Optimization of the Gas Content Evaluation Method for Deep Coal Seams Based on Diffusion Models

Area of Science:

• Earth Science
• Petroleum Engineering
• Geochemistry

Background:

• Accurate gas content determination is vital for coalbed methane (CBM) exploration and development.
• Traditional on-site desorption methods for gas content evaluation are prone to significant errors, particularly in deep CBM reservoirs.
• Complex CBM occurrence states and extended diffusion times in deep reservoirs exacerbate errors in conventional gas content assessment.

Purpose of the Study:

• To compare the performance of various gas diffusion and lost gas prediction models in deep CBM reservoirs.
• To propose and evaluate a modified method for calculating gas loss that accurately describes methane diffusion.
• To identify the initiation time and key factors influencing gas loss during CBM sampling.

Main Methods:

• Comparative analysis of existing gas diffusion and lost gas prediction models.
• Development and validation of a novel modified curve fitting (MCF20) method for gas loss calculation.
• Evaluation of the proposed method against pressure-preserved coring data.

Main Results:

• The bidisperse diffusion model demonstrated the best fit for gas diffusion in deep CBM reservoirs.
• Using the start of core extraction as the zero point for gas loss calculation is more geologically accurate for deep coal seams.
• The proposed MCF20 method significantly reduces errors and shows strong agreement with pressure-preserved core extraction results, outperforming USBM and ACF methods.

Conclusions:

• The MCF20 method is proposed as the optimal approach for accurate gas content evaluation in deep CBM reservoirs.
• Gas loss is strongly correlated with escape time, Ro, fixed carbon content, and vitrinite content during rope core extraction.
• Accurate estimation of lost gas is critical for reliable CBM resource assessment and development planning.