Daniel J. Soeder

Porosity and Permeability of Eastern Devonian Gas Shale

Etude de huit carottes de schistes a gaz provenant du bassin des Appalaches (Etats-Unis). Determination de la permeabilite, de la porosite et du potentiel productif des schistes

The Marcellus Shale: Resources and Reservations

The Marcellus Shale is an organic-rich, sedimentary rock formation in the Appalachian Basin of the northeastern United States that contains significant quantities of natural gas. Published estimates of the amount of gas that may be recoverable from the Marcellus Shale have been higher than 1.42 trillion cubic meters, or 50 trillion cubic feet [Engelder and Lash, 2008]. The recovery of commercial quantities of gas from a low-permeability rock like the Marcellus became economically possible with the application of directional drilling technology, which allows horizontal boreholes to penetrate kilometers of rock, combined with staged hydraulic fracturing to create permeable flow paths into the shale. Each hydraulic fracturing treatment may use more than 11 million liters of water (3 million gallons), which must then be recovered from the ground to allow gas flow [Harper, 2008].

Porosity, Permeability, and Pore Structure of the Tight Mesaverde Sandstone, Piceance Basin, Colorado

Special core analyses on 44 tight Mesaverde sandstone samples from the U.S. DOE Multiwell Experiment (MWX) were combined with petrographic investigations to relate the porosity and permeability of the cores to the pore structure of the rocks. Core analysis was performed on 1-in. (2.54-cm) -diameter horizontal plug samples with a computerized steady-state-flow measuring device that routinely measures gas flow rates with a resolution of better than 10/sup -6/ std cm/sup 3//s. All samples were selected from intervals expected to be gas-productive on the basis of wireline well logs and were taken from the portion of the interval that showed the lowest gamma ray log response. The core plugs were measured for dry permeability to gas, relative permeability at various water saturations, porosity to gas, and PV compressibility. Petrographic samples were taken directly off the plug ends and were analyzed with both an optical microscope and a scanning electron microscope (SEM). The petrographic study was explicitly directed toward observing the flow paths and pore structure deduced from the core analysis data.

Porosity and Permeability of Tight Sands

Detailed analyses of more than 50 core samples of western tight sands have resulted in several unanticipated observations that are set forth in this paper. Core analyses performed under stress representative of producing conditions provided data on porosity, pore volume compressibility, stress dependence of permeability to gas, and slope of the Klinkenberg plot (permeability at constant net stress vs. the inverse of pore pressure). Scanning electron microscope (SEM) and petrographic microscope analyses were performed on samples cut from the ends of core plugs tested. The microscopic studies were explicitly directed toward observing the <0.1 micron flow path openings deduced from permeability data. All samples were from depths that were either known to be gas producers or judged likely to be producers on the basis of wireline log analysis.

Shale Gas Development in the United States

Although natural gas has been obtained from organic-rich shales in the United States since the first commercial gas well was produced in 1821 to provide gas light to four commercial establishments and a mill in the small town of Fredonia, New York, large-scale shale gas production is a recent phenomenon. Assessments of the geological and engineering challenges of shale gas resources were performed in the 1970s and 1980s, as new domestic energy sources were sought in response to an oil embargo imposed upon the United States, and the resulting “energy crisis” that followed. The amount of natural gas present in the shales was found to be significant, but commercial production had to await advances in drilling and completion technology that came about in the 1990s. The new technology allowed for the economic development of this resource in the 21st Century.

Pore Geometry in High- and Low-Permeability Sandstones, Travis Peak Formation, East Texas

Core analysis data have been correlated with petrographic observations of pore geometry for a number of sandstone samples from the Travis Peak Formation in East Texas. The results have also been compared with pore structure and core analysis data that had been obtained earlier for tight sands from Rocky Mountain basins. 14 refs., 13 figs., 2 tabs.

Clay: Geologic Formations, Carbon Management, and Industry

The term clay is used interchangeably for the particles and the minerals, the latter commonly referred to as clay minerals to distinguish them from the clay particle size. Clay sediments are typically deposited in quiet-water environments, settling out as fine-grained mud, which may then be buried and lithified into shale. The quiet-water depositional environments are favorable for deposition of organic material as well, which over geologic time, may result in the shale becoming a source rock for petroleum and natural gas. Hydrocarbons have traditionally been produced from porous and permeable reservoir rock, where they had migrated from source rock and become concentrated in geologic traps. The recent development of “unconventional resources” like shale gas and tight oil has allowed hydrocarbons to be produced directly from the source rock. Conventional natural gas and oil reservoirs that have been depleted of hydrocarbons provide a viable option for secure carbon storage because there is a known trap and seal. CO2 can potentially be used for enhanced recovery of the hydrocarbons and for pressure management in shale, to minimize the loss of permeability that comes from increased net stress during drawdown. As a side note, natural clay-rich geomaterials can be used in agriculture, industrial processes, and for clay liners in chemical and radioactive waste disposal sites. The purpose of addressing geomaterials here is to give the readers an idea of the enormous breadth of each subject and point them toward other resources for additional information.

Groundwater Quality and Hydraulic Fracturing: Current Understanding and Science Needs: Groundwater

Hydraulic fracturing (fracking) is a process used for the stimulation and production of ultra-low permeability shale gas and tight oil resources. Fracking poses two main risks to groundwater quality: (1) stray gas migration and (2) potential contamination from chemical and fluid spills. Risk assessment is complicated by the lack of predrilling baseline measurements, limited access to well sites and industry data, the constant introduction of new chemical additives to frack fluids, and difficulties comparing data sets obtained by different sampling and analytical methods. Specific recommendations to reduce uncertainties and meet science needs for better assessment of groundwater risks include improving data-sharing among researchers, adopting standardized methodologies, collecting predrilling baseline data, installing dedicated monitoring wells, developing shale-specific environmental indicators, and providing greater access to field sites, samples, and industry data to the research community.

The Assessment of Instruments for Detecting Surface Water Spills Associated with Oil and Gas Operations

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A modeling study of air migration from a drilling well to the surrounding aquifer in Appalachia

We investigate the migration of high pressure air in a fractured groundwater aquifer during the process of drilling a shale gas well using the air hammer technique. We consider a scenario where the high pressure air used for the drilling process could leak into the aquifer. A three dimensional conceptual numerical model was developed using TOUGH2 to quantify the spatial and temporal impact of such a leak on the surrounding aquifers. We have also studied the sensitivity of the numerical model to the air pressure used during the drilling process. Our simulations indicate that the air migrated through the fractures and was released approximately 300 m away from leaking area in just a few hours.