干热岩储层增渗递减趋势下热能抽采的蒙特卡洛预演分析

Monte carlo analysis of heat extraction simulation under the decreasing trend of permeability enhancement in the hot dry rock reservoir

  • 摘要: 世界范围内持续半个世纪的增强型地热系统(Enhanced Geothermal System, EGS)的现场试验显示出了低孔低渗干热岩开发的巨大风险。考虑产出不确定性传递溯源,实现人工热储渗透率分布的随机性表征以及基于此的系统产出预演,才能给出当前增渗造储技术条件下EGS热能产出的一般性客观评价。在完善前人压裂域预设构造的基础上,基于蒙特卡洛思想,通过热能抽采的遍历模拟实现同一增渗条件下热能产出的预演分析。首先,基于人工造储的增渗量级和扩展范围确定渗透率整体的递减趋势,利用转向带法模拟生成随机分布,叠加递减趋势和随机分布构造出具有整体确定性和局部随机性的渗透率分布样本;其次,建立多物理场耦合模型,由统一的初边值条件对所构造的渗透率分布样本展开热能抽采模拟,并总结了热能产出的评价指标;最后,由数值模拟结果得出每个样本的产出评价指标,分析样本产出指标的分布特点。基于产出指标的统计分析,指出工质载热运移过程中能否有充足的流量扫掠经过足够的热储空间,决定了EGS的不同产出模式。当以产出流量20 kg/s和服务年限15 a为临界指标时,失败样本所占的比率约为1/4,即在最大增渗5个数量级,扩展范围达350 m的条件下,EGS系统仍面临着严重的产能风险。有效构筑EGS人工热储的2个关键点为足够空间的有效联通增渗域与避免形成优势渗流通道,即干热岩开发要求储层增渗技术具有更高的可控导性,当前技术条件下对EGS的建造决策应持保守谨慎态度。

     

    Abstract: Over the past half-century, field tests for enhanced geothermal system (EGS) worldwide have revealed the significant risks associated with the development of low porosity and low permeability hot dry rock (HDR) reservoirs. In view of the transmission and traceability of production uncertainty, the realization of stochastic characterization of permeability distributions and the corresponding output simulation preview can provide a general and objective evaluation of EGS production under the current permeability enhancement technology. Based on the Monte Carlo method, a traversal simulation of geothermal energy extraction is conducted to achieve a predictive analysis of thermal energy production under the same enhanced permeability conditions. Firstly, the overall decreasing trend of permeability is determined based on the magnitude and expansion range of permeability enhancement in artificial reservoirs, and the turning bands method (TBM) is used to generate random distributions, superimposing decreasing trends and random distributions to construct permeability distribution samples with overall determinism and local randomness. Secondly, a multiphysics coupling model is established, and geothermal extraction simulations are conducted on the constructed permeability distribution samples under unified initial and boundary conditions. Finally, the production evaluation indicators for each sample are derived from the numerical simulation results, and the distribution characteristics of the production indicators are analyzed. Based on statistical analysis of the production indicators, this paper highlights that enough flux which can sweep adequate stimulated reservoir volume (SRV) is the key to different production patterns. When setting the critical values of 20 kg/s of production flow and a service life of 15 years, the failure rate of samples is approximately 1/4. This indicates that, even under conditions of maximum permeability enhancement by 5 orders of magnitude and an expansion range of 350 m, the EGS system still faces significant production capacity risks. The two key points for effectively constructing EGS artificial reservoirs are the effective connection of enough high-permeability domains and the avoidance of dominant seepage channels. That is, the development of HDR requires reservoir enhancement technologies with higher controllable technique. Thus, under the current technological conditions, a conservative and cautious attitude should be maintained towards the decision of EGS construction.

     

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