Monte Carlo Simulation and Well Testing Applied in Evaluating Reservoir Properties in a Deforming Longwall Overburden

Abstract

During longwall mining, the intact strata start to deform and fracture as the mining face progresses. Gob gas ventholes (GGVs) are drilled from the surface over a longwall panel before mining to capture methane from the fractured zone. Due to fracturing and bedding-plane separations, reservoir properties change extensively. This poses a major problem for venthole designers and methane control engineers and may become a safety and health concern for underground work force due to unexpected methane emissions: it is difficult to predict the location of major strata separations and their temporal magnitudes to best locate the ventholes. Measurements obtained at different times during longwall mining may not be helpful for this purpose as strata deformation is a dynamic process and the results from different tests may not be lumped together to analyze the data collectively. This article uses a combination of Monte Carlo (MC) simulation and well testing methods to analyze multiple data sets obtained from a GGV at different longwall face locations. The aim was to determine the magnitude of average strata separation before conducting well test analyses to determine the properties of a deformed reservoir. MC simulation was used to process cross-correlated and uncertain data distributions obtained from measurements to generate a set of normally distributed values for each data type. These values were further used to project the amount of strata separation to the timing of well test. Finally, well-test analyses were used to interpret test data and to evaluate reservoir properties. h Net pay (m, ft) k Permeability (md, md) kh Flow capacity (md.m, md.ft) k/μ Mobility kh/μ Transmissivity P Pressure (kPa, psia) w Channel thickness (m, ft) q j jth flow rate (m 3 /d, bbl/d) q n nth flow rate (m 3 /d, bbl/d) r e External radius or radius of investigation (m, ft) r w Wellbore radius (m, ft) s Apparent skin factor t Time (h, h) t a Pseudo-time (h, h) t n nth flow period or superposition time Pseudo-pressure for gas (kPa 2 /Pa.s, psia 2 /cp)

Sequential Gaussian co-simulation of rate decline parameters of longwall gob gas ventholes

“…Karacan [7] predicted radii of investigations for operating GGVs as 578 and 2818 ft, depending on the locations, by use of pressure transient analyses of multirate drawdown test techniques. The results given in this paper also compare, within the order of magnitude, with the radius of investigation (~4000 ft) of a well in a completely different setting and known to produce from a bedding-plane separation [9] in a bounded gob reservoir.…”

“…The properties of fractured zones, mainly permeability, are determined through conventional pressure-and rate-transient well test analyses techniques that are used systematically and routinely for oil and gas [2][3][4][5][6]. Results showed that permeabilities of bedding plane separations can be as high as 150 Darcies, with average permeabilities (including fractures and intact formations) within the slotted casing interval of GGVs varying between 1 Darcy and 10 Darcies [7][8][9].…”

Coal mine methane: A review of capture and utilization practices with benefits to mining safety and to greenhouse gas reduction

“…Karacan and Goodman (2011b) analyzed the multiple rate data of a GGV tested in such an environment. However, since the existence of bedding plane separation was known and its thickness and location were known, the model was set up for an infinite acting fracture where the flow field was linear towards the wellbore in the separation zone.…”

Geostatistical modeling of the gas emission zone and its in-place gas content for Pittsburgh-seam mines using sequential Gaussian simulation

“…During mining of N-F panels, average methane production from GGVs is about 200 Mscfd, as opposed to 500-600 Mscfd from GGVs of G-K panels. This difference may be related to various factors discussed in Karacan (2009a) and differences in properties of the gob as a reservoir (Karacan, 2009c;Karacan and Goodman, 2011b). Similarly, methane rate from BF-2 averages around 2500-3000 Mscfd, where it is 1500-2000 Mscfd from BF-3.…”