利用CO2循环开采高温废弃气藏地热机制及潜力评价研究

Depleted high temperature gas reservoirs have great potentials for geothermal exploitation. The geothermal resource in a typical medium-size gas field can be equivalent to over 10 million tons of standard coal in terms of thermal energy. The existing well network and surface facilities in depleted gas field can be fully utilized for geothermal development, which can reduce capital investment and production cost. In general, water, as a conventional heat transmission fluid, has a good capability for heat exchange, but its low injectivity and possible damage to reservoir rocks may restrict its application in deep gas reservoirs with low permeability. Supercritical CO2 has been considered as an alternative to water since it has superior mobility and high heat capacity. This project intends to study the mechanisms and feasibility for enhanced geothermal exploitation from depleted high temperature gas reservoirs ( > 100℃) via CO2 injection, and to assess the recoverable geothermal potentials. The flow behavior and heat mining efficiency of different heat transmission fluids (i.e. water and CO2) will be investigated experimentally under high temperature and pressure conditions of typical hot gas reservoirs, and the interactions of water-CO2-formation fluids and rocks will be studied to verify their influence on flow and heat transmission and mining efficiency. An integrated numerical simulation model will be established, in coupling with wellbore flow-heat transmission, reservoir flow-heat mining and fluid-water-rock interaction models, to provide a comprehensive evaluation of the heat mining potential and capacity from the depleted high temperature gas reservoirs. The research project can provide guidelines for the selection of heat transmission fluids and the optimization of injection-production schemes in order to enhance the geothermal exploitation and development.

高温废弃天然气藏具有很大的地热开发潜力，一典型中型气田的地热资源量可达上千万吨标准煤。开采地热期间可利用气田原有井网和地面设施，降低资金投入和开发成本。水作为常规携热介质，具有较好的热交换能力，但其应用的普遍性受到可注性和地层伤害的限制。超临界CO2因其独特热物性和很高的流动性，可作为水的替代选择。本项目拟针对高温废弃气藏储层（>100℃），进行CO2作为携热介质开采高温废弃气藏地热机制及潜力评价研究。主要研究内容包括，通过水与超临界CO2在地层内的渗流和热交换实验，研究携热介质在地层中的渗流特征和采热规律，分析CO2和水与地层流体和岩石的相互作用及其对采热效率的影响；建立基于多现象耦合的携热介质渗流-采热模型及井筒流动-热交换模型，形成高温废弃气藏地热开采潜力综合评价体系，研究不同注采方案下的采热效率和最终采热率，为携热介质优选、井网和注采参数优化及经济可行性评价提供基础数据。

针对高温废弃气藏，基于不同携热介质热交换实验结果，建立了地热开发模拟模型，开展注水和注CO2采热机制和潜力研究，分析了地质、注采工艺参数及CO2-岩石-流体作用对采热速率的影响，进行了方案优化和采热-发电经济性分析，揭示了气藏地热资源开发的可行性。根据考虑携热介质物性随温度压力变化的高温废弃气藏地热开发模拟模型，分析了储层构造和物性及注采方案对水和CO2采热能力的影响。模拟结果表明，对于1000×500×50m尺度规模，渗透率10mD（10E-3μm2），储层温度150ºC的高温气藏，单一注采井组的注水采热速率为2MW，40年可累计采热2.69E13 KJ（相当于9.2万吨标准煤）；而注CO2采热速率可达3.8MW，40年累计采热量可达4.83E13 KJ（16.5万吨标准煤）。采热影响因素分析结果表明，当气藏边底水能量强、渗透率大、储层温度高及水侵严重时，可考虑注水开发。对于低渗气藏，由于CO2具有较高的可注性和流动性，其采热速率和能力高于水，应优先考虑。地层水蒸发和盐析分析结果表明，当地层水饱和度和矿化度较高时，会发生较为严重的盐析，造成注入井附近储层堵塞，降低CO2采热速率（降幅可达24%），因此提出了预注入低矿化度水的方法，降低盐析及减缓地层堵塞。CO2–地层水–岩石地化反应砂岩及数值模拟结果表明：对于砂岩储层，地化反应对储层物性和CO2采热速率影响不大；对于碳酸盐岩储层，CO2注入引起的地化反应，会导致方解石和白云石溶解，增加储层渗透率，提高CO2采热速率。针对典型的低渗高温废弃气田，建立了注CO2循环地热开发模型，优化了注采井网和参数，进行了技术和经济可行性分析。结果表明，对于储层温度120ºC，埋深3000m，规模约为1500×1000×50m的目标区块，采用3注6采井网， 4500t/d循环注入CO2，运行30年，年均地热发电为6.20E6 kW∙h，成本约为0.66元/(kW∙h)，与当前常规煤电和其他发电技术的成本（0.3-1.3元/kW∙h）相比，注CO2循环开发高温废弃气藏地热具有一定的经济竞争能力和环保效果。