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June 2007
Chapter 6
Formation Evaluations, Logging
6.1 InHistoricaprovided locating wexploratitruck-mou
This procoperationscontent. Wpurposes.sensor sparoof and f
The advelarger-diabrought ocompaniethe oil anentered thbegan to t
Now that they meangas produunderstanparametegas-produFormation Evaluations, Logging 289
troductionlly, the wireline logging services employed by the coal industrymeans to assist in mapping the coal, measuring its thickness, andater tables in formations above the coal. The wells drilled for coal
on were typically cored continuously from the surface by small,nted drilling rigs drilling boreholes of less than 4 in. diameter.
edure enabled the mine owners to analyze the coal from the coring to answer their questions concerning rank, mineral matter, and BTUireline logging in these wells was essentially done for qualitative
The mineral logging industry uses small-diameter tools with shortcing to provide sharp contrasts between the coal and the surrounding
loor rock.
nt of extracting methane before mining for coal required use of ameter borehole for optimum production. This methane production alsoil and gas exploration companies into the coal industry; these
s were more comfortable evaluating the wireline logs that were used ind gas industry. It was only natural that the major wireline companiese coalbed methane (CBM) exploration and production arena. Coringake a back seat as the primary evaluation tool.
we have quantitative coalwireline log measurements available, what do and how do we quantify the key parameters for evaluating coal for itsction properties? This chapter is dedicated to helping the reader
d wireline log measurements and how to determine some of the keyrs that need to be understood to successfully analyze a CBMction play. The critical reservoir parameters addressed in this chapter
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Coalbed Methane: Principles and Practices
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on wireline logs are: Coal thickness. Gas content. Coal tonnage and volumetric gas in place. Cleat Evalua Natura Moder
In this cha(nuclear, ereact in co
6.2 BBefore wewireline lo
6.2.1 Do
The downgreatest cooften creaare well washed-ouWhen thisborehole wcould be ipart of anyborehole eation Evaluations, Logging June 2007
permeability.tion of sands near the prospective CBM target.l fracture orientation. n-day stress orientation.
pter, we will evaluate four major categories of wireline logging toolslectrical, acoustic, and magnetic resonance) and determine how theyal.
orehole Environment evaluate wireline logs, we should examine the environment in whichgs are run.
wnhole Environment
hole environment is not friendly to wireline measurements. Thencern is the borehole shape because irregularly shaped boreholes can
te misleading wireline log measurements. As an example, coalbeds thatcleated can often be eroded by the drilling process, creating at section of borehole that shows as an enlarged caliper measurement. occurs, the wireline measurements that rely on good contact with theall may be reading a mixture of formation and mud properties, which
nterpreted as showing coal where there really is no coal. An integral interpretation of wireline logs is a consideration of the rugosity of thenvironment and its relationship to the individual measurement devices.
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Coalbed Methane: Principles and Practices
June 2007
6.2.2 Wireline Logging
Wireline logging measurements are recorded at various distances from thebottom of the logging tool, as shown in Fig 6.1. This means that if there is anysticking of the logging tool as the log is being recorded, the irregular toolmovemena loggingfollows: aporosity, acable is betool startsis reacheddifferent dthe wirelinprinted onmeasurem
Each logimportanteach sensoshows an quad commeasuremFormation Evaluations, Logging 291
t will affect each sensor in a different place on the log. For example, if tool is 75 ft long, the measurement sensors will be distributed ast 8 ft for resistivity, 35 ft for bulk density and caliper, 50 ft for neutronnd 70 ft for gamma ray. If the tool physically stops while the wirelineing extracted from the well, the pull on the cable will increase until the
moving again or the maximum tension that can be applied to the cable. If the tool starts moving, each sensor will have been stationary at aepth in the wellbore. As the log is recorded, the surface equipment ine logging unit delays each measurement so all the measurements are
depth. Tool pulls can cause a mismatch of the position of a particularent.
ging service company manufactures its own logging tools. It is to examine the log heading for information related to the position ofr in areas of wireline tension pulls or washed-out boreholes. Fig. 6.2
example of a quad combination log through several coalseams. Thebination log consists of the three porosity tools and a resistivityent device discussed in this chapter.
