天然气水合物原位补热降压充填开采方法三维数值模拟研究

3D NUMERICAL EVALUATION OF GAS HYDRATE PRODUCTION PER-FORMANCE OF THE DEPRESSURIZATION AND BACKFILLING WITH IN-SITU SUPPLEMENTAL HEAT METHOD

天然气水合物是未来极具潜力的新型高效清洁替代能源。在分析水合物开采面临的瓶颈问题的基础上,提出了一种全新的天然气水合物开采方法——原位补热降压充填开采法。该方法将氧化钙(CaO)粉末注入天然水合物储层,降压开采天然气,天然气水合物分解产生的水和氧化钙粉末迅速反应,产生的大量热量补充天然气水合物的分解热。本文利用基于有限体积法的新型天然气水合物模拟器,构建三维地质模型对该方法进行产能数值模拟评价。模拟结果表明相较于常规水平井方法以及水平井结合压裂开采方案,该方法对生产的促进效应明显,尤其是与水平井结合压裂开采方案相比该方法的累积产气量明显提高,但累积产水量没有显著变化,开采效率显著提升。施工工艺中裂缝等效渗透率和氧化钙注入量两个关键参数的敏感性分析结果表明在压裂过程中,压裂技术的增产效果会随着等效渗透率的提高而逐渐减弱。除此之外,氧化钙注入量越大,增产效应越明显,并且提高氧化钙注入量只会提高产气量,不会显著提高产水量,所以理论上注入量越大,产气效率越高。与此同时,该方法在不同渗透性能的天然气水合物储层中均有一定的适用性,其中针对低渗储层的促进效应更为显著。综合上述结论,本文从三维模型理论计算的角度定量化验证了原位补热降压充填开采方法的潜在价值,期待为将来的水合物试采工作提供一定参考。

Natural gas hydrate(NGH)is a promising clean alternative energy resource for world in future.Based on the analysis of the challenges in the commercial exploitation,the depressurization and backfilling with in-situ supplemental heat method had been proposed to enhance the gas production of methane hydrate reservoir.In this method,the calcium oxide(CaO)powder is injected into the hydrate reservoir,and the natural gas is exploited by depressurization.The water produced by the decomposition of natural gas hydrate will react with the calcium oxide powder rapidly,which would provide amounts heat for supplement thermal energy of the decomposition of natural gas hydrate.This novel method is evaluated by a numerical simulator based on the finite volume method in this work.A three-dimensional reservoir model was constructed.The simulation results indicate that comparing with the conventional horizontal well method and the horizontal well combined fracturing method,this method has a better production performance.Comparing with the horizontal well combined fracturing method,the cumulative gas production of this method is improved,but the cumulative water production has changed slightly simultaneously.Therefore,the recovery efficiency has been significantly improved.The results of the sensitivity analysis of the equivalent permeability of fractures and the mass of CaO injection show that the increasing effect of fracturing on gas production declines with the improvement of equivalent permeability of fractures.In addition,the greater the amount of injected calcium oxide,the more obvious the effect of increasing production.Increasing the amount of injected calcium oxide only increase the gas production,but not significantly increase the water production.Therefore,theoretically the larger the injection,the higher the gas production efficiency.Simultaneously,the feasibility of this method has been testified in reservoirs with different flow capacity.Herein,the improving effect on low-permeability reservoir is more obviously than other cases.Based on the above conclusions,this work quantitatively verifies the potential value of the depressurization and backfilling with in-situ supplemental heat method from the perspective of the theoretical calculation of the three-dimensional model,which looks forward to providing the reference for following work of hydrate recovery.