Application of Leaky Aquifer Type Curves for Coalbed Methane Characterization

Application of Leaky Aquifer Type Curves for Coalbed Methane Characterization D.O. Cox; D.O. Cox Questa Engineering Corporation Search for other works by this author on: This Site Google Scholar P.R. Onsager P.R. Onsager Questa Engineering Corporation Search for other works by this author on: This Site Google Scholar Paper presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, September 2002. Paper Number: SPE-77333-MS https://doi.org/10.2118/77333-MS Published: September 29 2002 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Get Permissions Search Site Citation Cox, D.O., and P.R. Onsager. "Application of Leaky Aquifer Type Curves for Coalbed Methane Characterization." Paper presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, September 2002. doi: https://doi.org/10.2118/77333-MS Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex Search nav search search input Search input auto suggest search filter All ContentAll ProceedingsSociety of Petroleum Engineers (SPE)SPE Annual Technical Conference and Exhibition Search Advanced Search AbstractMany coalbed methane (CBM) reservoirs are part of larger aquifer units that also contain water-saturated sandstones. To account for connections to aquifers, this paper presents "leaky aquifer" type curves for radial flow in leaky aquifers with wellbore storage and skin, and for vertically fractured wells in leaky aquifers. The type curves are similar to those with a constant pressure boundary, and in some cases, the characteristic flat derivative of infinite-acting radial flow may be completely masked. Examples from several CBM basins are presented that demonstrate the methodology.IntroductionCoalbed methane (CBM) has developed into an important part of the U.S. gas supply over the last 20 years. In 1982, production from CBM reservoirs was virtually nonexistent, but recent tabulations show CBM reservoirs accounted for 7% of the total U.S. dry gas production and 9% of U.S. dry gas reserves in 20001.Coalbed methane reservoirs are considered unconventional gas reservoirs, for three main reasons2. First, in addition to being the reservoir rock, the coal is also the source rock for the gas in most cases. Secondly, the gas is stored by adsorption in the coal, rather than through the normal mechanism of compression by increased pressure. Finally, coal beds contain natural fractures, called cleats, which are commonly filled with water at the time the reservoir is discovered. For this reason, most CBM reservoirs require dewatering before they produce commercial volumes of gas.In some instances, water influx from other aquifer units can inhibit the dewatering of the coal and thereby limit coalbed methane recovery3. For this reason, methods to characterize the degree of connection between the coal and other units are needed.It has long been recognized in the groundwater industry that many aquifers have imperfect seals, and are in connection to other aquifer units through low permeability, "leaky" confining layers. In the late 1950's, Hantush and Jacob4,5, and Hantush6–8 published a series of papers with techniques to account for different boundary conditions and varying degrees of connection between aquifers. The Hantush-Jacob methodology is known in the groundwater industry as a "leaky aquifer" model. The idealized Hantush-Jacob leaky aquifer model of interest here is the one that assumed a constant pressure boundary at the top of the confining layer. This assumption is equivalent to having much greater horizontal permeability in the supporting aquifer than the vertical permeability of the confining layer. In 1969, Neuman and Witherspoon9 devised a more general model to account for finite permeability and volume in the aquifer providing pressure support.The leaky aquifer approach entered the petroleum literature for investigating the effectiveness of seals for gas storage projects in the 1960's and early 1970's10–12, but has had little use in the petroleum industry for pressure transient analysis. Instead, most of the petroleum-industry papers assume generalized multi-layer models with varying properties in each layer. Most of these papers consider well testing in layered formations with crossflow where all layers are open at the well13–17. In most cases with CBM wells, only the coal is producing, and the adjacent sandstone units that are in communication with the coal are commonly cased off. This type of boundary condition was considered in the general case with the properties of all layers being known by Witherspoon and Neuman12, Streltsova18, and Ehlig-Economides and Ayoub19. A recent paper by Guo, et al.20 considered either constant pressure or no-flow boundaries on the confining layer; this formulation is basically the same as that of Hantush and Jacob4. Keywords: application, radial flow, storage, leaky aquifer type curve, aquifer system, fractured well, coefficient, type curve, aquifer, leakance Subjects: Formation Evaluation & Management, Drillstem/well testing This content is only available via PDF. 2002. Society of Petroleum Engineers You can access this article if you purchase or spend a download.

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