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Coalbed Methane: Principles and Practices
292 Form
Fseation Evaluations, Logging June 2007
ig. 6.1Combination toolstring with the location of the different nsor measurement points.
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Coalbed Methane: Principles and Practices
June 2007
MD
Coal
GRCorrelation
GAPISP(SPA)
MVCALI
0
-80
6
200
20
16
DepthResS(DFL)Resistivity
OHMMResM(HMRS)
OHMMResD(HDRS)
OHMM
1000
1000
1000
1
1
1
RHOBBulk Density
PEF1000
10000
1
0.000
DECP
Porosity
PHID
PHIN(NPHI)0
0
1
1
DT(DTC)40140
Sonic
Fig. 6.2Formation Evaluations, Logging 293
5550
5600
Quad combination log in a coal.
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Coalbed Methane: Principles and Practices
294 Form
6.3 Tool Measurement Response in Coal
6.3.1 Natural Gamma Ray
The naturradioactivnaturally general tygenerally(NORM)groundwmeasuremreadings aray respoOccasionawas depomeasuremation Evaluations, Logging June 2007
al gamma ray tool measures bulk gamma rays emitted from thee minerals in the immediate vicinity of the wellbore. Most of the
occurring radioactivity in sedimentary formations comes from threepes of minerals: thorium, potassium, or uranium. Clay minerals
contain large amounts of naturally occurring radioactive minerals. Uranium, being a more soluble mineral, can be transported byater moving through the formation. Typically, a gamma rayent is interpreted as follows: the high readings are shales and the lowre potential reservoirs. Coals usually have a very low natural gammanse because the concentration of clay minerals is low (Fig. 6.3).lly, a coal will have some radioactive material (typically uranium) thatsited by groundwater movement (Fig. 6.4), making the gamma rayent much higher.
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Coalbed Methane: Principles and Practices
June 2007
Fig. 6.3Formation Evaluations, Logging 295
The natural gamma ray tool response in a typical coal.
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Coalbed Methane: Principles and Practices
296 Form
Enhancedis a recomreduces thto help hcoal-thick
Fig. 6.4ation Evaluations, Logging June 2007
vertical resolution processing of the natural gamma ray measurementmended practice in CBM applications. This processing mathematicallye vertical resolution of the measurement, sharpening the bed boundaryighlight the detail within the coal and result in a more accurateness measurement.
The natural gamma ray tool response in a hot coal.
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Coalbed Methane: Principles and Practices
June 2007
6.3.2 Spontaneous Potential
The spontaneous potential (SP) measurement is a voltage potential differencecreated by three phenomena: salinity difference between the borehole fluid andreservoir fluid, streaming potential, and electrochemical invasion. The mostcommon sborehole reservoir. are tradedas a voltatrack). Whthat the sawater. Whborehole shale conresponse, the SP defa good qu
In coals, Slikely dueA greatepermeabilthan 10 ftSP measucorrection
6.3.3 Re
Resistivityeach serviof these twFormation Evaluations, Logging 297
ource of this SP is the salinity difference between connate water andfluids. SP is generated by fluids moving from the borehole to theElectrochemical SP effects are most common in carbonates where ions between the reservoir rock and the borehole fluids. The SP is measuredge in reference to the zero baseline value in shale (Fig. 6.3, first logen the SP measurement deflects to the left of the baseline, it indicateslinity of the borehole fluid is lower than the salinity of the formationen the SP measurement deflects to the right of the shale baseline, the
fluid salinity is greater than the salinity of the formation water. Thetent of the formation tends to decrease the magnitude of the SPas do thin beds, hydrocarbons, and low permeability. The magnitude oflection (no matter which way it goes) times the thickness of the coal isalitative indication of permeability.1
P deflection tends to reflect the bulk permeability in the coal. It is most to a combination of salinity difference and streaming potential effects.r SP deflection observed across from a coal indicates greaterity in a coal. When measuring production potential of coalbeds less thick, one should consider applying some thin-bed corrections to therement either by software bed-resolution enhancement or chart-books to arrive at the most realistic SP response.
sistivity Measurements
tools come in two general categories, induction or laterolog. Whilece companys resistivity devices may differ in name, they are all in oneo categories.
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Coalbed Methane: Principles and Practices
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The choice of resistivity tools is usually based upon the salinity of the boreholefluids. Induction tools are typically run in wells with less than 30,000 ppmchlorides in the drilling mud. In saltier mud systems, the dual laterolog tool is thetool of choice.
