Devonian Shale Gas Production Research Articles

Devonian shale gas production: mechanisms and simple models : J Pet Technol V43, N4, April 1991, P470–482

This paper shows that, even without consideration of their special storage and flow properties, Devonian shales are special cases of dual porosity. The authors show that wile neglecting these properties in the short term is appropriate, such neglect in the long term will result in an under-estimation of shale gas production.

Devonian Shale Gas Production: Mechanisms and Simple Models

Summary We show that, even without consideration of their special storage and flowproperties, Devonian shales are special cases of dual porosity. We also showthat, while neglecting these properties in the short term is appropriate, suchneglect in the long term will result in an under-estimation of production. Introduction Tremendous accumulations of gas occur in the Devonian shale formations ofthe eastern U.S. Because the formations are so tight, gas can be produced onlywhen extensive networks of natural fractures exist. In areas where fracturingis sufficient to promote production, the gas basically will flow theproduction, the gas basically will flow the short distance from storage in thetight rock matrix to the fracture network and then through the fracture networkto the producing well. Only minute quantities of gas flow directly from therock to the well. These production characteristics make shale gas productioncharacteristics make shale gas reservoirs examples of so-called dual-porositybehavior. The problem with the term "dual-porosity" in shales, however, is that shales have very little open porosity. Unlike conventional gasreservoirs, where gas is stored in the open pore space of the rock, shalesstore an enormous amount of gas in a sorbed (adsorbed or absorbed) state. Thebehavior of fluid flow in the shale matrix also deviates from that of aconventional gas-reservoir/rock matrix. The permeability of conventionalreservoir rock is permeability of conventional reservoir rock is really amacroscopic average of viscous open-channel flow occurring on a pore level. Inshales, the conduits are generally so small that only a few molecules can slipthrough the openings at any given time. Open-channel flow cannot exist for themost part; thus, flow through the shale matrix results predominantly frommolecular diffusion. predominantly from molecular diffusion. The objectives ofthis paper are to clarify the roles that these unusual storage and flowproperties have in the general dual-porosity properties have in the generaldual-porosity system; to estimate their effects on the production behavior ofcommon Devonian shale production behavior of common Devonian shale wellsheuristically; and to show that even without the unusual storage and flowbehaviors, Devonian shale gas reservoirs are special cases of dual porositybecause of the tight nature of the rock matrix. This paper does not present newpredictive models for the behavior of shale reservoirs, nor does it usesophisticated numerical models normally required for predictions of theperformances of these nonlinear systems. The objectives of this paper areachieved through the application and intuitive comparison of the most simplemodels available for shale reservoirs. Theory Dual-Porosity Equation Formulation. Devonian shale gas reservoirs areexamples of the classic dual-porosity, fractured-reservoir model. In thisdual-porosity model, the reservoir is composed of matrix elements andfractures. Fig. 1 shows two examples of very idealized fractured reservoirs. The matrix, represented by individual blocks in Fig. 1, is a portion of thereservoir that can store large quantities of gas, but it does not have theconductivity to transport gas for long distances. The fractures, whichpartition the matrix elements, can transport the partition the matrix elements, can transport the gas but have limited storage capability Production fromdual-porosity reservoirs differs from the production of conventionalsingle-porosity systems, where gas flows directly from the formation to thewell. In fractured reservoirs, gas flows through the fracture network to thewell. The fracture network, in turn, is being constantly recharged by flow fromthe matrix elements. Given a sample control volume in a fractured reservoir(Fig. 1a or 1b), a control volume mass balance can be performed. − = 0, Ac t ..................(1) where v f = average flow velocity through the fractures with respect to anentire cross section, A c = external surface area of the control volume, qt =total mass flow rate from the matrix to the fractures within the controlvolume, and delta x delta y delta z = magnitude of the control volume. The plussign in Eq. 1 results from the assumption that flow from the matrix to thefracture is a positive quantity. For conceptual simplification, it is assumedthat density and pressure are approximately constant in the control volume. Because the total matrix recharge includes flow from all the individual matrixelements, the total matrix flow rate can be rewritten as qt = N (q ma), where qma is the average volumetric flow rate per element, and N is the total numberof elements in the control volume.

