This paper (SPE 52607) was revised for publication from paper SPE 36737, first presented at the 1996 SPE Annual Technical Conference & Exhibition, Denver, 6-9 October. Original manuscript received for review 25 October 1996. Revised manuscript received 17 August 1998. Paper peer approved 1 September 1998. Summary In naturally fractured formations such as coal, permeability is sensitive to changes in stress or pore pressure (i.e., changes in effective stress). This paper presents a new theoretical model for calculating pore volume (PV) compressibility and permeability in coals as a function of effective stress and matrix shrinkage, by means of a single equation. The equation is appropriate for uniaxial strain conditions, as expected in a reservoir. The model predicts how permeability changes as pressure is decreased (i.e., drawdown). PV compressibility is derived in this theory from fundamental reservoir parameters. It is not constant, as often assumed. PV compressibility is high in coals because porosity is so small. A rebound in permeability can occur at lower drawdown pressures for the highest modulus and matrix shrinkage values. We have also history matched rates from a boomer well in the fairway of the San Juan basin by use of various stress-dependent permeability functions. The best fit stress/permeability function is then compared with the new theory. P. 539

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CopW!Qhf 1SS6 Sceiaty C4Petrolwm Engineers, k This pcpar was prqmred for p+esentatwn at the 19% SPE Annual Technical Conferorw am Exh!bitmn iwtd m Damfer, Co&ade U S A S-9 OUcWr 199S TM POWwas aaktad for preaanlatmn by an SPE Progrsm Ccfnm!ftee following ravmw of !nformatna mnlmmd m an abstract submmd by tha author(s) ConlonN of lha Pqx!r a$ prosanled, hava not ban r.vmwad by tlw Scaety of Potrolawn Eogmeors and are vA@ 10 ccfroclm+l by tlw author(s) Tha m@l@r@ qs praaonlad do98 not !wco$sar,ty reflect any posilon of tha SOWty of Pelrohum Engmears it ufksrs 0! members Papers WOWWd a! SPE meetngs qm lubIact !0 publtitmn rovmw by Edflorml Ccmmmeaa of the Scaety of POtrOlOUm Engmers PwmIsmc+I to copy IS feslricfod to an abslracl of not more Ihan 30iJ Words. llluWatKnIa may nol ba cxmod Th@ qbstracl should cnnlam consmcuo. ! q duwfwadgmant of wkro mnd by w+wr"fha Papar w-s piesentad Wnlo L!br8rmri 'SPk P O Box 83383S R!chardsm TX 750S3-3S3S U S A, ffix 01-21 4-9S2-9435 AbstractIn naturally fractured formations, such as coal, permeability is sensitive to changes in stress or pore pressure (i.e., effective stress). This paper presents a new theoretical model for calculating pore volume compressibility and permeability in coals as a fun~on of effective stress and matrix shrinkage, using a single equation. The equation is appropriate for uniaxial strain conditions, as expected in a reservoir. The model predicts how permeability changes as pressure is decreased (i.e., drawdown). Pore volume compressibility is derived in this theory from fundamental reservoir parameters. It is not constant, as often assumed. Pore volume compressibility is high in coals because porosity is so small. A rebound in permeability can occur at lower drawdown pressures for the highest modulus and matrix shrinkage values, We have also history matched rates from a "boomer" well in the fairway of the San Juan basin using various stressdependent permeability functions. The best fit stress-permeability function is then compared with the new theory.

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SPE Members Abstract This paper presents the results of a compositional simulation study to investigate the relationship between the displacement mechanism, the minimum enrichment requirement, and the oil recovery efficiency for a rich gas system. The simulation results indicated that the displacement mechanism over a wide range of solvent enrichment was governed by a condensing/vaporizing process; however, the relative importance of the condensing or vaporizing mass transfer processes, and their impact on the overall displacement efficiency, was a function of the enrichment level. For displacements simulated at or above the slim tube minimum enrichment (ME) level (determined from the recovery breakover point on a recovery versus enrichment plot), recovery was high and the process behaved very similar to a classical condensing mechanism. Displacements simulated below the slim tube ME were less efficient; however, both condensing and vaporizing mass transfer processes contributed to the overall recovery processes contributed to the overall recovery efficiency. At enrichment levels considerably above the slim tube ME, the effect of the condensing mass transfer process on the displacement became less significant and the behavior approached that of a classical vaporizing mechanism. The slim tube technique for determining the ME requirement based on simulation results was further compared with two other computational methods: one based on Hutchinson and Braun's multiple contact mixing, and the other based on displacement behavior at the limit of zero dispersion. For this rich gas system, the minimum enrichment level predicted by these methods was significantly higher than the slim tube ME and corresponded to conditions of thermodynamic miscibility development by a classical vaporizing mechanism. Despite such a high enrichment requirement needed to achieve thermodynamic miscibility, the oil recovery did not increase significantly beyond that achieved at the slim tube ME. Simulation results for the condensing/vaporizing system were also compared with those predicted by a classical condensing mechanism. These comparisons showed that:for a classical condensing mechanism, the thermodynamic and the slim tube methods predicted the same minimum enrichment requirement;at low dispersion the two mechanisms showed similar displacement behavior; andhigher values of dispersion had a bigger impact in reducing the displacement efficiency of the condensing/vaporizing mechanism compared with the classical condensing process. Introduction Hydrocarbon miscible gas flooding has long been recognized as a viable enhanced recovery method by the petroleum industry. To improve the economic and technical success for projects involving rich gas injection, it is essential to have a good understanding of the displacement mechanism and minimum enrichment requirement for achieving an efficient displacement. Traditionally, the displacement mechanism of rich gas drives has been assumed to be via a condensing process in which the intermediate components of the solvent enriched the reservoir oil toward the point of miscibility. However, the work of Stalkup point of miscibility. However, the work of Stalkup in 1965 and recent investigations have challenged this traditional concept for some rich gas displacements. Zick showed that a very efficient oil recovery can be achieved through a combined condensing/vaporizing mass transfer interaction between the reservoir fluid and the injected solvent. His study was based on limited phase behavior experiments and equation of state (EOS) calculations for a rich gas system. P. 35