Matrix

Fracture

Matrix

Butt Cleats

Face Cleats

RRRRReeeeesssssererererervvvvvoir Engoir Engoir Engoir Engoir Engininininineeeeeering for Gering for Gering for Gering for Gering for GeeeeeolooloolooloologggggistsistsistsistsistsArticle 12 - Coalbed Methane Fundamentals by Kamal Morad, P. Eng., Ray Mireault, P. Eng., and Lisa Dean, P. Geol., Fekete Associates Inc.

Historically, gas emissions from coal havebeen a nuisance and a safety hazard duringcoal mining operations, causing numerousexplosions and deaths. But today, coalbedmethane (CBM) is an increasingly importantsource of the world’s natural gas productionwith many countries, including Canada,actively developing this unconventionalenergy source.

Currently, CBM accounts for 10% of U.S.natural gas production with the size of theresource (OGIP) estimated at 700 TCF. Themost active areas of production are the SanJuan Basin in New Mexico, the Powder RiverBasin in northeast Wyoming / southeastMontana, and the Black Warrior Basin inAlabama.

In Canada, CBM is still in the early stages ofdevelopment, yet it already accounts forabout 1% of total gas production. The WesternCanada Sedimentary Basin contains themajority of Canada’s estimated 600 TCF ofCBM resource potential. Formations ofgreatest interest are the Mannville, whichtends to produce water as well as gas (a“wet” coal) and the Horseshoe Canyon,which usually produces gas with virtually nowater (a “dry” coal).

In general, coal is classified into four maintypes depending on the quantity and types ofcarbon it contains as well as the amount ofheat energy it can produce. These are:

1. Lignite (brown coal) – the lowest rankof coal; used as fuel for electric powergeneration.

2. Sub-bituminous coal – properties rangebetween lignite and bituminous coal.

3. Bituminous coal – a dark brown to black,dense mineral; used primarily as fuel insteam-electric power generation.

4. Anthracite – the highest rank; a harder,glossy, black coal used primarily forresidential and commercial spaceheating; it may be divided further intopetrified oil, as from the deposits inPennsylvania.

Note that graphite, which is metamorphicallyaltered bituminous coal, is technically thehighest rank of coal. However, it is notcommonly used as fuel because it is difficultto ignite.

COAL CHARACTERISTICSCOAL CHARACTERISTICSCOAL CHARACTERISTICSCOAL CHARACTERISTICSCOAL CHARACTERISTICSCoals are recognized on geophysical logs

because of several unique physicalproperties. The coals typically have very lowgamma, low density, and high resistivityvalues.

Similar to conventional naturally fracturedreservoirs, coal is generally characterizedas a dual-porosity system because it consistsof a matrix and a network of fractures (Figure12.1). For both groups, the bulk of the in-place gas is contained in the matrix. However,matrix permeability is generally too low topermit the gas to produce directly throughthe matrix to the wellbore at significantrates.

In a naturally fractured system, most of theproduced gas makes its way from the matrixto the fracture system to the wellbore. Ifthe well has been hydraulically frac’d, gas mayalso travel from the natural fracture systemto the man-made fracture system to thewellbore. With both conventional naturallyfractured reservoirs and CBM reservoirs, thenatural fracture system has high permeability,

relative to matrix permeability, but verylimited storage capacity.

In coal terminology, natural fractures arecalled “cleats”. The cleat structure consistsof two parts: face cleats and butt cleats(Figure 12.2). Face cleats are typicallycontinuous fractures that go across thereservoir. They are considered the mainpathway for gas production.

Butt cleats are discontinuous, perpendicularto the face cleats and generally act as a feedernetwork of gas into the face cleats.

The effective porosity, permeability, andwater saturation are all properties of thecoal “cleat” system. Since coal permeabilityis a property of the cleat space, it is affectedby the structure and characteristics of thecleat network, e.g., the dominant fractureorientation, fracture continuity, frequency,and width.

The effective permeability of the cleat systemis also influenced by the contrast betweenface and butt cleat permeabil ity. CBMreservoirs are generally considered to beanisotropic systems, where the effectivepermeability is the geometric average of faceand butt cleat permeability. Permeabilityanisotropy creates elliptical drainage areasand should be taken into account whenplacing wells in CBM development projects.

DIFFERENCES WITH CONVENTIONALDIFFERENCES WITH CONVENTIONALDIFFERENCES WITH CONVENTIONALDIFFERENCES WITH CONVENTIONALDIFFERENCES WITH CONVENTIONALRESERVOIRSRESERVOIRSRESERVOIRSRESERVOIRSRESERVOIRSA good starting point to understanding theproduction characterist ics of coalbedmethane reservoirs is by considering thedifferences to conventional gas production.The most significant differences are:

• In a conventional reservoir, the majorityof the gas is contained in the pore spacebut in a CBM reservoir, the majority ofthe gas is adsorbed (bonded to the coalmolecules) in the matrix.