The mosinductiondiffer, inte
Generallysecond logresistivityfluid-filled
With dualfloor rockmeasured 30 ft thick
Modern ineffects of 12 ft rancoal becaubest indica
In salty m(Fig. 6.5)having a twith no ination Evaluations, Logging June 2007
t common resistivity devices run for CBM applications are-based tools. While the principles of measurement behind the toolsrpreting the resistivity log response in coal is similar.
, coal tends to exhibit rather high resistivity measurements (Fig. 6.3, track). Coal, in its purest form, is a good insulator and has very high. Impurities in coal such as clays, pyrites, volcanic minerals, and cleating tend to reduce the resistivity in coals.
induction-type resistivity measurements, the resistivity of the roof and encasing the coal can have a significant impact on the resistivityin the coal; these shoulder beds should be considered in coals less than.
duction logs and dual laterolog tools can be processed to reduce thethe shoulder beds so that vertical-bed resolution can be reliably in thege. Older electrical logs can be very confusing when used to evaluatese these logs have a much coarser vertical resolution and are not thetor of coal thickness.
ud systems, the dual laterolog has been used to indicate permeable coal from non-permeable coal (Fig. 6.6). Permeable coal is observed asypical invasion profile while the tight coal shows very high resistivityvasion.
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Coalbed Methane: Principles and Practices
June 2007
Fig. 6.5D
GammDual Laterolog in Coalbeds
GAMA
Q LO0
CAL4.5
INCQ SH
6
-4.5Formation Evaluations, Logging 299
ual laterolog tool response in a well-cleated coal.
X700
X600
X700
DENSITY
POUNDSDEN CORR
a Ray Spectral Density Log DLL/MicroguardMA
PING
200
IPER-.5
HESORT
16
.5
GM/CCDEN POROS
3.02.0
g=2.65 -.10.30 PE QUAL TENSION010000
GM/CC .25-.25SDL PE 9-1
100PE CORR 100
OHM-MLL DEEP
2000.2
OHM-MLL SHALLOW
2000.2
MICROGUARD
OHM-M 2000.2 TENSION010000
X600
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Coalbed Methane: Principles and Practices
300 Form
Fig. 6.6D
6.3.4 Mi
MicrologmeasuremThe two mnormal remicrolog permeablethe degreedirectly de
DENSITYGAMMAA
Q LO0
CAL4.5
6
-4.5
MICROGUARDGamma Ray Spectral Density Log DLL/Microguard
Dual Laterolog in Coalbeds
INCQ SHation Evaluations, Logging June 2007
ual laterolog tool response in a non-cleated coal.
cro-Resistivity Measurements
resistivity measurement is a very shallow, non-focused resistivityent, taken from a rubber pad about the size of a human hand (Fig. 6.7).easurements on the microlog tool are the normal and inverse. The
sistivity reads slightly deeper than the inverse measurement. Thehas historically been used as an indicator of mud cake across from zones. In relation to coal, the microlog can be an excellent indicator of of cleating in coalbeds (Fig. 6.8).2 Although cleat permeability is nottermined using the microlog, many studies have demonstrated a good
POUNDS
PING
200
IPER -.5
16
.5
GM/CCDEN POROS
3.02.0
SS PU-.10.30 SDL PE DEN CORR
010000
GM/CCTENSION
.25-.25100
OHM-MLL DEEP
2000.2OHM-M
LL SHALLOW2000.2
OHM-M 2000.2TENSION
010000
HESORT
POUNDS
X800
X700
X800
X700
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Coalbed Methane: Principles and Practices
June 2007
correlation between the cleated footage and production. Fig. 6.9 shows anexample of the variation of permeability indications among coalbeds in the samewellbore.
Fig. 6.7Formation Evaluations, Logging 301
The microlog pad.
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Coalbed Methane: Principles and Practices
302 Form
Non-cleated or fractured
Fig. 6.8coal.ation Evaluations, Logging June 2007
Fractured or poorly cleatedLow to fair permeability
Well cleatedGood permeability
Thin laminated coal sequenceProbably some fracturingMost likely low permeability
Microlog tool response characterization chart for identifying cleating in
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Coalbed Methane: Principles and Practices
June 2007
Fig. 6.9the sameFormation Evaluations, Logging 303
Microlog response showing differences in permeability between coals in wellbore.
