裂缝分布对CO_(2)-EGS热提取的影响研究

Effects of fracture distribution on heat extraction through CO_(2)-EGS

地热能是一种极具潜力的可再生能源,已引起广泛关注。在深部地热储层中,人工改造后形成的复杂裂缝网络为热提取提供了重要通道,裂缝空间分布将会直接影响热提取率。为探究裂缝不同空间分布对采热性能的影响,以幂律分布的裂缝网络为基础,采用TOUGH2MP-FLAC^(3D)框架下建立的THM耦合模型,系统研究不同裂缝长度指数(a)、密度(β)的裂缝网络对新型增强型地热系统(CO_(2)-EGS)采热性能的影响。并以热突破时间、EGS寿命、产热率与总产热能以及产热效率5种评价指标对储层热性能进行详细评估。结果表明,在恒速注入的情况下,裂缝长度指数a越大,长裂缝占比越小,注采井之间形成的贯穿裂缝数量越少,裂缝宽度越大,致使生产温度、产热率降低越快,更早达到热突破,从而缩短EGS寿命,降低总产热能。当a相同时,裂缝密度β越大,裂缝数量越多,生产温度与产热率降低越慢,延长热突破时间与EGS寿命,提高产热量。热突破时间最高可增加15.65 a,EGS寿命增加约10 a,总产热能增加约22.77%。而当长度指数a增长时,热突破时间最多缩短了13.1 a,总产热能降低20.8%。因此,长裂缝占比提高和裂缝密度增加有助于提高注采井之间裂缝的连通性,促进流体对流换热,更好地发挥裂缝在热开采中的作用,提高采热量。研究结果为干热岩造缝增渗改造提供一定的理论指导。

Geothermal energy,a promising source of renewable energy,has attracted considerable attention.In deep geo-thermal reservoirs,complex fracture networks formed by artificial stimulation provide predominant channels for heat ex-traction.Therefore,the spatial distribution of fractures directly affects the heat extraction efficiency.This study aims to explore the effects of different spatial distributions of fractures on the heat recovery performance.Based on the fracture network following a power-law distribution,this study systematically investigated the effects of the fracture network with different values of fracture length index(a)and density(β)on the heat recovery performance of a new enhanced geothermal system with CO_(2)as injection fluid(CO_(2)-EGS)using the THM coupling model established under the TOUGH2MP-FLAC^(3D)framework.Furthermore,this study presented a detailed evaluation of the thermal performance of CO_(2)-EGS using five evaluation indicators:heat breakthrough time,CO_(2)-EGS life,heat production rate,total heat produc-tion,and heat production efficiency,obtaining the following results.Under a constant injection rate,an increase in a cor-responded to a smaller proportion of long fractures,a smaller number of penetrating fractures between the injection and production wells,a larger fracture width,and higher decreasing rates of the production temperature and heat production rate.These led to earlier heat breakthroughs,thereby shortening the CO_(2)-EGS life and reducing the total heat production.In the case of a constant a,a greater fracture densityβwas associated with a greater number of fractures and lower de-creasing rates of the production temperature and heat production rate.These prolonged the thermal breakthrough time and CO_(2)-EGS life and improved heat production.Specifically,the heat breakthrough time,EGS life,and total heat pro-duction could increase by up to 15.65 a,about 10 years,and about 22.77%,respectively.In contrast,increasing a de-creased the thermal breakout time and total heat production by 13.1 a and 20.8%,respectively.Therefore,increasing the proportion of long fractures and fracture density is instrumental in improving fracture connectivity,promoting convect-ive heat transfer of fluids,improving the effects of fractures in heat recovery,and enhancing heat production.The results of this study can serve as a theoretical guide for the hydraulic fracturing of hot dry rocks to generate fractures and en-hance their permeability.