Key techniques for precise measuring gas content in deep coal mine: In-situ pressure- and gas-preserved coring

Abstract

Gas content serves as a critical indicator for assessing the resource potential of deep coal mines and forecasting coal mine gas outburst risks. However, existing sampling technologies face challenges in maintaining the integrity of gas content within samples and are often constrained by estimation errors inherent in empirical formulas, which results in inaccurate gas content measurements. This study introduces a lightweight, in-situ pressure- and gas-preserved corer designed to collect coal samples under the pressure conditions at the sampling point, effectively preventing gas loss during transfer and significantly improving measurement accuracy. Additionally, a gas migration model for deep coal mines was developed to elucidate gas migration characteristics under pressure-preserved coring conditions. The model offers valuable insights for optimizing coring parameters, demonstrating that both minimizing the coring hole diameter and reducing the pressure difference between the coring-point pressure and the original pore pressure can effectively improve the precision of gas content measurements. Coring tests conducted at an experimental base validated the performance of the corer and its effectiveness in sample collection. Furthermore, successful horizontal coring tests conducted in an underground coal mine roadway demonstrated that the measured gas content using pressure-preserved coring was 34% higher than that obtained through open sampling methods.

1. Introduction

Energy is a vital foundational resource for national development, and the exploration of energy resources is progressively advancing into greater depths [1,2]. Coal is an important component of the national energy structure, and the development of abundant deep coal resources has attracted more attention [3,4]. Gas, as a byproduct of coal mining resources, has abundant reserves and is also a high-quality clean energy source [5–7]. However, the development of gas faces serious disaster phenomena such as gas outbursts and gas explosions [8,9]. Gas content in coal mines is crucial for assessing gas outbursts, evaluating deep coal seam reserves, and guiding resource development planning, with accurate measurement relying on high-fidelity samples [10]. There are two primary methods for sampling in deep coal mines: open-type coring and sealed-type coring [11].

The open-type coring method collects samples at the borehole opening, stores them in sealed tanks, and measures desorption and crushed desorption gas content through natural desorption experiments. The time square root method is commonly used to measure lost gas, although its accuracy is influenced by sampling time, coal sample properties, and drilling fluid conditions [12,13]. Li et al. [14] analyzed gas concentration and loss in cylindrical coal samples, showing that while a square root relationship applies during the early desorption stage, it introduces substantial errors as the loss time extends. A simplified dynamic model was proposed to accurately estimate coal seam gas content, addressing errors in lost and residual gas calculations while significantly improving measurement efficiency [15]. However, the model's reliability depends on its alignment with the site-specific desorption characteristics of coal samples, while increasing mining depth amplifies gas loss during deep-hole sampling, potentially leading to greater estimation errors in loss gas content.

The sealed-type coring method retrieves samples from the bottom of the borehole while preserving the in-situ pressure environment, ensuring stable pressure during transfer to prevent gas loss. This approach aims to overcome the limitations of open-type coring by maintaining sample integrity and improving measurement accuracy.