No abstract

No abstract

No abstract

Adsorption plays an important role in carbon dioxide sequestration and methane recovery processes in deep coal seams. If the effects of coal physical properties on its gas adsorption capacity are considered, it increases with vitrinite content and pressure and reduces with liptinite, mineral matter, moisture contents, and temperature and follows a U-shaped variation with carbon content. Furthermore, CO 2 has higher adsorption capacity compared to other gases. There are two main methods to estimate the amount of gas adsorbed in a coal seam, which are called direct and indirect methods. The latter method is more common for coal. In the direct method, the volume of gas released from a coal mass into a sealed desorption canister is measured, and under the indirect method, adsorption isotherms and empirical relations are used. However, any of those methods are unable to count the effect of swelling-induced strain and, therefore, fail in measuring the absolute adsorption in coal. Moreover, among various adsorption models, Langmuir and DubninRadushkevich (D-R) equations are the most widely using models. However, since gas can be absorbed through both adsorption and absorption processes for coal, it is important to have an additional term to count the missing absorption mechanism in both the Langmuir and D-R models. Finally, a new descriptive model for gas adsorption capacity of coal as a function of effective factors is proposed. The new model is based on the existing D-R equation, which was modified by inserting a new expression for the term of micropore capacity. Two types (CO 2 and N 2 ) of gas adsorption data for coal from five different locations (British Colombia and Alberta in Canada and Victoria, Sydney and Bowen in Australia) at three different temperatures (273, 296.5, and 318 K) were considered for the model development. According to the model results, new gas adsorption equations can fairly well accurately predict the adsorption capacity in coal.

In exploration for shale gas, experimental methane-sorption measurements represent a valuable source of information for resource estimates and for reservoir-modeling studies. Here, the main difficulty is the relatively low adsorption capacity of shales (typically 10% of the sorption capacity of coals), as well as the fact that the measurements need to be performed over a wide range of pressures and temperatures characteristic of past or present geological conditions. In this work, we demonstrate the capabilities of an adapted manometric apparatus to reliably measure excess sorption isotherms at pressures of up to 30 MPa and temperatures up to 423 K on carbonaceous shales. This is accomplished with an experimental design comprising separate heating zones for the sample cell and for the rest of the apparatus. An experimental and mass-balance approach is presented to quantify the temperature gradient existing between the two heating zones, as well as the thermal expansion of the sample cell, and to account for these in the calculation of the excess sorption. We demonstrate that the analysis of the helium-void-volume data over a large temperature range can be interpreted with respect to the thermal expansion of the sample and, in some cases, changes in pore-volume accessibility to helium. We propose to perform blank-expansion tests with non-adsorbing specimens (e.g., steel cylinders) as a quality check to eliminate device-specific artifacts resulting from unknown measurement uncertainties or from uncertainty in the equation of state. Two evaluation procedures are presented to quantitatively account for the blank tests in the final result of sorption measurements on shale samples. As an example, methane-sorption isotherms for carbonaceous shale at 311, 338, 373, and 423 K are presented. By use of a Monte Carlo algorithm to simulate the propagation of the experimental uncertainties, the final estimated uncertainty in excess sorption resulting from systematic errors was found to be 6 0.007 mmol/g at 25 MPa. The consideration of the blank-expansion tests in the mass balance further reduces the systematic error, at least to a point at which an excellent intralaboratory consistency is obtained. The estimated uncertainty resulting from random errors was found to significantly overestimate the actual precision of the experimental setup, and an explanation is provided with respect to experimental design. A datareduction approach using an excess-sorption function based on a Langmuir-type absolute-sorption model was found to provide an excellent representation of the measured sorption data. By means of simplified model calculations we demonstrate that the excess-sorption formalism is a sufficient, simple, and adequate approach to applications in shale-gas-resource estimation. The uncertainties pertaining to representativeness of experimental sorption data of in-situ reservoir conditions are briefly discussed.