Abstract: High-temperature depleted gas fields are characterized by large burial depth, low permeability, and abundant geothermal energy, which can be exploited via recycling CO₂. To realize the field application of CO₂ and the efficient exploitation of geothermal energy, CO₂ can be injected before geothermal development to improve natural gas recovery and restore gas reservoir pressure. However, there is still a lack of systematic research on the process of subsurface CO₂ geothermal development, the thermal cycle of CO₂ power generation on the ground has not been reported in the literature, and the economics of CO₂ geothermal development and power generation need to be analyzed in details. For this reason, firstly, a geothermal exploitation model is established based on the conditions in a typical high-temperature depleted gas field, and an analysis is performed on the temperature and pressure changes and geothermal development rate of the reservoir in each stage of CO₂ geothermal development throughout the whole process. Then, this paper proposes the organic Rankine cycle and CO₂ direct cycle power generation methods, and optimizes the thermal cycle process of power generation. Finally, the cost and economics of CO₂ geothermal development and power generation are evaluated using the levelized power generation cost calculation method. The results show that for the depleted gas reservoir with a volume of 1 500 m×1 000 m×50 m at 120℃, 11.75×10⁸m³ CO₂ can be stored during the enhanced gas recovery and pressure build-up stage; the method of CO₂ injection and production has a great influence on CO₂ storage in the geothermal development stage. However, the CO₂ geothermal development rate can be maintained at about 10 MW within 30 years. The organic Rankine cycle system has a maximum power generation rate of 132.7 kW, while the CO₂ direct cycle system can be up to 718.5 kW; the purchase cost of CO₂ has a great impact on the cost of power generation, and when the price is lower than 7-10 US dollars/t, the cost of geothermal power generation via recycling CO₂ injection from low-permeability depleted gas reservoirs is equivalent to the cost of existing coal-fired power generation.

Key words: depleted gas reservoir, geothermal development via CO₂ recycling, CO₂ geological storage, organic Rankine cycle, CO₂ direct cycle

摘要： 高温废弃气田埋藏深、渗透率低、地热能丰富，适合循环注采CO₂开发。为实现CO₂现场应用及地热高效开发，可在地热开采前注入CO₂提高天然气采收率并恢复气藏压力。但对地下CO₂采热过程还缺少系统研究，利用地面CO₂发电热力循环过程也未见相关报道，CO₂采热发电的经济性更需详细分析。为此，首先基于典型高温废弃气田建立了地热开发模型，全过程分析了地下CO₂采热各阶段储层温压变化和采热速率；随后提出了有机朗肯循环和CO₂直接循环发电方式，对发电热力循环过程进行了优化计算；最后采用平准化发电成本计算方法，对CO₂采热发电成本进行了评价。研究结果表明，对于120℃、1 500 m×1 000 m×50 m的废弃气藏，提高采收率和压力恢复阶段可埋存CO₂ 11.75×10⁸m³；CO₂注采方式对地热阶段CO₂埋存量影响较大，但30年内CO₂采热速率均可维持在约10 MW；有机朗肯循环系统发电速率最高为132.7 kW，而CO₂直接循环系统可达718.5 kW；CO₂购买费用对发电成本影响较大，价格低于7~10美元/t时，低渗废弃气藏注CO₂采热发电成本与现有煤电成本持平。

关键词: 废弃气藏, CO₂采热, CO₂地质封存, 有机朗肯循环, CO₂直接循环