Geologic Model for Identification of Higher Devonian Shale Gas-Production Potential

Areas of higher gas production within the Devonian shales of the Appalachian basin may be identified with the aid of an integrated geologic model. Lithology and fractures combine to form complex reservoir systems. The search for these reservoirs is aided by integration of log lithofacies analysis and interpretations of seismic data. Gas Research Institute - sponsored research has shown that in areas where siltstones and silty shales occur, they contribute most of the gas production. In black shale - dominated areas, thin silt lamination, in certain sequences, appears to provide permeability for gas flow. These lithological sequences relate to lithofacies, which can be mapped, providing the means of predicting occurrence and thickness. Natural fractures provide higher permeability pathways for gas flow and often are associated with Devonian structural trends. These trends are frequently the result of reactivation of preexisting basement structures. Although some fractures may have been formed during early tectonic events, many may be due to relatively recent reactivation along older zones of weakness. Reactivated faults often propagate upward through the Paleozoic section and are identifiable using single-fold seismic data. The structural expression of the fault zones is often broad, low-relief clinoforms. These are internally complex as identified onmore » seismic sections. A geologist may look for areas of higher probable gas production by locating areas that exhibit elements of the geological model. Through the integration of core, logs, seismic, and well-test and production data, a localized reservoir model may be constructed aiding in the siting and completion of wells.« less

Identification of Gas-Productive Shale Intervals in Upper Devonian of Southern Appalachian Basin: ABSTRACT

Devonian shale gas production is influenced strongly by lithology as well as gas saturation. Petrographic analysis reveals five rock types in the shale interval: (1) clean sandstones and siltstones, (2) shaly sandstones and siltstones, (3) silty shales, (4) gray shales, and (5) black shales. The permeability of each is a function of pore geometry. Rocks with the highest permeabilities are characterized by intergranular and chaotic pore systems. The lowest permeabilities occur in strongly laminated pore systems. Pore geometry, lithology, and mineralogy are controlled by depositional environment. Thus the distribution of rock types, their permeability, and mineralogy are ordered, i.e., nonrandom.

Interpretation of Natural Controls on Devonian Shale Gas Production from Seismic Data: ABSTRACT

Columbia Gas and the Gas Research Institute are studying a set of Devonian shale gas wells in southwestern West Virginia to determine the geologic controls on shale gas production. Approximately 25 line-mi of Vibroseis seismic data were recorded to (1) evaluate the regional geologic setting and structural style of the area; (2) evaluate relationships among the regional geology, a high producing shale gas area, and a postulated fault zone; (3) evaluate the local geologic setting of three study wells; and (4) attempt to recognize stratigraphic controls on shale gas production. Several initial conclusions concerning production controls have been reached. (1) Basement faulting extended up through the shale section, and resultant fracturing influenced gas production in the northern area of the seismic survey. (2) Reflection patterns in the shale contain information on the lithologic character of the shale. Local lenses of silt or sandy shale are probably present within the more fine-grained shale section. These lenses may be geologic features with increased permeability. (3) Areas of dimmed reflection energy on seismic lines correlate with areas of high gas reserves, suggesting that these areas are fractured shale. Information from the seismic survey is being integrated with core, log, and well-test data to understand the active controls on shale gas production in the study area. End_of_Article - Last_Page 1439------------

Importance of Regional and Local Structure to Devonian Shale Gas Production from Appalachian Basin: ABSTRACT

Over 1 Tcf of high-quality natural gas has been produced from Devonian shales during the past 100 years. The shale is its own source, seal, and reservoir; natural features within the shale form the reservoir. Organic shale is the prime requisite for production, but productivity relates to the presence of open fractures so that one can presume that abnormally productive trends correspond to open fracture zones. This presumption is supported by the direct association of linear zones of high production with linear basement structure detailed in three fields studied. The blanketlike nature of the lower production ubiquitous in southern West Virginia and eastern Kentucky probably relates to a fracture facies developed within the shale and controlled by shale lithology and Appa achian tectonics. End_of_Article - Last_Page 1671------------