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Coalbed Methane: Principles and Practices
304 Form
6.3.5 Nuclear Measurements
Nuclear measurements are divided into two categories: tools using high-energygamma rays and tools using high-energy neutrons. The high-energy gamma raytools measure the electron bulk density of the formation, while the high-energyneutron to
High-enerneutron tocombinati
Most CBMshown goproximatedisplay thwells within coal is tcoaly shalthird log tthe borehois then caalong theborehole wfront of ththe bulk dfluid the tmeasurembulk-densenvironme
An integrmeasuremgood coal
The neutrresponds tation Evaluations, Logging June 2007
ols respond to the hydrogen index of the formation.
gy gamma ray tools are commonly called density tools; high-energyols are referred to as neutron tools. Typically, when they are run in
on, the log is called a density-neutron log.
evaluation is performed with the density log. Many studies haveod correlation between the bulk-density3,4 measurement and the analysis and gas content in coal. Most pre-1988 density logs did note curve measurement below 2 g/cc, so quantitative evaluation work in older density logs is limited. The range of bulk-density measurementsypically between 1.2 g/cc and 2 g/cc. Gas content has been measured ines with a bulk density up to 2.6 g/cc. The bulk-density log (Fig. 6.3,rack) is a high-energy gamma ray tool that requires good contact withle wall for the most accurate measurement. The porosity measurement
lculated from the bulk density, assuming a matrix density. Washouts borehole wall can create pockets of mud between the tool and the
all. The density tool will measure the bulk density of everything ine pad. If there is formation and water in front of the measurement pad,ensity of the formation will be reduced by the volumetric portion ofool encounters. The end result is that washouts cause the bulk-densityent to spike to low bulk-density values. Thus, if one uses only a
ity measurement for coal identification without regard for the boreholent, coal thicknesses could be greatly overestimated.
al measurement with modern density logs is the photoelectric (PE)ent. PE measurement is an excellent measure of lithology as well as aidentifier. The PE measurement typically reads below 1.0 in coals.
on porosity tool, often referred to as a compensated neutron tool,o the hydrogen index of the matrix rock adjacent to the tool (Fig. 6.10).
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Coalbed Methane: Principles and Practices
June 2007
Coal, by its chemical composition, has one of the highest hydrogen index valuesof common minerals encountered in sedimentary deposits. Thus, in coal, thecompensated neutron (CN) tool records a very high apparent porosity. The CN isa good tool to identify coal in either open holes or cased wellbores.
The CN pbecause itmeasuremneutron p
Cor
G0
-80
6
Fig. 6.10Formation Evaluations, Logging 305
orosity tool is not typically used as an indicator of other coal properties is usually run in combination with the density log. The bulk-densityent is more accurate in the low-density end of the measurement and theorosity is most accurate in the low-porosity measurements. In
MD
2400
2450
Coal
GRrelation
APISPMV
CALI
200
20
16
DepthResS
Resistivity
OHMMResMOHMMResD
OHMM
1000
1000
1000
1
1
1
RHOBBulk Density
PEF3
10.000
1
0.000
DECP
Porosity
PHID
PHIN0
0
1
1
1
Compensated neutron log response in coal.
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Coalbed Methane: Principles and Practices
306 Form
cased-hole applications, the CN tool can be used quite efficiently to drive coalproperty calculations by correlating the neutron porosity with the bulk-densitymeasurement in a well where both tools are run (Fig. 6.11).
For convedensity pmeasuremfilled. Whreading shfilled. In coal, i.e.,
Fig. 6.11and bulkation Evaluations, Logging June 2007
ntional wireline log displays, the neutron porosity is overlain with theorosity. A rule of thumb when interpreting these two porosityents is that when the curves overlay each other, the formation is fluiden there is a crossover effect observed, wherein the neutron porosityows lower porosity than the density porosity, the pore space is gas
coals, this is not necessarily the case. However in some areas, a drycoal that produces mostly gas and very little water initially, has the
A Crossplot showing the relationship of compensated neutron porosity density.
