缝洞型碳酸盐岩地下储气库高速注采渗流特征及库容动用机理

Flow behaviors and storage utilization mechanisms of fractured–vuggy carbonate UGSs during high-rate injection and production

四川盆地是中国重要的天然气产能基地,目前已开发相当规模的碳酸盐岩气藏,该类气藏储层缝洞系统发育并具有超低孔隙度、低渗透率特征,气藏枯竭后可改建为地下储气库(以下简称储气库),以满足天然气高峰使用时期和冬季天然气保供的需求。但国内尚无此类型储气库成熟的建库经验,而缝洞型储气库的高速注采渗流特征是验证建库和用库的可行性的重要依据。为此,以中国石油西南油气田公司筹建的首座缝洞型储气库——牟家坪储气库为例,基于地震、露头等数据资料,设计了具备储层特征的缝洞组合方案,并构建了三维数字岩心模型,然后基于模型开展数值仿真模拟,计算其在高速开采过程中各区域气体压力和流速变化规律,最后从微观尺度分析了缝洞型储气库的渗流特征和库容动用机理。研究结果表明:①储层渗透率越高,缝洞内气体动用越快,渗流滞后的效应越显著;②同一缝洞组合条件下,气体流速和开采量与开采压力梯度成正比,开采压力梯度对气体渗流特征有着显著影响;③缝洞系统中存在狭窄裂缝,裂缝开度越小,缝洞间渗流滞后效应越明显;④缝洞组合和形态结构越复杂,渗流滞后越显著。结论认为,该方法能够为研究缝洞型碳酸岩盐储气库注采渗流特征和库容动用机理提供重要的理论基础,还可为同类型储气库建设提供技术支撑。

The Sichuan Basin is an important natural gas production base in China,where carbonate gas reservoirs have been developed on a considerable scale.This type of gas reservoir are characterized by a developed fracture–vug system,ultra low porosity and low permeability.Depleted gas reservoirs can be rebuilt into underground gas storage(hereinafter referred to as UGS)to meet the demands of natural gas supply during peak natural gas consumption in winter.Unfortunately,however,there is no mature experience at home that can be used for the construction of fractured–vuggy carbonate gas-reservoir UGSs.In addition,the flow behaviors in fractured–vuggy UGSs during high-rate injection and production are an important basis for verifying the feasibility of UGS construction and operation.Taking the first fractured–vuggy UGS(Moujiaping UGS)built by PetroChina Southwest Oil&Gasfield Company as an example,this paper designs a fracture–vug combination scheme with reservoir characteristics based on the seismic and outcrop data,and establishes a 3D digital core model.Then,numerical simulation is carried out by using the model,and the variation laws of gas pressure and flow rate in different areas in the process of high-rate production are calculated.Finally,the flow behaviors and storage utilization mechanisms of fractured–vuggy UGSs are analyzed on a microscopic scale.And the following research results are obtained.First,the higher the reservoir permeability,the faster the gas utilization in fractures and vugs,and the more significant the flow hysteresis effect.Second,under the condition of the same fracture–vug combination,the gas flow rate and production rate are proportional to the production pressure gradient.The production pressure gradient has a significant impact on gas flow behaviors.Third,narrow fractures present in the fracture-vuggy system,and smaller fracture aperture has a more significant impact on the flow hysteresis effect.Fourth,the more complicated the fracture–vug combination and the morphological structure,the more significant the flow hysteresis.In conclusion,this mothed can provide an important theoretical basis for researching the flow behaviors and storage utilization mechanisms of fractured–vuggy carbonate UGSs in the process of injection and production,as well as a technical support for the construction of similar UGSs.