超临界CO_(2)聚能压裂开发煤层气可行性研究

Feasibility on exploitation of coalbed methane by SC-CO_(2)shock fracturing

煤层气资源的高效开发承担着保障煤矿高效生产、减少瓦斯事故的安全责任以及补充清洁能源供应、助力“双碳”目标实现的重要使命。提出了采用超临界CO_(2)聚能压裂技术开发煤层气的新思路,该技术利用高压超临界CO_(2)动态致裂煤体,配合“聚能冲击+油套同注”等工艺方式实现高压超临界CO_(2)多次、持续注入,使储层产生一定规模且不受地应力控制的复杂裂缝网络,具有无水、环保、安全、高效等技术优势。利用自主研发的真三轴超临界CO_(2)聚能压裂实验系统开展了高阶煤和低阶煤的室内压裂实验,并与常规水力压裂、超临界CO_(2)压裂煤岩井筒压力和成缝特征进行对比。结果表明:与水力压裂和超临界CO_(2)压裂相比,超临界CO_(2)聚能压裂煤岩裂缝迂曲度更高,可形成主裂缝、微裂缝与层理缝相互沟通的网状裂缝;随着聚能压力的增加,煤岩裂缝形态变得复杂,针对特定煤岩存在合理的聚能压力范围确保形成网状裂缝的同时减少煤粉的生成;超临界CO_(2)聚能压裂技术适用于原生结构煤,现场应用中对专业设备需求少、可操控性强,常规CO_(2)压裂设备配合聚能工具即可满足施工要求;超临界CO_(2)聚能压裂致裂能量充足、对井筒的伤害程度小、经济成本低,可作为煤层气老区盘活和新区改造的新型储备技术;建议现场应用中采用超临界CO_(2)聚能压裂与水力压裂相结合的复合压裂工艺,确保形成网状裂缝、扩大改造范围的同时具有长期的导流能力。通过研究验证了超临界CO_(2)聚能压裂开发煤层气的可行性,可望为煤层气高效开发−CO_(2)地质埋存一体化提供一种新思路。

The efficient exploitation of coalbed methane(CBM)bears the safety responsibility of ensuring the efficient production of coal mines and reducing the gas accidents.At the same time,it undertakes the important mission of supply-ing the clean energy and contributing to the realization of the dual carbon goal.A novel idea of using supercritical carbon dioxide(SC-CO_(2))shock fracturing to stimulate CBM was proposed.This technique utilizes the high-pressure SC-CO_(2) to dynamically fracture the coal body,and cooperates with the energy shock and tube-casing injection to realize the multiple and continuous injection,so that the coal reservoir creates complex fracture networks with a certain scale that are not con-trolled by in-situ stress.This technique has some advantages such as water-free,environmental protection,safety and high efficiency.A self-developed true triaxial SC-CO_(2) shock fracturing experimental system was used to conduct the laborat-ory fracturing experiment of high-rank and low-rank coal rocks,and the borehole pressure characteristics and fracture pat-terns were compared with the conventional water fracturing and the SC-CO_(2) fracturing.The results show that the tortuos-ity and complexity of coal fractures in the SC-CO_(2) shock fracturing are higher than that of the conventional water fractur-ing and the SC-CO_(2) fracturing.Consequently,the fracture networks in which main fractures,micro-fractures and bedding fractures connected with each other are created.With the increase of shock pressure,the fracture becomes branched.There is a reasonable range of shock pressure for specific coal to ensure the generation of fracture networks and reduce the pul-verized coal.The SC-CO_(2) shock fracturing is suitable for primary structured coal.In the field applications,it requires less professional equipment and has strong controllability.The conventional CO_(2) fracturing equipment and shock tools can meet the fracturing requirements.The SC-CO_(2) shock fracturing has the advantages of sufficient fracturing energy,little damage to the wellbore,and low economic cost.It can be used as a new reserve technique for revitalizing old CBM areas and reforming new CBM areas.It is suggested that the combination of the SC-CO_(2) shock fracturing and the water fractur-ing should be adopted in the field application to ensure the generation of fracture networks,the expansion of the scope of stimulation and the long-term conductivity.This research verifies the feasibility of the SC-CO_(2) shock fracturing to exploit CBM,which is expected to provide a new idea for the integration of efficient CBM exploitation and CO_(2) geological storage.