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Coalbed Methane: Principles and Practices
June 2007
neutron porosity recording a lower porosity than the density porosity, Fig 6.12.
A wet copressure inthe neutrominerals p
Another cdevices tmeasuringtools meadown by irate of gaproportionThe formavalues of
Fig. 6.12porosity Formation Evaluations, Logging 307
al (a typical coal that produces water as the mechanism to reduce the cleat system for gas desorption) is often observed as a stacking ofn and density porosity. This observation may vary based upon theresent, geographic area, and service-company logging tool response.
ategory of neutron tools, the pulsed neutron tools, are small-diameterypically run through casing for formation evaluation. Instead of high-energy neutrons elastically reflected to the logging tool, these
sure the gamma rays given off when a high-energy neutron is slowednelastic atomic scattering then captured by atoms in the formation. Themma ray decay from a short, high-energy neutron burst is inverselyal to the formation capture cross-section called the formation sigma.tion sigma is inversely proportional to the formation resistivity. Higher
sigma correspond to lower resistivity. The pulsed neutron tools usually
An example of a dry coal tool response from the density-neutron measurements.
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Coalbed Methane: Principles and Practices
308 Form
have dual detectors in which the total count rate ratio is calibrated to neutronporosity. The inelastic count rates available from these tools are good indicatorsof mineralogy by determining the hydrogen yield and carbon-oxygen ratio fromthe spectral decay (Fig. 6.13).
Fig.ation Evaluations, Logging June 2007
ZONE_C
l
l
DepthMD
00.6 G/CC
00.6RHOB
Hydrogen YieldYHI
Filtrate
Water
PEF
Coal
50 00.5
CH Gas Effect
NPOP(N/A)
PHIA_CCoal
Undercall
Gas Effect
PHIS_C
EPOP_C
PHID_C
DECPMPHI(N/A)
PorosityPHIN(NPHI)
[V6HL_GP/DN[V6 HL_GP]
200
CorrelationGPAPISP
2000
CALI2080
POP(N/A)144
00.5
00.5
00.5
00.5
00.5
00.5
MINV(N/A)
MINOP(N/A)
NetPay
PAY
TEN(TBIS)LBS 100000
500.000
500.000
500
sed Hole
100.001
OHMM10000.1
OHMM10000.1
OHMM10000.1PT(ILD)
ResM(IUM)
ResS(SGRD)
KW_Cter Perm Reslsth
6.13Pulsed neutron log response in coal.
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Coalbed Methane: Principles and Practices
June 2007
6.3.6 Acoustic Measurements
Acoustic tools come in two varieties, a monopole sonic and a dipole sonic. Themonopole sonic tools are typically used when measuring compressionalslowness.transmitttransmittslowness possible torientatiodeterminetypically brun in eith
GRCorrelati
SP(SPA
CALI
0
-80
6
Fig. 6.14Formation Evaluations, Logging 309
Dipole sonic tools offer both a monopole transmitter and a dipoleer (Fig. 6.14). Most modern dipole sonic tools have two shearers at an orthogonal orientation, which allows an X and Y shearmeasurement. When coupled with a navigational package, it is ofteno detect the orientation of the modern stress field and open fracturesn. Occasionally, the magnitude of the stress field differences can bed. Sonic logs identify coals by their long transit times, which wille longer than most any other formation in the well. Sonic tools can beer open holes or cased wellbores.
RHOBBulk Density
PEF(PE)1
0.000
3
10.000
on
)200
20
16
Depth
MD
4750
4800
ResS(DFL)Resistivity
ResM(HMRS)
ResD(HDRS)
1
1
1
1000
1000
1000Coal
PHIN(NPHI)Porosity
PHID(DPHI)1
1
0
0
DT(DTCF)Sonic
DTS_EST340
340
40
40
MINVMicrolog
MNOR0
0
100
100
Dipole Sonic tool response in coal.
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Coalbed Methane: Principles and Practices
310 Form
6.3.7 Magnetic Resonance Measurements
The magnetic resonance imaging tool (MRIL) is a porosity device that measuresonly the pore space filled with fluid; its porosity measurement is independent ofthe lithology of the formation. Hence, it is the only porosity device that canaccuratelycoal is priexample, measuremis reflectiv
Fig. 6.15ation Evaluations, Logging June 2007
measure the porosity in a coal (Fig. 6.15). The porosity measured inmarily the cleat porosity. Some coals do have matrix porosity, forthe coals in the Powder River basin. But in general, the bulk-densityent is used for a gross coal thickness and the coal with MRIL porositye of the net coal thickness.6
MRIL tool response in coal.
