Evaluation of the Adsorbed Gas Amount in a Shale Reservoir Using the Three Compositions Adsorbing Methane (TCAM) Method: A Case from the Longmaxi Shale in Southeast Chongqing, China

To evaluate the amount of adsorbed methane in a shale reservoir under different conditions of temperature and pressure, the Longmaxi shale from the Py1 well of southeast Chongqing was selected as a case to establish the three compositions adsorbing methane (TCAM) method based on the results of total organic carbon, X-ray diffraction, and methane isothermal adsorption analyses. The amounts of methane adsorbed by organic matter (OM), clay, and other minerals were calculated under different conditions of temperature and pressure, followed by the evaluation of the occurrence characteristics of adsorbed gas in a shale reservoir. A comparison of data from experiments and the TCAM method with two shale samples revealed the uncertainty of TCAM. The results from the two samples range from 0.02 to 0.08 m(3)/t and from 0.01 to 0.09 m(3)/t, with average values of 0.05 and 0.04 m(3)/t, respectively. The methane adsorption capacities of OM, clay, and other minerals at temperatures ranging from 30 to 70 degrees C were 26.04-38.76, 1.83-2.76, and 0.43-0.53 m(3)/t, respectively, with average values of 32.86, 2.38, and 0.48 m(3)/t, respectively. These values are significantly reduced and are more likely to be affected by the temperature. The contributions of OM, clay, and other minerals to the total adsorbed gas are 34.83, 46.74, and 18.43%, respectively, on the basis of the limited data available. The efficient exploitation of adsorbed gas in a shale reservoir requires an increase in the reservoir temperature or a reduction in the reservoir pressure to lower than 10 MPa. The TCAM method requires the shale reservoir to be in a relatively stable sedimentary environment, to have the same diagenetic evolution, and to have the same type of OM. Once the TCAM model is established, it can be applied to the same marine shale reservoir with a similar diagenetic evolution in the same area.