Water-Uptaking Feature For The Shales Of The Lower Silurian Longmaxi Formation In Northern Guizhou,China

A large amount of water injected during hydraulic fracturing is retained within the matrix or fractures of shale reservoirs,which can impede gas production.Thus,understanding the water-uptaking feature of shale is beneficial to optimize shale gas development programs and improve shale gas production.In this work,the petrology,geochemistry,pore structure characterization and water sorption experiments were used to comprehensively investigate the mineral composition,geochemistry,pore structure and water sorption characteristics for the shales of the Lower Silurian Longmaxi Formation in northern Guizhou,China.Furthermore,the main controlling factors of water vapor adsorption(WVA)were discussed in depth.The mineral composition,organic matter characteristics and pore characteristics of the Longmaxi shales in YC3 and XY1 wells were comparatively studied using various experimental methods.The total organic carbon(TOC)contents of the YC3 shales averaged 0.30wt.%,categorized as organic-lean shale,dominated by clay minerals,with relatively developed inorganic pores.In contrast,the TOC contents of the XY1 shales averaged 5.31wt.%,categorized as organic-rich shale,with abundant organic pores and larger specific surface area and pore volume values.A combination of univariate regression and multiple regression analysis was used to discuss the main control factors of WVA in shale.It was found that the main control factors of WVA were different at different relative humidity(RH)stages.At a low RH,monolayer vapor molecules adsorbed onto the pore surface via strong intermolecular bonding provided by clay minerals.With increasing RH,multilayer adsorption and capillary condensation began,and clay minerals gradually lost their dominant status.At a high RH,the pore structure controlled the behavior of WVA via capillary condensation,and porosity/pore volume was the main controlling factor.The contribution of clay minerals to WVA was quantified by four standard clay minerals,including smectite,kaolinite,chlorite,and illite.The four clay minerals were ranked according to the magnitude of WVA as follows:smectite(142.62 mg/g)>kaolinite(32.43 mg/g)>illite(7.58mg/g)>chlorite(7.20 mg/g).For the Longmaxi shales in the study area,illite-smectite mixed-layer mineral is the main WVA site within clay minerals.Pore size distributions derived from WVA exhibit unimodal distributions,which are generally great than that obtained from N₂ physisorption due to the existence of clay minerals.The spontaneous imbibition(SI)and WVA experiments were combined to study the water sorption characteristics of shales,and the process of fracturing fluid loss was divided into SI stage and WVA stage.The total mass of absorbed water in the SI and WVA experiments was positively correlated,and the different values of absorbed water between SI and WVA were due to differences in the microfractures,the driving force and the pore-filling sequence.