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Coalbed Methane: Principles and Practices
June 2007
Based upon the assumption that there exists a good correlation between cleatporosity and permeability in coal, the magnetic resonance measurement is a veryvaluable tool for providing permeability information in multi-seam coal plays.
6.3.8 Ele
Over the long way MEI toolsgive a repfour or six
The resoluin some cacoal (Fig informatioplay. Presfracture ccleating inearly whethrough thFormation Evaluations, Logging 311
ctrical Imaging
past decade, micro-electrical imaging (MEI) technology has come ain its capability to image high-resistivity formations, such as coalbeds. have an array of micro-resistivity buttons mounted on multiple pads toresentative view of the inside of the borehole. These tools have either arms carrying the micro-electric pad array.
tion of these devices is on the order of 0.1 in.; therefore, it is possibleses to see the difference between cleated coal (Fig 6.16) and fractured
6.17). In poorly cleated coal, cleat orientation may be observed. Then derived from electrical imaging is critical in understanding any CBMent-day stress orientation, borehole breakout, fracture identification,onnection from the coals to adjacent sands along with partings, andformation in the coal are essential elements of information to obtain
n developing a CBM play. Much of this information is made availablee analysis of micro-electrical imaging logs.
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Coalbed Methane: Principles and Practices
312 Form
Fig. 6.16partings.ation Evaluations, Logging June 2007
An electrical-image log in thin coals showing individual cleats and thin
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Coalbed Methane: Principles and Practices
June 2007
Fig. 6.17Formation Evaluations, Logging 313
An electrical-Image log in a thick coal that is highly fractured.
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Coalbed Methane: Principles and Practices
314 Form
6.4 Wireline Log Evaluation of CBM Wells To make a meaningful assessment of the CBM potential of a particular prospect,the analyst must consistently apply a methodical, structured evaluation.Understanding the key parameters in CBM reservoir evaluation early in thelifecycle odevelopmmeasuremwith analy
6.4.1 Co
The grossgeneral wi
1. Bu2. Ga3. Ne4. So5. Sh6. Re
All of theborehole iwireline lboreholes minimize ation Evaluations, Logging June 2007
f a project can help the analyst make pertinent decisions on projectent.7 Many of these key parameters can be determined with wirelineents. A methodical process for utilizing wireline logs in conjunctiontic data to evaluate the CBM potential is the next topic of discussion.
al Identification
thickness of a particular coalseam is determined by following thesereline log measurement cutoffs:
lk-density measurements less than 2 g/cc.mma ray measurements less than 60 API.utron porosity measurements greater than 50%.nic transit time greater than 80 s/ft.ear transit time greater than 180 s/ft.sistivity greater than 50 m2/m.
preceding cutoffs must to be determined locally. The condition of then which the wireline log was recorded must be considered when usingogs for the identification of coal. As mentioned previously, rugosecan give a false indication of coal. Using multiple coal indicators helpsthe negative effects of borehole rugosity on coal thickness.
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Coalbed Methane: Principles and Practices
June 2007
6.4.2 Coal Tonnage
A convenient measure to assess analog CBM projects compares coal tonnage peracre. Since no two CBM fields are identical, there is no reason for two CBMfields with similar coal tonnage per acre to be identical. However, coal tonnageper acre gcoal tonnaCoal tonn
whereCTp
RHO
6.4.3 Pr
The proxicontent, mOf primarmoisture coal sampbetween mlogs.3 It ibetween thproximatemeasuremshould be Formation Evaluations, Logging 315
ives a starting place with stimulation treatment design. Determiningge in the project area is the first step to quantify the available resource.age is calculated using Eq. 6.1.
