Abstract
In coal seam gas (CSG) wells, water is periodically removed from the wellbore in order to keep the bottom-hole flowing pressure at low levels, facilitating the desorption of methane gas from the coal bed. In order to calculate gas flow rate and further optimize well performance, it is necessary to accurately monitor the liquid level in real-time. This paper presents a novel method based on autocorrelation function (ACF) analysis for determining the liquid level in CSG wells under intense noise conditions. The method involves the calculation of the acoustic travel time in the annulus and processing the autocorrelation signal in order to extract the weak echo under high background noise. In contrast to previous works, the non-linear dependence of the acoustic velocity on temperature and pressure is taken into account. To locate the liquid level of a coal seam gas well the travel time is computed iteratively with the non-linear velocity model. Afterwards, the proposed method is validated using experimental laboratory investigations that have been developed for liquid level detection under two scenarios, representing the combination of low pressure, weak signal, and intense noise generated by gas flowing and leakage. By adopting an evaluation indicator called Crest Factor, the results have shown the superiority of the ACF-based method compared to Fourier filtering (FFT). In the two scenarios, the maximal measurement error from the proposed method was 0.34% and 0.50%, respectively. The latent periodic characteristic of the reflected signal can be extracted by the ACF-based method even when the noise is larger than 1.42 Pa, which is impossible for FFT-based de-noising. A case study focused on a specific CSG well is presented to illustrate the feasibility of the proposed approach, and also to demonstrate that signal processing with autocorrelation analysis can improve the sensitivity of the detection system.
Highlights
Mine gas is one of the most serious safety hazards in coal mining [1]
The results have shown the superiority of the autocorrelation function (ACF)-based method compared to Fourier filtering (FFT)
The latent periodic characteristic of the reflected signal can be extracted by the ACF-based method even when the noise is larger than 1.42 Pa, which is impossible for FFT-based de-noising
Summary
Mine gas is one of the most serious safety hazards in coal mining [1]. It is reported that 19 gas accidents with 100 fatalities or more have occurred in Chinese coal mines from 1950 to 2012 [2].The exploitation of coal seam gas (CSG) would reduce the incidence of coal-gas outbursts, as CSG pre-drainage can reduce the gas content and pressure in coal [3]. Mine gas is one of the most serious safety hazards in coal mining [1]. It is reported that 19 gas accidents with 100 fatalities or more have occurred in Chinese coal mines from 1950 to 2012 [2]. The exploitation of coal seam gas (CSG) would reduce the incidence of coal-gas outbursts, as CSG pre-drainage can reduce the gas content and pressure in coal [3]. CSG is produced by lowering the pressure within the coal seam so that methane is released from the coal in the form of gas and brought to the surface accompanied by the water [5]. CSG production is closely related to water drainage which is a prerequisites for reducing the reservoir pressure and desorption of adsorbed gas [6,7]. Too fast a dewatering rate can lead to an unfavorable ultimate
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