uGeneral statement

uFigure captions

uType-producing areas

uProduction controls

uBibliography

uGeneral statement

uFigure captions

uType-producing areas

uProduction controls

uBibliography

uGeneral statement

uFigure captions

uType-producing areas

uProduction controls

uBibliography

uGeneral statement

uFigure captions

uType-producing areas

uProduction controls

uBibliography

uGeneral statement

uFigure captions

uType-producing areas

uProduction controls

uBibliography

Figure Captions

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Type Producing Areas 

Four NW-SE trending “type producing areas” (TPAs) can be delineated within the field, based upon similarities in reservoir characteristics and production behavior, which are broadly consistent within each TPA, but change dramatically across three internal field boundaries which separate the TPAs (Figure 1). Production parameters useful for delineating the TPAs and their boundaries include:

• Cumulative gas production (Figure 1)

• Peak gas producing rate

• Current gas producing rate

• Cumulative water production

• Gas to water ratio

• Composition of produced gas (Figure 2)

• Cumulative oil production (see Figure 7) 

Within each TPA, wells tend to exhibit certain distinctive characteristics in their production behavior through time, as illustrated in the “Type Production History Curves” that have been developed using selected wells from each of the four areas (Figures 3, 4, 5, and 6). Some key characteristics of the type curves and production from each TPA are:   

TPA1: Initial Production: 150 MCFGD, 25 BWPD, Peak gas rate: 350 MCFGD, reached after 7 years of incline; Cumulative after 12 years: 1 BCFG, 37 mbbl (water); Gas/water ratio: >20; Gas quality: 900-1100 Btu.   

TPA2: IP: 200 MCFGD, 0.2 BWPD; Gas rate declines rapidly then stabilizes at 75 MCFGD, low gas and water rates throughout well life; Cumulative after 12 years: 270 MMCFG, 1 mbbl (water); G/W ratio:>200. Gas quality: 1100-1300 Btu; low CO₂, some oil production. Many of these wells produce no water. Most of the wells (80%) reporting oil production are in this area.   

TPA3: (a.k.a. “The High Rate Fairway”) IP:1000 MCFGD, 275 BWPD; Gas rate inclines quickly, reaching a peak rate of 3700 MCFGD within 1 year; Water rate declines to 30 BWPD within 3 years; Cumulative after 12 years: 13 BCFG, 227 mbbl (water); G/W ratio: 20-200; Gas quality: 650-900 Btu, >10% CO₂.   

TPA4: IP: 200 MCFGD, 250 BWPD; Peak gas rate of 700 MCFGD is attained after 12 years of incline; Cumulative after 12 years: 2.2 BCF, 845 mbbl (water); G/W ratio: <20; Gas quality: 900-1100 Btu.   

Using these type profiles, each of the wells in the Fruitland Coal gas field was classified into one of these four type categories, based on its individual production history. A color-coded map showing the distribution of wells in each category aids in the delineation of the TPA boundaries (Figure 8). Companies operating gas production wells in the field have used different drilling, completion, and operation methods to optimize production, dependent upon the reservoir characteristics of the portion of the field in which the well is located.

Controls of Production Behavior 

Variations in production behavior can be linked to underlying geological controls, especially thermal maturity, coal thickness, organic petrology, and hydrodynamics. These variables affect many of the primary characteristics of the reservoir, such as hydrocarbon (oil and gas) content and composition, fluid pressure, isotherm shape, water saturation, and cleat permeability. Interpretation of these interrelationships is complex, as the effects of multiple controls may be overprinted at any given location. Thermal maturity clearly has the greatest impact on reservoir quality. Coal rank increases from south to north, ranging from 0.45 Ro,vit (subA) in TPA1 to 1.6 Ro,vit (mv) in TPA4. The highest gas production rates occur within TPA3 (a.k.a. “The High Rate Fairway”). The sinuous, but sharp, southern boundary of TPA3 (B2/3) is associated with a coal rank transition occurring around 0.75 – 0.80% Ro,vit. South of B2/3 (within TPA2), the reservoir is underpressured, permeability and production rates are low, and wells produce wet gases (rich in C₂₊) with little or no water. Most of the wells that report oil production from the Fruitland Coal are also located in this area (Figure 7). North of B2/3 (within TPA3) the reservoir is overpressured (although now depleted in many areas), permeability is high, and wells produce dry CH₄ together with up to10% (or more) CO₂ and large volumes of water. Peak gas rates increase 10-100X across the B2/3 boundary. Overpressuring within TPA3 has been interpreted as a relict of thermal maturation and hydrocarbon generation (Meek and Bowser, 1993).   

Boundary B1/2 may also be related to maturity, but if so, the effect is more subtle. The location of B1/2 is less sharp than B2/3 but is roughly coincident with a 0.60% Ro,vit contour, which corresponds approximately to the entry of coal into the window of oil expulsion. The presence of a distinct oil phase in the fracture network in TPA2 may impede the flow of gas and water to the production well; this could account for the generally low gas production rates. Moreover, occluded oil in the organic matrix suppresses the methane sorption capacity; this decreases gas content (Levine, 1991). Gas and water production rates tend to be significantly higher in TPA1 than TPA2, indicating higher permeability.   