CTpA = 1359.7 * h * RHOB (6.1)
A = coal tonnage per acreh = coal thickness, feetB = minimum bulk density in the coal, g/cc
oximate Analysis
mate analysis is a routine coal analysis to derive the mineral matteroisture content, volatile matter, and fixed carbon content of the coal.
y interest to the CBM project development are the mineral matter andcontent. Mineral matter, often called ash content, is residue after thele has been burned. When compared, one notices a good correlationineral matter content and bulk-density measurement from wireline
s possible to derive the proximate analysis using the correlationse wireline log measurement of bulk density and the constituents of the analysis. Because each coal is unique, the modeling between coreents and wireline log measurements provides essential information thatobtained early in the lifecycle of the project.7
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Coalbed Methane: Principles and Practices
316 Form
6.4.4 Gas Content in Coal
Determining gas content in coal is the primary goal of CBM reservoir analysis. Itis essential to have representative measurements of the initial gas content in adistribution of coals as well as in the organic-rich shale around the wellbore. Thisinformatioand the winitial gas
When expeffects oconsideracommonly
whereGC
M
ComparincalculatiunderstanCoupling Langmuirresponse.8the CBM ation Evaluations, Logging June 2007
n is used to construct a linear correlation between initial gas contentireline log measurement of bulk density. From the basic correlation,content can be derived for a larger area than just the pilot project.
anding the gas content algorithm for a more descriptive case, thef pore pressure from dewatering the coal need to be taken intotion. Eq. 6.2 is the characterization of the Langmuir isotherm most used to model the gas content through the lifecycle of the well8.
GC_L = Lc * [1- Mc+ Ac] / [PR/(PR+Pc)] (6.2)
_L = Langmuir desorbed gas content, scf/tonLc = Langmuir constant, scf
c = Moisture content in the coal, %Ac = Ash content in the coal, %Pc = Langmuir pressure, psiPr = Reservoir pressure, psi
g the measured gas content at initial reservoir conditions with theon Langmuir gas content is another step in CBM reservoirding. The use of the Langmuir isotherm is discussed in Chapter 3.the calculated desorbed gas content using wireline log data with the isotherm is a powerful tool to help users understand CBM production This is especially important through the later stages in the lifecycle ofreservoir when it is time to select infill drilling locations.
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Coalbed Methane: Principles and Practices
June 2007
6.5 Gas-In-Place CalculationsTotal gas-in-place (GIP) calculations are derived by multiplying the project area,coal tonnage, and the gas content together.
whereG
GCCTp
6.6 RThe recovisotherm trecovery content atengineericurve. Theinterferenchigh, prorecovery fgood firstcan be cal
where
GC_
GCFormation Evaluations, Logging 317
GIP = GC_L * CTpA * A (6.3)
IP = Gas in place, scf_L = Langmuir gas content, scf/tonA = Coal tonnage per acreA = Total area in acres
ecovery Factorery factor, discussed in Chapter 3, is estimated using the Langmuiro obtain gas content at initial and abandonment reservoir pressure. Thefactor is estimated as the ratio of the initial gas content and the gas abandonment. The gas content at abandonment pressure is not a strictng calculation because it falls on the steep portion of the isotherm actual recovery factor will be a combination of drainage patterns, welle, production operations, and economic variables. When gas prices are
jects can be economic longer than with low gas prices. The actualactor determination may be decades in the future, but this method is a approximation of the recovery factor. The recovery factor (Eq. 3.12)culated using the units described in this chapter as Eq. 6.4.
R = GC_A / GC_L (6.4)
R = Recovery factorA = Gas content at abandonment pressure from the Langmuir
isotherm, scf/ton_L = Initial desorbed gas content, scf/ton
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Coalbed Methane: Principles and Practices
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6.7 Drainage Area Calculations8
As a CBM project matures, infill drilling may be deemed necessary for optimumgas recovery. In Lifecycle 4, mature asset development, many wells havenumerous years of production history. The cumulative gas production can beused to cagas-in-plaGIP must
whereCDG
When a ccalculated
The assumthe reserproductiolocations.
6.8 CSuccessfuthe singlelifecycle opermeabilcalibrate wireline lcoalseamation Evaluations, Logging June 2007
lculate the volumetric drainage area by rearranging the volumetricce calculations. Note that the units of cumulative gas production andbe the same.
CDA = Cumulative gas production / GIP (6.5)
A = Current drainage Area, AcresIP = Original recoverable Gas-in-place calculation per acre
ircular drainage pattern is assumed, the drainage radius, DR, can be from Eq. 6.6.
