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Saurabh S.,Southern Illinois University Carbondale | Harpalani S.,Southern Illinois University Carbondale | Singh V.K.,Southern Illinois University Carbondale | Singh V.K.,Core Laboratories
International Journal of Coal Geology | Year: 2016

Significant increases in permeability of coal with continued production of coalbed methane (CBM) is a well-accepted phenomenon, particularly in the San Juan basin in the US and Surat basin in Australia. Modeling this increase is either based on the resulting increase in fracture porosity of coal or the associated changes in stresses as a result of the sorption-induced strain. This paper combines the experimental results of a study that measured sorption-induced coal matrix volumetric strain with depletion and a model proposed to estimate the associated changes in stress. The overall changes in stress, resulting from the combined effect of the poro-mechanical behavior and sorption-induced strain, were estimated by introducing a Biot-like coefficient. Plotting the stress path followed during depletion along with the failure envelope for the coal-type clearly showed that shear failure of coal is possible due to anisotropic loading resulting from a large reduction in the horizontal stresses. This would explain the large increases in permeability, typically observed in CBM operations. Finally, a permeability model was developed using the Biot-like coefficient, and assuming transversely isotropic behavior of coal. A comparison of the experimental and modeled permeability results showed that the model works well as long as coal does not fail. However, it demonstrates that a permeability model, incorporating failure of coal, is warranted for reliable prediction of permeability variation. © 2016 Elsevier B.V. Source


Gentzis T.,Core Laboratories
Energy Sources, Part A: Recovery, Utilization and Environmental Effects | Year: 2011

This study documents a comprehensive investigation of the in-situ stress magnitude and orientation data from borehole breakouts, mini- and micro-fracture tests, bulk density logs and other stress data, for strata in or adjacent to the Mannville Group and Edmonton Group coals in a part of the west-central Alberta Basin, Canada. The horizontal in-situ stress orientation data can be used to plan horizontal well trajectories in the coal strata of interest to intersect natural fractures and cleats possessing the highest permeability. Horizontal stress orientation data can also be used to assess the effects of well trajectory or borehole collapse risks during drilling, completions, and production operations. Subtle variations in the vertical total stress in the study area may reflect similar trends in the mean effective stress. However, this would need to be confirmed with more complete horizontal stress magnitude and formation pressure data. Copyright © Taylor & Francis Group, LLC. Source


Gentzis T.,Core Laboratories
Energy Sources, Part A: Recovery, Utilization and Environmental Effects | Year: 2011

This study describes the main technical challenges encountered while drilling the first horizontal coalbed methane well ever attempted in Canada. The well was drilled in the deep (>1,300 m), low-permeability (1-3 md) coals of the Lower Cretaceous Mannville Formation in the central Alberta Plains. The study also discusses the investigation that followed the occurrence of a few unexpected events during the drilling phase of the well, as well as the stability analysis of the horizontal borehole conducted. Suggestions are made on how to best mitigate the risk of getting the drill pipe stuck and potentially losing the hole along with drilling equipment. The findings are very instructive and should have an application in geological settings similar to those found in the Alberta Basin. Any modifications on how to drill horizontal wells in coal seams present at great depths and under high in-situ stresses must take into account the geomechanical properties of the target seam through triaxial testing, which should include obtaining the non-linear Hoek-Brown failure criterion and testing the coal core for very low confining stress. The lab-derived strength data may also have to be downgraded in order to predict, more realistically, the radius of the yielded zone around the wellbore and the horizontal borehole stability using geomechanical software, such as STABView and FLAC. © 2011 Copyright Taylor and Francis Group, LLC. Source


Gentzis T.,Core Laboratories
Energy Sources, Part A: Recovery, Utilization and Environmental Effects | Year: 2011

Borehole stability analysis was conducted using STABView and FLAC geomechanical modeling for a horizontal coalbed methane well in the San Juan Basin. The objective was to determine whether the coal seam at 3,000 ft depth would yield under certain drilling and production conditions. Triaxial tests using 1-inch diameter core plugs were performed to construct the Mohr-Coulomb and Hoek-Brown failure envelopes. A reduction of the peak coal strength by approximately 30% was needed to correct the scale-dependent strength to a value that was thought to be more representative for a 4.75-inch diameter horizontal borehole. No yielding was predicted at underbalanced and overbalanced drilling conditions when the uncorrected M-C and H-B lab strength data were used. A 30% reduction in peak strength predicted no yielding during overbalanced drilling and only minor yielding during underbalanced drilling or production. The maximum extent of the yielded zone at BHP of 100 psi predicted by STABView was 37% over gauge. FLAC gave directionally similar but slightly more conservative results compared to STABView. A reasonable amount of yielding and subsequent detachment of the coal is expected along the horizontal well at the highest drawdown pressures. Stability analysis showed that drilling a horizontal well in Coal A overbalanced would be possible. Drilling the same coal seam underbalanced will produce a rim of yielded coal but no catastrophic failure, provided that the coal is not highly fractured. Copyright © Taylor & Francis Group, LLC. Source


Gentzis T.,Core Laboratories
International Journal of Coal Geology | Year: 2013

The Paleocene-age Wasatch and Fort Union Formations in the study area contain numerous coal seams that are interbedded with shale, sandstone, and conglomerates. Individual seam thickness ranges from 2 to 15ft (0.6-4.6m) and aggregate thickness ranges from 64 to 163ft (19.5-49.7m). The coals are of sub-bituminous to high-volatile C/B bituminous rank (Ro,ran=0.47 to 0.59%) and are found at depths from <2000 to 6000ft (610-1579m). Gas content ranges from 115 to 263scf/t or 3.59-6.92cm3/g (ARB), which is higher than expected for the coal rank. The coals appear to be close to full saturation, based on a comparison of adsorption isotherms with desorption data. The coals pinch-out updip in the subsurface forming a stratigraphic trap, which may explain the high gas content and the mixed biogenic-thermogenic origin supported by isotopic analysis. Gas composition is 97% methane with 3% inerts, and the gas has a heating value of 990BTU/ft3. The coals are relatively clean (average ash content is 15wt.%) and have fast diffusivity (63% of total gas desorbs in <16h). A valid pressure transient test in a coal seam at 3140ft (957m) depth showed permeability to be 14mD (un-stimulated). The coal reservoir was normally-pressured reservoir (0.44psi/ft or 9.7kPa/m) but was heavily damaged (skin of +23). Another coal seam located at 4100ft (1250m) had a modeled permeability of 2.6 to 4.6mD, which was estimated from long-term water production. Potential recoverable resource is thought to be 700BCF (billion cubic feet or almost 20 billion m3) with wells drilled on 80-acre spacing, which translates to a gross EUR of 1.4BCF per well for the 500 well locations identified in the developable acreage. Based on numerical simulation using the simplistic FeketeTM model, peak coal gas rate is expected to be 320MCF/D (9014m3 per day) after 36months, and production should remain at that level for 12months before declining at a rate of 10% per year. Conventional gas potential also exists in numerous intervals within the Upper Fort Union and Wasatch Formations sandstones at depths shallower than 2000ft (526m). Few wells drilled decades ago, having perforations in these sands, showed IP (initial production) rates ranging from 500MCF/D (14,084m3 per day) to 12MMCF/D (338,028m3 per day). A 24-inch (60cm) diameter pipeline that has an operating pressure of 800-1000psig, a tap, and a capacity of 275MMCF/D (7.74 million m3 per day) crosses the study area. The pipeline is connected to the Colorado Interstate Gas mainline, which provides gas to markets in the eastern United States. © 2012. Source

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