Coal thickness trends and the stratigraphic pinchout of major coal seams in the lower Fruitland have a strong impact on B3/4. A protracted stillstand of the Pictured Cliffs shoreline in the predominantly regressive facies sequence resulted in a vertical aggradation of the beach and marginal marine sands (Pictured Cliffs Formation) and associated back-barrier peat-forming environments (Fruitland Formation) near B3/4 (Fassett, 2001). Toward the southwest, on the landward side of B3/4, the net coal isopach reaches its greatest value in the basin, ~80-100 feet. Northeast of B3/4, the net coal isopach thins dramatically, due to the seaward pinchout of several major coal seams in the lower part of the Fruitland Formation. Stratigraphic pinchout of the lower Fruitland coals is interpreted as the primary control on the B3/4 boundary.   

The influence of hydrogeology is greatest in TPA4 but is minor elsewhere. Wells in TPA4 have very low gas to water ratios, many being less than 1 (mcf of gas per bbl water). Some of the wells near the coal outcrop have produced over 6 million barrels of water and have g/w ratios as low as 0.2. The proximity of these wells to the outcrop and their very long dewatering times indicates that there is a probable connection of the coal gas reservoir to the outcrop where meteoric recharge is taking place. Published water chemistry data support a meteoric recharge model for coals in TPA4 (Kaiser and Swartz., 1990). Wells in TPA3 dewater relatively quickly and, based on production history data, appear to be hydrologically isolated from TPA4. B3/4 is inferred to be a hydrodynamic boundary that isolates the lower Fruitland coals in TPA3 from the coals in the northern part of the field and therefore from connection to the recharging aquifer.   

Structural geology has little discernible influence on reservoir behavior in the Fruitland Coal field, except along the northwestern basin margin, where beds are dipping at high angles toward the basin center. Previously published interpretations that gas production rates are influenced by tectonically induced fracture trends, hingelines, or other structural features are difficult to substantiate. Although other authors have implied that regional production trends may be influenced by structural features, none has convincingly made the case. Stratigraphic variations, including coal rank related variations, and coal thickness trends created during deposition appear to be the primary controls on the regional scale production trends.

Selected Bibliography 

Fassett, J.E., 1988, Geometry and depositional environment of Fruitland Formation coal beds, San Juan basin, New Mexico and Colorado: anatomy of a giant coal-bed methane deposit, in J.E. Fassett, ed., Geology and coalbed methane resources of the northern San Juan basin, Colorado and New Mexico: Denver, Rocky Mountain Association of Geologists, p. 23-38.

Fassett, J.E., 2001,Competing models for the Fruitland Formation coal and coal-bed methane system of the San Juan Basin, New Mexico and Colorado (abstract): GSA Abstracts with Programs, v. 33, no. 6, p. 57.

Kaiser, W.R., and T.E. Swartz, 1990, Hydrology of the Upper Cretaceous Fruitland Formation and the producibility of coal-bed methane, San Juan Basin, Colorado and New Mexico (abstract): AAPG Bulletin, v. 74, p. 690.

Kaiser, W.R., T.E. Swartz, and G.J. Hawkins, 1991a, Hydrology of the Fruitland Formation, San Juan basin, in W.B. Ayers Jr. et al., Geologic and hydrologic controls on the occurrence and producibility of coalbed methane, Fruitland Formation, San Juan basin: Chicago, Gas Research Institute Topical Report GRI-91/0072, p. 195-241.

Kaiser, W.R., W.R., W.B. Ayers Jr., W.A. Ambrose, S.E. Laubach, A.R. Scott, and C.M. Tremain, 1991b, Geologic and hydrologic characterization of coalbed methane production, Fruitland Formation, San Juan basin, in W.B. Ayers Jr. et al., Geologic and hydrologic controls on the occurrence and producibility of coalbed methane, Fruitland Formation, San Juan basin: Chicago, Gas Research Institute Topical Report GRI-91/0072, p. 273-301.

Kaiser, W.R., and W.B. Ayers Jr., 1994, Coalbed methane production, Fruitland Formation, San Juan basin: geologic and hydrologic controls, in W.B. Ayers, Jr. and W.R. Kaiser, eds., Coalbed methane in the Upper Cretaceous Fruitland Formation, San Juan basin, New Mexico and Colorado: University of Texas at Austin, Bureau of Economic Geology Report of Investigations 218, p. 187-207.

Kaiser, W.R., T.E. Swartz, and G.J. Hawkins, 1994, Hydrologic framework of the Fruitland Formation, San Juan basin, in W.B. Ayers Jr. and W.R. Kaiser, eds., Coalbed methane in the Upper Cretaceous Fruitland Formation, San Juan basin, New Mexico and Colorado: University of Texas at Austin, Bureau of Economic Geology Report of Investigations 218, p. 133-163.

Levine, J., 1991, Coal petrology with applications to Coalbed methane R & D, Short Course Note, Coalbed Methane Symposium, Tuscaloosa, Alabama.

Meek. R.H., and P.D. Bowser, 1993, The transition between the low-rate and high-rate producing areas of the Fruitland coal, San Juan basin, New Mexico (abstract): AAPG Bulletin, v. 65, p. 1455.

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