中美常压页岩气赋存状态及其对可动性与产量的影响——以彭水和阿巴拉契亚为例

Occurrence of normally-pressured shale gas in China and the United States and their effects on mobility and production:A case study of southeast Sichuan Basin and Appalachia Basin

美国阿巴拉契亚盆地Ohio组页岩、Marcellus组页岩等常压页岩气区已获得商业化开采,中国川东南地区五峰组—龙马溪组页岩虽已初步开发,但由于埋深大、储层物性差等原因导致产出效益没有美国典型页岩气田好。常压页岩气因储层深度导致绝对压力存在差异,从而对气体赋存状态产生影响,进而对含气性与可动性产生显著影响,因而亟须对中美常压页岩气储层条件、气体赋存状态、含气性、可动性差异等开展定量化研究。选取典型常压的中国彭水地区隆页1井五峰组—龙马溪组页岩与美国阿巴拉契亚盆地Marcellus组页岩与Ohio组页岩作为研究对象,以体积法为基础,综合考虑页岩储层温度、储层压力、成熟度、水、油对吸附程度与最大吸附能力的影响,以及温度、压力、孔隙度与含水饱和度对确定游离气量的影响,对三组页岩储层含气性进行评价,并以三组页岩储层压力作为起始压力,以5 MPa的降压幅度进行降压生产模拟,在分别阐明降压生产中吸附、游离气产出过程的基础上,揭示中美常压页岩气产量差异的根本原因:国内五峰组—龙马溪组页岩相比于阿巴拉契亚盆地Marcellus组页岩吸附能力较弱,吸附气量较低,游离气含量较低,导致总产气量也明显较低;而相比于Ohio组页岩,常压的五峰组—龙马溪组页岩埋深大,温度、压力高,由此造成吸附气采出程度极低,含气孔隙度略低导致游离气采出程度较低,二者综合导致总产气量也明显较低。

Normal pressure shale gas plays such as the Ohio Formation Shale and Marcellus Formation Shale in the Appalachian Basin in the U.S.have been developed commercially.Although Wufeng-Longmaxi Shale in southeast Sichuan Basin in China has been initially developed,its production efficiency is not obvious and its cost is high due to the large burial depth and poor reservoir properties physical properties.In the normal pressure shale gas formation,the absolute pressure changes at different depths,which influences the gas occurrence state,and then has a significant influence on gas content and mobility.Therefore,it is urgent to carry out quantitative research on the differences of normal pressure shale gas reservoir conditions,gas occurrence state,gas content and mobility between China and the US.In this study,the Wufeng-Longmaxi shales of Well-LY1 in Pengshui of China and the Marcellus shale and Ohio Shale of Appalachian Basin in the United States are selected as the research objects under normal pressure.Based on the volume method,and with the considering of the influence of the temperature,pressure,maturity,water and oil on the adsorption capability and maximum adsorption capacity,and the temperature and pressure,porosity and water saturation on the determination of free gas volume,the gas content of three groups of shale reservoirs is evaluated.The reservoir pressures are considered as the initial pressures and the simulation are conducted based on a pressure drop of 5 MPa.On the basis of clarifying the desorption process of adsorption and free gas in the depressurized production respectively,the fundamental cause of the difference in normal pressure shale gas production between China and the U.S.is revealed.Compared to the Marcellus Shale in Appalachian Basin,the lower adsorption capacity,porosity and free gas of the Wufeng-Longmaxi shales result in low production of adsorbed gas and free gas.However,compared to the shale of the Ohio Formation,the deeper burial,higher temperature and higher pressure in the Wufeng-Longmaxi shales result in the extremely low recovery rate of adsorbed gas.Its lower porosity also contributes to the lower free gas production.