Potential New Sources of Natural Gas

The U.S. DOE's gas resource program aims at resolving existing technological barriers to effective recovery of natural gas from Devonian shale and methane from coalbeds. Upon completion of these projects, DOE expects the technology developed jointly with industry to result in wide-scale recovery and use of these new sources of natural gas. Introduction Natural gas continues to be one of the major sources of energy produced and used in the U.S. Declining gas reserves and curtailment of supplies have reemphasized the major influence this energy source has on the U.S. economy. The U.S. DOE is investigating several options for increasing the supply, including a program for unconventional gas recovery (UGR). Four UGR projects currently are being assessed: western tight gas sands, geopressured reservoirs, Devonian shales, and methane from coalbeds. Both the Devonian shale and methane from-coalbeds projects are paramount in this assessment, since they underlie a large section of the U.S. The eastern (Devonian) shales contain a vast, essentially unexplored volume of natural gas. This area could represent new gas recovery from approximately 250,000 sq miles throughout the U.S. Studies by the government and industry have been focused on shale characterization to determine the magnitude of potential gas reserves and technology development needed to improve current state-of the-art stimulation techniques. The initial R and D results have shown promise and point out the technology needed for successful development. The goal of the methane-from-coalbeds project is to provide natural gas from coal seams. While coal itself is recognized as a major energy source, it also contains vast quantities of methane gas. This methane source is not new, since coal mine operators have been aware of its presence and release into the atmosphere during mining operations. Technology studies are being conducted to learn the production potential of this methane and to show how this gas potential of this methane and to show how this gas may be put to widespread use. Devonian Shales The Devonian shales of the Appalachian, Michigan, and Illinois basins have produced natural gas since the 1800's. These shales in the eastern U.S. (Fig. 1) contain a high volume of gas. Independent estimates of the recoverable gas range from 3 Tcf to several hundred times that amount. To date, the gas produced from these shales has been limited to an produced from these shales has been limited to an estimated 2.5 Tcf because of the unpredictable behavior and economics shown by existing Devonian shale wells. Similarly, these uncertainties have restricted private-sector R and D funding and development of technology needed for Devonian shale gas production. JPT P. 703

Devonian Shale Gas Production, Photolineaments, and Geology in Central Appalachian Basin of West Virginia: ABSTRACT

Remote-sensing methods involving photolineament mapping have been used for hydrocarbon prospecting with varying success. Much natural gas in the Appalachian basin is in formations of very low primary permeability; therefore production is from fractures. Natural fracture zones often appear as photolineaments on various types of remote-sensing images. In some parts of the basin, photolineaments have been demonstrated to be loci of improved production of natural gas. Photolineaments are also often indicative of surface and subsurface fractures and stress fields. A test was made of these relations for Devonian shale gas production in the central part of the Appalachian basin. Photolineaments were derived from satellite and aircraft images at a scale of 1:250,000; fracture dat were measured on surface outcrops and in one core; and natural gas production data were open flows as given on drillers' logs. Fractures generally parallel nearby photolineaments, but the relation is not simple. Natural open flows tend to be lower on or near photolineaments than between them, whereas open flows of stimulated wells do not appear to be related to photolineaments. Such relations may be due to venting of gas through natural fractures, and the stimulated wells, having new fractures, release previously trapped gas. The disparity between results from this project and those of others may be due to several factors: the allochthonous sedimentary sheets present in the area of some other projects do not extend into the central part of the Appalachian basin; the lithologies differ i significant ways; and the methods used for mapping and the types of photolineaments differ. End_of_Article - Last_Page 847------------

Devonian Shale Gas Production in Central Appalachian Area: GEOLOGICAL NOTES

Devonian Shale Gas Production in Central Appalachian Area: GEOLOGICAL NOTES

Developments in Appalachian Area During 1941: ABSTRACT

Developments in Appalachian Area During 1941: ABSTRACT