Wettability Evaluation Of Marine-continental Transitional Shale

As an important parameter of rock physical properties,wettability not only determines the distribution law of oil and gas in porous rock,but also greatly affects the recoverability and development difficulty of oil and gas.For the purpose of studying the shale wetting characteristics of the Shanxi and Taiyuan Formation in Well Mouye 1 in Zhongmou Block,Southern China North Basin,this paper uses contact angle measurement and spontaneous imbibition to characterize the shale wettability,and discusses the main influencing factors of shale wettability to establish a model for predicting the wettability of marine-continental transitional shale,which will help to further deepen shale gas accumulation mechanism and improve shale gas reservoir evaluation.The main knowledge gained is as follows:The shale of Shanxi and Taiyuan Formation are strongly oil-wet or weak-medium water-wet,showing mixed-wet characteristics.The contact angle measurement results show that,whether it is a mechanically polished block sample or an argon ion polished block sample,kerosene will spread quickly on the surface of the shale and be sucked in,and the oil contact angle is about 0 °,indicating that the Shanxi-Taiyuan Formation shale is strongly oil-wet.In contrast,deionized water will form a stable hemisphere on the surface of the shale.The mechanically polished Shanxi and Taiyuan shale deionized water has a contact angle between 20.7 ° and 39.2 °,with an average value of 26.6 °,which is moderately water-wet characteristics,and argon ion polishing shale deionized water has a contact angle between 52.6 ° ~ 99.6 °,the average value is 81.7 °,showing weak water-wet characteristics.In addition,the wettability characterization results of the spontaneous imbibition experiment are consistent with the contact angle.The imbibition results of parallel and vertical bedding also reveal that shale wetting has a certain directional dependence.According to comprehensive evaluation,the water-wet capacity of different lithofacies shale is: clayey shale> continental quartz shale> clay and continental quartz shale.The contact angle results also reflect external factors,that is,the wetting liquid and the surface roughness of the sample have an important influence on the contact angle measurement.The smoother the shale surface,the higher the salinity of the formation water and the greater the water contact angle,the weaker the shale's water-wet characteristics,and the kerosene wettability is much higher than the deionized wettability.In addition,shale wettability is also affected by internal factors(rock material composition and reservoir physical properties).The water contact angle of shale is positively correlated with TOC content,pyrite content,and BJH pore volume,and with clay mineral content,quartz content showed a negative correlation.The total gas content is an indirect influencing factor of shale wettability,which is indicative.The greater the gas content,the greater the water contact angle and the worse the water wettability.Based on the above knowledge,a shale wettability prediction model based on mineral composition and reservoir properties is established.Vertically,the shale wettability changes have a certain regularity.Because of the frequent vertical changes of the marine-continental transitional shale and the wide range of changes in reservoir physical properties and gas content,the water contact angle varies greatly and the wettability is complicated.Overall,the change trend of shale contact angle in vertical direction is basically consistent with the change trend of organic matter content,total pore volume change and gas content,but it shows the opposite change trend with quartz content and clay mineral content.Combining shale wettability and gas-bearing characteristics can provide a new reference and basis for the evaluation of marine-continental transitional shale reservoirs and the optimization of favorable intervals.