多分支井地热系统CO_(2)与水的取热效果对比

Comparison of heat extraction performance between CO_(2) and water in a multilateral-well geothermal system

多分支井地热系统是开采地热资源的新方法,具有单井注采循环、井眼与储层接触面积大、井眼与裂缝沟通能力强等优势,其中取热工质的采热能力与稳定性是决定地热系统取热效果的关键因素。为了深入分析CO_(2)与水两种常见工质的取热效果,建立了多分支井地热系统井筒—储层耦合的流动传热模型,数值模拟了中、高温热储条件下,CO_(2)与水在多分支井地热系统储层和井筒中的流动传热规律。研究结果表明:①CO_(2)在中心保温管上返过程中会降压膨胀做功,产生显著温降效应,温度降低超过60℃;②CO_(2)在中心保温管和环空内密度差异大,产生的浮力作用可实现自主循环取热;③在150~200℃的中温储层中,当CO_(2)的注入质量流量为水的2.6倍时,CO_(2)和水从储层中采出的热量相当,但CO_(2)的浮力作用优势明显,具有更好的取热效果;④在超过200℃的高温储层中,水的黏度降低、热容升高、密度差异增大,其取热功率显著高于CO_(2),并且水的循环压力损耗明显降低,减弱了CO_(2)的浮力作用优势,因此水具有更好的取热效果。结论认为,流动传热模型和数值模拟证实不同温度地热地区应选用不同工质类型取热,为下一步地热资源的有效利用提供了理论支撑。

Multilateral-well geothermal system is a novel geothermal exploitation method,and it has the advantages of single-well injection and production cycle,large contact area between wellbore and reservoir and strong connectivity between wellbore and fracture.The heat extraction capacity and stability of heat extraction medium are the key factors influencing the heat extraction performance of the geothermal system.In order to deeply analyze the heat extraction performance of two common working fluid,i.e.,CO_(2)and water,this paper establishes a wellbore-reservoir coupling fluid flow and heat transfer model for the multilateral-well geothermal system.In addition,the flow and heat transfer of CO_(2)and water in the reservoir and wellbore of multilateral-well geothermal system under the condition of medium-and high-temperature thermal reservoir are numerically simulated.And the following research results are obtained.First,when CO_(2)returns upward through the central insulation pipe,it expands to do work with the drop of its pressure,so as to generate a remarkable temperature drop effect with a temperature drop over 60℃.Second,the buoyance effect induced by the great difference of CO_(2)density in the central insulation pipe and the annulus can realize independent heat extraction through circulation.Third,as for medium-temperature(150℃-200℃)reservoir,the heat extracted by CO_(2)from the reservoir is equal to that by water when the injection mass flow rate of CO_(2)is 2.6 times that of water,but CO_(2)has an obvious advantage of buoyancy effect,so its heat extraction performance is better.Fourth,in a high-temperature(above 200℃)reservoir,the viscosity of water reduces and its heat capacity and density difference increase,so its heat extraction rate is much higher than that of CO_(2).What's more,the circulating pressure loss of water decreases greatly,which weakens the buoyancy advantage of CO_(2).Therefore,water has abetter heat extraction performance in high-temperature reservoirs.In conclusion,the flow and heat transfer model and the numerical model verify that different types of heat extraction medium shall be selected in different geothermal areas with different temperatures,which provides theoretical support for the effective utilization of geothermal resources in the next step.