Unconventional natural gas resources including shale and tight gas reservoirs hold most of natural gas reserves around the world. However, these resources pose a great challenge in reserve estimation, modeling flow behavior, reservoir simulation, drilling, history matching, and well testing. Because of high complexity of rock composition, poor petrophysical properties and tightness of the pore system gas-rock interactions can no longer be ignored since they provide crucial information about the flow and recovery processes. Gas adsorption is one of the most important mechanisms describing gas-rock interaction.

In this work, adsorption of CH4 and CO2 on the organic-rich Qusaiba Shale, Saudi Arabia was studied. XRD analysis showed that the shale consists mainly of clays, Quartz and K-feldspar. SEM characterization shows that the shale has a wide range of pore types and sizes. The total organuic carbon (TOC) of the studied samples has an average of 6.10 wt% and Tmax is 417°C which indicates the shale immaturity.

The results of the experimental study showed that the CO2 content in the natural gas has a noticeable effect on the desorption of the natural gas from the shale rock. The studied shale sample has no CH4 adsorption at 50°C and it increases slightly to 16 mg/g after increasing temperature to 100°C and further increased to 145 mg/g at 150°C which is attributed to opening of the pores that previously blocked by organic matter when the temperature increases at 50°C and 100°C. Increasing CO2 fraction in the mixture from 0% to 10% CO2 the total maximum gas uptake increased to approximately 24%, 26% and 31% at 50°C, 100°C and 150°C respectively. The pure CO2 adsorption on was the approximately 85 mg/g and increased to 555 mg/g at 150°C and 44 bar which reflects the high shale attraction toward CO2. Also, The TOC and shale mineralogy affected the adsorption behavior of CO2 and CH4 on the studied shale sample causing the adsorption behavior to be endothermic i.e. increasing adsorption with increasing temperature. The presence of water bearing clay minerals that are sensitivity to temperature causing damage to crystal structure which resulted in huge increase in the adsorption uptake at high temperatures. The adsorption isotherms analysis confirmed that Langmuir isotherm is not the suitable model to be used for adsorption for all studied gases at the studied temperatures. The temperature changes have projected a tremendous effect on the desorption process of the natural gas.

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