Key Factors Controlling Gas Adsorption Capacity Of Shale And Quantitative Characterization

Tighter energy demand-and-supply and harder-to-get conventional oil and gas has made unconventional oil and gas, including those from shale, coal seams, tight sandstones and etc., more attractive than ever before in the industry. Among the unconventionals, shale gas has been proven the most promising by the shale gas boom in the US for the past couple years.Shale gas occurrence mode are divided into free gas, absorbed gas and dissolved gas. Previous studies on the percentage of absorbed gas in total produced shale gas show that the percentage varies in different shale plays but is usually high and ranges between 40%~85%. In addition, the existence of absorbed gas will also increase the preservation ability gas in shale. Based on the above understanding, the existence of adsorption phase is believed to be an important factor for successful development of shale gas. As a result, it is important to reveal key factors controlling the absorption capacity of gas in shale. In the past, the evidences for the key factors are almost comprehensive embodiment of two or more factors. For example, the relationship between gas adsorption capacity and TOC may conclude other effects from maturity of organic matter, temperature, humidity, and so on. Undeniably, it is more convictive to reveal the factors individually. In this paper, we designed and carried out a series of parallel absorption experiments to improve the research.This research carried out a series of parallel experiments and numerical modeling to reveal key factors, including TOC and maturity of organic matter, gas composition, temperature, pressure and humidity, that have impact upon gas adsorption capacity of shale. The results show that:(1)Pores with diameter less than 3nm are the major contributor to the specific surface area of shale, accounting for 80% of the total. In addition, micropores with diameter less than 2nm are generated in large numbers during the thermal evolution of organic matter, which causes substantial increase in the specific surface area and adsorption capacity.(2)Shale absorbs more CO2 than CH4, implying that injection of CO2 could enhance CH4 recovery.(3)Perfect simulation of Langmuir model indicates gas adsorption on shale is in a monolayer way. Geological application of adsorption potential model exhibits the adsorption capacity of shale increases first and then decreases with rising burial depth under combine effect of temperature and pressure of geological conditions, which is different from changing pattern of storage capacity for free gas that improving gradually with increasing burial depth with a given constant porosity. This inconsistence of the two tendencies makes it possible for mutual conversion of absorbed gas and free gas – good for shale gas preservation.(4)During the thermal evolution of organic matter, hydrophilic NSO functional groups will gradually degrade causing the humidity of shale to be reduced and adsorption capacity for gas to increase.(5)Maturity of organic matter, gas composition and geological condition(including temperature & pressure, humidity) all has impact upon the adsorption capacity of shale. Among them, the humidity plays a less important role and is easily overshadowed by other factors, such as the abundance of organic matter.