Inference of strata separation and gas emission paths in longwall overburden using continuous wavelet transform of well logs and geostatistical simulation

Abstract

"Inference of strata separation and gas emission paths in longwall overburden using continuous wavelet transform of well logs and geostatistical simulation" (2014). USGS Inference of strata separation and gas emission paths in longwall overburden using continuous wavelet transform of well logs and geostatistical simulation Prediction of potential methane emission pathways from various sources into active mine workings or sealed gobs from longwall overburden is important for controlling methane and for improving mining safety. The aim of this paper is to infer strata separation intervals and thus gas emission pathways from standard well log data. The proposed technique was applied to well logs acquired through the Mary Lee/Blue Creek coal seam of the Upper Pottsville Formation in the Black Warrior Basin, Alabama, using well logs from a series of boreholes aligned along a nearly linear profile. For this purpose, continuous wavelet transform (CWT) of digitized gamma well logs was performed by using Mexican hat and Morlet, as the mother wavelets, to identify potential discontinuities in the signal. Pointwise Hölder exponents (PHE) of gamma logs were also computed using the generalized quadratic variations (GQV) method to identify the location and strength of singularities of well log signals as a complementary analysis. PHEs and wavelet coefficients were analyzed to find the locations of singularities along the logs. Using the well logs in this study, locations of predicted singularities were used as indicators in single normal equation simulation (SNESIM) to generate equi-probable realizations of potential strata separation intervals. Horizontal and vertical variograms of realizations were then analyzed and compared with those of indicator data and training image (TI) data using the Kruskal-Wallis test. A sum of squared differences was employed to select the most probable realization representing the locations of potential strata separations and methane flow paths. Results indicated that singularities located in well log signals reliably correlated with strata transitions or discontinuities within the strata. Geostatistical simulation of these discontinuities provided information about the location and extents of the continuous channels that may form during mining. If there is a gas source within their zone of influence, paths may develop and allow methane movement towards sealed or active gobs under pressure differentials. Knowledge gained from this research will better prepare mine operations for potential methane inflows, thus improving mine safety.Published by Elsevier B.V.

Analysis of gob gas venthole production performances for strata gas control in longwall mining

“…In the Upper Pottsville Formation, these potential intervals are generally concentrated around the interfaces of sandstone and limestone with shale-rich formations, within shales and especially along the Pratt coal interval, from which the GGVs are expected to capture the gas. 17 After mining of the E-1 panel was completed, GGVs still continued to produce high-quality methane but at lower rates. This flow behavior shows that there were still very permeable flow paths and gas availability within the strata influenced from mining even after the panel was completed.…”

“…In fact, a study based on the well logs of this area inferred that there will be bedding plane separations and horizontal fractures in Pratt coals and in the shale-sandstone transitions in deeper sections. 17 In order to control the gas that may be released in these zones and migrate towards the mine through fractures and bedding plane separations, six GGVs were drilled along the E-1 panel at different locations from panel start and from the tailgate (Fig. 5).…”

“…16 conducted studies on the effect of different geotechnical factors on characteristics of fracturing, gas sources, and gas flow paths for longwall operations in the United Kingdom. More recently, Karacan and Olea 17 predicted the potential intervals of strata separations using continuous wavelet transform (CWT) and generalized quadratic variations of well logs. The CWT matrix of coefficients was analyzed to locate the frequency and space parameters of singularities, which are precursors for discontinuities and weaknesses in the strata that are precursors for separation.…”

Enhancing Safety in the Polish High-Methane Coal Mines: an Overview

“…Through theoretical research and many investigations, this zone in the overlying strata is divided into three vertical zones named caving, fracture and constrained, respectively. Many researchers conducted the rules for determining these zones [1, 2,6,7,11,12,25,26,36,46,50,51]. A detailed analysis of the development of research on the determination of these zones was presented by Majdi et al [21].…”

A shared borehole approach for coal-bed methane drainage and ground stabilization with grouting

“…While ground stabilization aims to reduce the deformation of the overburden strata by backfilling and sealing mining-induced voids or fractures such as the gob area, 1 bedding separation zones, 2,3 and caved zones, 4-6 effective gas drainage relies on the generation of as many pressure-relief fractures as possible. [7][8][9][10][11] Thus, controlling measures for ground subsidence and gas emissions are generally implemented separately. Examples include ground stabilization using grout injection, as tested in the Chinese, 2,3 Australian, 12 and Polish 13 coal industries, and overlying seam CBM drainage through surface boreholes, as applied in the USA 14,15 and China, 16 The grout injection technique involves injection of fly ash slurry into mining-induced bedding separations (above a fractured zone) or into the gob area through surface boreholes (typically referred to as overburden grout injection 2,3 and gob area grout injection, 17 respectively), thereby reducing strata deformation and surface subsidence.…”