressibility makes it possible to get high temperature at the bottom of theinjection well, while lager expansion coefficient makes the temperature drops rapidlyduring the extraction process. Here, a classically idealized “five-spot” model coupledwellbore is set up according to the geological and geothermal conditions andparameters of the central depression of Quantou formation of Songliao basin. Ourpurpose is to compare the heat extraction efficiency of CO2with that of water, andevaluate the advantages and disadvantages using CO2. Simulation results show that:the net heat extraction rate of water is higher than that of CO2at the same flow rate;but the pressure drop of underground cycle for water is more than that for CO2and itwill become greater with the increase of flow rate; and CO2can achieve the automaticcircle by a thermosiphon and the negative pressure difference can be greater with thetiny flow rate; CO2is more suitable than water to the geothermal system of a tinycirculation pressure difference and low injection temperature; and CO2is moresuitable than water as the working medium of heat transfer in the low temperatureheat and low permeability reservoir.