Evaluation and correction of prediction model for free gas content in deep coalbed methane :a case study of deep coal seams in the eastern margin of Ordos Basin

The Ordos Basin is rich in deep coalbed methane resources, the development of which is helpful to increase the reserve and production of oil and gas and achieve the goals of carbon peaking and carbon neutrality. Focusing on the key problem of predicting free gas content in deep coal seams, the paper proposes a new free gas content prediction model based on the validation and evaluation of the existing free gas content prediction model, when relying on pressure-retaining coring data from the eastern margin of the basin at the depth from 1911.60 m to 1924.84 m. The results show that the gas content of 9 pressure-retaining scaled coring samples is ranged from 4.22 m3/1 to 16.35 m3/1 ; the free gas content is ranged from 1.24 m3/t to 2.56 m3/t. accounting for 11. ()0% to 36.17% of the total gas content. The difference in gas content is obviously affected by ash content, and the samples with high ash content and high density tend to have low gas content, characterized by overscituration in the middle coal seam and undersaturtition in the upper and lower coal scams. Comprehensively considering the methane gas density, water saturation and porosity of the reservoirs under certain temperature and pressure conditions, in combination with the pore space occupied by adsorbed methane, a prediction model of free gas content in deep coal seams has been constructed based on the effect of adsorption swelling. As predicted using the model, the free gas content is ranging from 1.29 m3, /t to 2.69 m3/1, accounting for 10. ()1 % to 38.54% of the total gas content, and the gas content rtitio (actual gas content/theoreticc.il maximum gas adsorption capacity) is within the range of 33.86% to 148.26%, which is highly consistent with the measured gas content in the pressure-retaining coring samples. In addition, the correlation model can be used to evaluate the gas content in deep coal scams, thus providing theoretical support for the accurate prediction and quantitative drainage and production of deep coalbed methane resources. © 2023 Science Press. All rights reserved.