• In a conventional reservoir, reservoirgas expands to the producing wells indirect response to any production-induced ressure gradient. But CBMreservoirs general ly require thatreservoir pressure be below somethreshold value to init iate gasdesorption.

• In a CBM reservoir, a gas molecule mustfirst desorb and diffuse through the coalmatrix to a cleat. It can then moveFigure 12.2. Example of coal cleat structure.

Figure 12.1. Coal is a dual-porosity system.

through the cleated fracture system andthe hydraulic frac-stimulation to thewellbore via conventional Darcy flow.

CBCBCBCBCBM GM GM GM GM GAAAAAS SS SS SS SS STTTTTORAORAORAORAORAGGGGGE CE CE CE CE CAPAPAPAPAPABABABABABILILILILILIT YIT YIT YIT YIT YThe primary storage mechanism in CBMreservoirs is adsorption of gas by the coalmatrix. Matrix surface area, reservoirpressure, and the degree to which the coalis gas saturated are the factors that determinethe in-place gas volume of a coal. Note thatthe smaller the coal particle size, the largerthe surface area.

The complete gas-in-place volumetricequation for a CBM reservoir is:

OGIP = A * h * b* GCi + (Ah i(1-Swi) / Bgi)

Where:

• A is drainage area,• h is net pay,• b is bulk density,• GCi is initial Gas Content,• i is porosity,• Swi is initial water saturation• Bgi is initial formation volume factor.

The first term represents the adsorbed gasin the matrix while the second term is thefree gas in the cleats. Since the pore volumein CBM reservoirs is in the order of 1% ofthe total volume, the free gas contributionto the total in-place gas volume is negligible.

As with all volumetric estimates, uncertaintyin the input data creates a range of possibleoutcomes for OGIP. Some common areas ofuncertainty for CBM projects include:

• The gas content of the coal,• The degree of heterogeneity and

complexity contained in CBMreservoirs,

• The impact of modell ing complexmultilayer coal/non-coal geometrieswith simple one- or two-sequencemodels.

CBM GAS DESORPTIONCBM GAS DESORPTIONCBM GAS DESORPTIONCBM GAS DESORPTIONCBM GAS DESORPTIONWhile the relationship between pressuredecline and gas production is essentially astraight line in a conventional reservoir(Figure 12.3), the depletion profile in a CBMreservoir is distinctly non-linear. For a givenpressure drop, a CBM reservoir will desorbsignificantly more gas when the startingreservoir pressure is low compared to whenreservoir pressure is high (Figure 12.4).

If the initial reservoir pressure is significantlygreater than the pressure required to initiatedesorption (the coal is under-saturated), andwater is initially present in the cleat system,then the initial production period mayproduce only water without any gas (Figure12.5). Depending on the degree of under-saturation, dewatering can last from a fewmonths to two or three years and cansignificantly affect the economics of theprospect.

If initial reservoir pressure is equal to thecritical desorption pressure (the coal is gas-saturated), then gas production will start assoon as reservoir pressure begins todecrease. This situation most often appliesto “dry” coals but can also apply to saturated“wet” coals.

The equation that is commonly used todescribe the relationship between adsorbedgas and free gas as a function of pressure isknown as the Langmuir isotherm. Theisotherm is determined experimentally andmeasures the amount of gas that can beadsorbed by a coal at various pressures. TheLangmuir isotherm is stated as:

V = VL * (P / PL + P)

Where:• VL, the Langmuir Volume, is the gas

content of the coal when reservoirpressure approaches infinity.

• PL, the Langmuir Pressure, is the pressurecorresponding to a gas content that ishalf (½) of the Langmuir volume. Thesteepness of the isotherm curve atlower pressures is determined by thevalue of PL.

CBM gas consists primarily of methane (CH4)but may also contain lesser percentages ofcarbon dioxide (CO2) and nitrogen (N2). Ascoal has the strongest affinity for nitrogen

Figure 12.3. Conventional gas P/Z plot.

Figure 12.4. Comparison of desorption volumes with changes in reservoir pressure.

D

D N

N

and the weakest affinity for carbon dioxide,the three gases adsorb/ desorb at differentrates from coal (Figure 12.6). Thus, it is notuncommon for the CO2 content of theproduced gas to decrease as gas is producedand reservoir pressure depletes.

CBM GAS TRANSPORT MECHANISMSCBM GAS TRANSPORT MECHANISMSCBM GAS TRANSPORT MECHANISMSCBM GAS TRANSPORT MECHANISMSCBM GAS TRANSPORT MECHANISMSAfter desorbing from the coal, gas in a CBMreservoir uses diffusion to travel throughthe coal matrix to the cleat system. The timerequired to diffuse through the matrix to acleat is controlled by the gas concentrationgradient, the gas diffusion coefficient, and the

cleat spacing. In general , greaterconcentration gradients, larger diffusioncoefficients, and tighter cleat spacing all actto reduce the required travel time. Onreaching a cleat, gas then travels the remainingdistance to the wellbore by conventionalDarcy flow. Since flow in a CBM reservoir isgenerally two-phase flow, fluid saturationchanges in the cleat system and consequentchanges in relative permeabilities becomeimportant.

