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USING MULTI-RATE DRAWDOWN WELL TESTS FOR ESTIMATION OF GOB GAS RESERVOIRS - MEASURMENTS CONCEPT and POLISH EXAMPLE FROM UPPER SILESIAN COAL BASIN

Jacek Hendel, Lukasz Banisz

Abstract

During longwall mining extraction, extremely high permeable spaces, called пїЅgobsпїЅ, are often created within crushed strata. Those gobs and abandoned: pathways, tunnels, ventilation entries or shafts, could be capacious storage deposits for coal-released gases, particular methane. This methane may be valuable source of both, energy and heat production, in active and post mining areas. Gases form crushed, fractured zones are explored by vertical boreholes drilled from surface, called gob gas ventholes (GGVs). GGVs are almost every time equipped with exhausters on wellhead for support gasses suction form strata to power engine/gas pipes. However, the main problem of commercial gob gas production is property estimation of Gas In Place (GIP) reserves. Prediction of GIP in gobs and overlying strata are extremely difficult because of complex relations of porosity and permeability of gobs and its changes in time. Lifespan of GGVs are also limited by multiple other factors e.g. boreholes location and diameter, surface elevation, distance between borehole and mining panel, time after coal extraction, atmospheric pressure etc. This article shortly presents concept of implementation multi-rate drawdown well test for gob gas reservoir estimation method for GIP estimations. Basic mathematical theory of well tests was presented. Within this article, authors use multi-rate drawdown well tests technique to estimated gas (methane) stored in gobs. Production data from gas well located in Upper Silesian Coal Basin were used. Authors used IHS Markit Well TestпїЅ 2016 software to estimated reserves and permeability and thickness of this specific reservoir. Sensitive analysis of porosity and coal seams thickness influence on gas stored amount was also made. Authors considered porosity values from 17% to 40% and seam thickness between 9 and 60 meters. As a conclusion, comparative results of implementation multi-rate drawdown well tests technique in American and Polish cases were presented.

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DOI resolver: doi.org/10.5593/sgem2017h/15/s06.082

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