地层温压条件下页岩吸附性能变化特征——以渝东北地区龙马溪组为例

Adsorptivity of shale under the formation temperature and pressure:a case of Longmaxi Formation in northeastern Chongqing

页岩气储层的吸附性是影响页岩含气性的关键因素。以渝东北地区下志留统龙马溪组富有机质页岩为例,通过变等温吸附实验,模拟在地层温压变化条件下页岩吸附性能的变化规律,为该区页岩气勘探提供理论依据。实验分析表明:有机质是控制龙马溪组页岩吸附性能的主要内在因素,有机碳含量与页岩吸附能力具有很好的正相关性;地层温压条件是控制页岩吸附性能的外在因素,温度和压力对于页岩吸附性能具有相互抑制的作用,随着温度和压力的升高,页岩吸附能力呈现出类似"抛物线"(先增大后减小)的变化轨迹,在温度和压力均相对较低时,压力起主导作用,在温度和压力均相对较高时,温度起主导作用。页岩吸附性能若要达到最佳状态,则需要埋深、温度和压力三者达到一种合适的匹配状态。实验模拟结果显示,渝东北地区龙马溪组页岩最佳理论吸附状态应在1 000～4 500 m的埋深条件下。

Adsorptivity is a typical characteristic of shale gas reservoir and a key impact factor of shale gas content.The change rule of shale adsorptivity under the formation temperature and pressure was simulated by a heterothermal adsorption experiment with a case of Lower Silurian Longmaxi shale in northeastern Chongqing,so as to provide theoretical basis for shale gas exploration in this area.The result shows that organic matter is the main internal factor which impacts shale adsorptivity of Longmaxi Formation and the TOC content has a very positive correlation with the adsorption capacity of shale.The geological conditions of temperature and pressure are the external factors which impact shale adsorptivity.Temperature and pressure are mutual inhibitory for shale adsorptivity.With the increase of temperature and pressure,the variation tendency of shale adsorption capacity is similar to parabola.When temperature and pressure are relatively low,pressure plays a leading role,while temperature plays a leading role when temperature and pressure are relatively high.Under the underground condition,the best adsorption state of shale needs to achieve an appropriate matching of temperature,pressure and burial depth).The experimental simulation results show that the optimum theoretical adsorption state of Longmaxi shale in northeastern Chongqing should be at the depth of 1 000-4 500 m.