As the gas is produced from a CBM reservoir,two distinct and opposing phenomena occur

that affect the absolute permeability of thecleat system:

1. As reservoir pressure decreases, itreduces the pressure in the cleats. Cleateffective stress (which is the differencebetween overburden stress and porepressure) increases and compressesthe cleats, causing cleat permeability todecrease.

2. As gas desorbs from the coal matrix,the matrix shrinks. Shrinkage causes thespace within the cleats to widen andthe permeability of the cleats increases.From the Langmuir isotherm (Figure12.4), the amount of gas desorbed for agiven pressure drop is relatively smallat high pressures. Thus in the earlystages of production, the compactioneffect is the dominant factor and cleatpermeability will tend to decreaseslightly. As production continues and gasrecovery becomes significant, matrixshrinkage will dominate and increasecleat permeability.

In “wet” coals, changes in the relativepermeability of the cleat system with changesin water and gas saturation must beconsidered in the Darcy flow equation tocorrectly predict well performance. Asi l lustrated by a typical set of relativepermeability curves (Figure 12.7), the relativepermeability to gas increases with decreasingwater saturation and vice versa.

CBM WELL PERFORMANCECBM WELL PERFORMANCECBM WELL PERFORMANCECBM WELL PERFORMANCECBM WELL PERFORMANCEThe production of CBM wells can begenerally divided into three separate phases(Figure 12.8):

• Dewatering phase (for under-saturatedreservoirs): In this phase, no gas isproduced (excepting in the transientnear wellbore region or in complexreservoirs).

• Negative decline: Water productioncontinues to decl ine while gasproduction increases.

• Production in this phase is generallydominated by the relative permeabilityof gas and water.

• Decline phase: Declining reservoirpressure is now the dominating factoralthough its impact is mitigated to someextent by a shrinking matrix andincreasing cleat permeabi l ity.Nonetheless, the gas production ratedecl ines as in conventional gasreservoirs, albeit at a slower rate ofdecline.

The water production forecast looks similarto a production forecast for a conventionalwater producing reservoir. Maximum waterproduction rates are achieved initially butdecline thereafter through a combination of

Figure 12.5. Desorption behaviour of under-saturated CBM reservoirs.

Figure 12.6. CBM gas storage capacities for N2, CH4, and CO2.

reservoir pressure depletion and decreasingrelative permeability to water.

The gas production profile displays both theinitial, dormant period followed by anincreasing production rate till it reaches apeak and then declines. Although reservoirpressure is monotonically declining throughthe l i fe of the simulation well , i t iscounteracted during the inclining productionperiod by increases in the relat ivepermeability to gas and in the absolutepermeability of the cleats.

As the water saturation approaches itsminimum value, declining reservoir pressuredominates and the well goes into the declinephase of its producing life. During this timeperiod, the declining production trendresembles conventional gas production.Note that a “dry” CBM reservoir exhibitsonly the declining portion of the productionpattern.

Given the scope and complexity of the inputsfor CBM reservoirs, simulation is generallyrequired to predict the deliverability and

cumulative production of CBM wells. Asimprovements in drilling, completion andproduction techniques advance, CBM willcontinue to be an increasingly importantsource of natural gas.

REFERENCESREFERENCESREFERENCESREFERENCESREFERENCESGas Research Institute. 1993. GRI ReferenceNo. GRI-94/0397, “A Guide to CoalbedMethane Reservoir Engineering,” Chicago,Illinois.

Jensen, D. and Smith, L.K. 1997. A PracticalApproach to Coalbed Methane ReservePrediction Using a Modified Material BalanceTechnique. International Coalbed MethaneSymposium, The University of Alabama,Tuscaloosa, Alabama, paper 9765, p. 105-113.

Lamarre, Robert A. 2005. “Coalbed Methane– A Non-Conventional Energy Source WhatIs It And Why Is It Important,” 25th AnnualNorth American Conference of the USAEE/IAEE, Fueling the Future.

Mavor, M.J . 1996. A Guide to CoalbedMethane: Coalbed Methane ReservoirProperties. Gas Research Institute Chicago,Illinois, GRI Reference No. GRI-94/0397,Chapter 4.

Schafer, P.S. and Schraufnagel, R.A. 1996. AGuide to Coalbed Methane: The Success ofCoalbed Methane. Gas Research InstituteChicago, Illinois, GRI Reference No. GRI- 94/0397, Chapter 1.

Steidl, P.F. 1996. A Guide to Coalbed MethaneReservoir Engineering: Coal as a Reservoir.Gas Research Institute Chicago, Illinois, GRIReference No. GRI-94/0397, Chapter 2.

Zuber, M.D. 1996. A Guide to CoalbedMethane: Basic Reservoir Engineering forCoal. Gas Research Institute Chicago, Illinois,GRI Reference No. GRI-94/0397, Chapter 3.

Figure 12.7. Relative permeability to gas and water.

Figure 12.8. CBM well production profile.