干热岩长期生产过程中天然裂缝损伤对导流能力演变影响研究

Effect of natural fracture damage on the conductivity evolution underlong-term production of hot dry rock resources

干热岩型地热是我国地热资源的重要组成部分,其开发利用对实现“双碳”目标具有重要意义。干热岩储层岩体以花岗岩为主,因其岩性致密,通常采用增强型地热系统(EGS)进行开发。作为循环工质流动换热的主要通道,人工裂缝和天然裂缝形变将造成导流能力演变,进而影响热储取热性能。现有导流能力研究对象多为人工裂缝,且多围绕基质弹性变形开展,并未考虑天然裂缝损伤影响。为揭示天然裂缝损伤作用效果,团队自主研发设计了高温高压岩心注采多场耦合实验平台,分析论证了实验系统的可靠性,设计了对应实验方案与实验流程。采用天然裂缝贯穿岩样,研究了常温下注采压差随排量和围压的变化规律,分析了高温下天然裂缝损伤特征,对比了不同排量,温差和注入方式下天然裂缝损伤对导流能力演变的影响。实验表明,冷流体注入导致天然裂缝体积较初始时刻显著增加,且破坏方式以弱胶结失效为主,在无围压条件下,损伤将引起裂缝隙宽和缝长增大,提升裂缝连通性,有助于改变裂缝导流能力,故压裂和取热方案设计中应对天然裂缝加以考虑;注采压差随围压和排量增大而增加,最大增幅可达0.6 MPa;高温生产下,注采压差改变量和导流能力演化率变化量最大值分别可达1.11 MPa和26.59%。较大排量和温差下,裂缝损伤特征更为明显。相较于连续注入,间歇注入方式下裂缝损伤程度更为显著。利用灰色关联分析得到了主控因素为温差,即热应力是造成裂缝附加导流能力演变的主要原因。本研究证明了天然裂缝损伤在干热岩长期生产过程中分析的必要性,为工程现场施工提供了一定指导。

Geothermal energy in hot dry rock formations is an important component of China’s geothermal resources,and its development is of significant importance for achieving the"dual carbon"goals.The reservoir rocks of hot dry rock formations are mainly granites.The lithology of granites is dense and is usually developed by Enhanced Geothermal System(EGS).As the main pathways for fluid flow and heat transfer in the circulation process,both artificial and natural fractures deformation can lead to the evolution of conductivity,thereby influencing the heat extraction performance of the thermal reservoir.Existing studies on conductivity mostly focus on artificial fractures,often centered around matrix elastic deformation,without considering the impact of natural fractures damage.To reveal the effects of natural fractures damage,a high-temperature and high-pressure rock core injection and extraction multi-field coupling experimental platform is independently developed and designed.The reliability of the experimental system was analyzed and verified,corresponding experimental schemes and procedures are designed.Natural fractures were used to penetrate the rock samples,study the variations of injection and extraction differential pressure with injection flow and confining pressure at room temperature.The characteristics of natural fractures damage at high temperatures were analyzed,and the impact of natural fractures damage on the evolution of conductivity under different injection flow,temperature difference and injection modes were compared.The experiments demonstrated that injecting cold fluid resulted in a significant increase in the volume of natural fractures compared to the initial state,primarily through weak cementation failure damage.Under no confining pressure conditions,damage caused an increase in fracture aperture and length,enhancing fracture connectivity and altering fracture conductivity.Therefore,natural fractures should be considered in the design of fracturing and heat extraction schemes.The injection and extraction differential pressure increased with increasing confining pressure and injection flow,with a maximum increase of up to 0.6 MPa.During high-temperature production,the maximum changes in injection and extraction differential pressure and conductivity evolution rate reached 1.11 MPa and 26.59%,respectively.Characteristics of fracture damage are more pronounced under higher injection flows and temperature differentials.Fracture damage is more significant under intermittent injection compared to continuous injection.Grey relational analysis identified the primary controlling factor as the temperature differential,indicating that thermal stress is the main cause of additional conductivity evolution due to fracture damage.This study highlights the necessity of analyzing natural fractures damage in the long-term production process of hot dry rock formations,providing valuable guidance for engineering field construction.