Control mechanism of pressure drop rate on coalbed methane productivity by using production data and physical simulation technology

Abstract

When determining the location of Coalbed Methane (CBM) wells, pressure variation is a key factor affecting CBM productivity. Understanding the relationship between reservoir pressure and gas production has become a central challenge in CBM development. In this study, drainage data from CBM wells located in a specific area on the eastern margin of the Ordos Basin were analysed to investigate the influence of geological parameters (e.g. coal seam thickness, burial depth) on productivity across different CBM well types. Subsequently, the effect of pressure drop rate on CBM production dynamics was studied.

To further explore this relationship, low-field nuclear magnetic resonance (NMR) experiments were conducted to simulate stepwise depressurization and methane desorption. These experiments addressed the impact of pressure drop rate on methane desorption from both primary and fractured coal structures. A numerical simulation model was then developed to evaluate CBM production performance under varying pressure drop rate constraints. This integrated approach considered key factors such as reservoir pressure and gas content to systematically assess the influence of pressure drop rate on CBM productivity. The main findings are as follows: 1) Pressure-production relationship: An exponential relationship was observed between the pressure decline curves of vertical and horizontal wells. The pressure drop stage corresponds to the stages of increasing and stable gas production. A higher pressure drop rate enhances early-stage production in vertical wells but adversely affects their long-term performance. In contrast, horizontal wells exhibit a pronounced decline in productivity under high drawdown rates. 2) Methane behaviour characterization: NMR simulations revealed distinct pressure-dependent behaviours for adsorbed and free methane. Adsorbed methane follows the Langmuir isotherm, and free methane exhibits a linear relationship with pressure. This distinction arises because adsorbed methane is mainly associated with micropore surface area, whereas free methane is governed by pore volume. 3) Desorption dynamics: NMR physical experiments showed that higher pressure drop rates hinder the desorption of adsorbed methane. The desorption efficiency was inversely proportional to the drawdown rate, largely due to the complex pore structures (e.g. ink-bottle-shaped pores) in the coal samples. Rapid pressure changes reduce methane desorption efficiency, and limit methane diffusion and migration. 4) Numerical simulation insights: The simulation results quantitatively characterized the impact of pressure drop rate on CBM productivity. Higher drawdown rates accelerate the vertical expansion of the pressure depletion cone, leading to an earlier production peak and higher initial gas output. However, over time, these wells exhibit weaker lateral expansion of pressure funnel, resulting in lower gas content and pressure gradients, and ultimately lower productivity in the later stages of drainage.