页岩气解析规律及赋存方式探讨

摘要

To improve the understanding of the rules of gas desorption and occurrence mode in shales,we conduct a case study in the southern Sichuan Basin based on practical gas desorption data of three wells.Results show that shale gas desorbed steadily from shale bedding,holes,cracks and lithology variations in rain-like,moniliform and linear patterns.The desorption curves increased sharply at first and then gradually decreased steadily,showing two inflection points.So do the variation of methane concentration and carbon isotope values of shale gas.The first inflection point is related to the lithological textures of shale itself and is the threshold for gas state.Before this point,the desorption gas was mainly in a free-gas state with large quantity and the desorption rate curves fluctuated greatly.After this point,the desorption gas was mainly in an adsorption state with steady and persistent desorption curves.These rules were further supported by the correlation analysis and theoretical deduction of shale gas reserves and actual production curves of shale gas.Calculation of the concentration of adsorbed gas in marine and continental shales indicated that they varied from 26.55％ to 87.47％ with an average of 46.29％.Sedimentary structures and preservation conditions of shale are the main controls on the shale-gas occurrence mode.%以四川盆地南部(川南地区)三口页岩气井自然解析数据为基础,对页岩气的解析规律及其赋存方式进行了系统的分析.结果表明,页岩气以微雨状、串珠状及线状从页岩层理、孔裂缝及岩性变化处持续稳定解析;解析曲线出现了急剧升高后缓慢降低的趋势,即出现了所谓的拐点;页岩气甲烷组分及碳同位素的变化亦表现了类似的规律.分析认为,第一拐点与页岩本身的岩性结构有关,是气体状态变化的界线,在该点之前,页岩气的解析气量大,解析速率曲线波动剧烈,页岩解析以游离气为主,此点之后,解析稳定而持久,吸附气占据主要部分.含气量相关性程度分析、理论推导及页岩气井实际产能曲线均印证了该规律的存在.最后,对海、陆相页岩吸附气所占比重进行了计算,其含量在26.55％～87.47％,平均46.29％,沉积构造及保存条件是决定页岩气赋存状态的主要因素.