DR = (CDA * 43560 / 3.14159)^0.5 (6.6)
ption of a circular drainage pattern is not always a direct reflection ofvoir condition; however, it is a reasonable way to compare then of wells when looking for permeability trends, offset, or infill
oal Permeability/Cleating l CBM production depends on good coal permeability. Permeability is-most important parameter that must be determined early in thef the CBM play. Chapter 4 discusses several methods for determiningity in single seams. When pressure transient test permeability is used tocertain wireline log measurement indications of permeability, theogs can be a very powerful analysis tool used to rank individuals that were not tested for permeability by pressure transient tests.
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Coalbed Methane: Principles and Practices
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Wireline log measurements used to indicate permeability are SP deflections froma shale baseline, microlog response, porosity measured by the magneticresonance imaging logs, and visual observations using MEI devices.
To quantify the microlog response in coal, the log response is first normalized forthe mud repoorly cle
In quantifvuggy car
To calibradeterminpermeabil
Maxim Micro
poorly Maxim Image
6.9 NThe preseformationconduits bto aquiferway to exdescribed coal with 2,000 BW
An additibreakout, Drilling-iBorehole Formation Evaluations, Logging 319
sistivity and then categorized as well cleated, moderately cleated, andated.2
ying MEI logs, most service providers treat cleating as they would abonate to export some volume fraction of cleats.
te the wireline log measurements to give a reasonable permeabilityation, the following procedure is recommended. Crossplot theity-ft, Kh, from the well testing against the following:um absolute value of the SP deflection times coal thickness.
log well-cleated footage + 0.75 * moderately cleated footage + 0.5 * cleated footage.um magnetic resonance porosity times the permeable thickness.
volume fraction of cleating times the thickness observed.
atural Fracturing and Stress Orientationnce of natural fracturing must be considered when evaluating the total analysis around the CBM prospect area. Natural fractures can serve asetween the coals and aquifers. If the coalbeds are somehow connecteds, the production analysis can be very confusing to interpret. The bestamine the wellbore for natural fracturing is to use MEI tools, aspreviously in this chapter. Fig. 6.17 shows an example of a fractureda fracture orientation 1020N. The production from this coal is overPD.
onal benefit of running MEI logs is the identification of boreholemodern-day fracture orientation, and modern-day stress orientation.nduced fractures tend to reflect the modern-day stress orientation.breakout is indicated by borehole elongation and usually occurs in the
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Coalbed Methane: Principles and Practices
320 Form
minimum stress orientation. The best situation occurs when the natural fractureorientation is in some oblique angle to the modern-day stress orientation.
6.10 MMechanicare commbehave achydraulic-the coal teinput wheMechanicompressiof a litholo
6.11 SThe goal wireline lWireline measuremidentifyinThese are throughouation Evaluations, Logging June 2007
echanical Rock Properties in CBM Evaluation al rock properties include Poissons ratio and Youngs modulus, whichonly used in hydraulic-fracture stimulation design. Coal does not
cording to the uni-axial strain model; therefore, it is difficult to model afracture treatment in coal. In general, a hydraulic fracture initiated innds to stay in the coal. Mechanical rock properties are a necessaryn analyzing post hydraulic-fracture treatment history matching.
cal rock properties can either be calculated from measuredonal and shear slowness using dipole sonic logs or derived through usegic model.
ummaryof this chapter was to show the value and utility of incorporatingogs as an integral component of modern CBM project assessment.logs are a very useful evaluation tool when calibrated with coreents, not only for gas content or estimating proximate analysis, but forg stress orientation, natural fracturing, and permeability in coalbeds.essential parameters to understand early in the lifecycle of the field andt the years as the project matures.
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Coalbed Methane: Principles and Practices
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References1Mullen, M.J.: "Log Evaluation in Wells Drilled for Coalbed Methane" RMAG Coalbed Methane San Juan Basin Symposium, 1988.
2Mullen, MCoalbed
3Mullen, Mthe Nortsented aposium,
4Mullen, MJuan Basity of Al
5Halliburto6Lipinski, 7Blauch, Mcal Solutpect," paCalgary,
8Coalbedware, C2003.Formation Evaluations, Logging 321
.J.: "Cleat Detection in Coalbeds Using the Microlog," RMAG Methane Symposium, Glennwood Springs, CO, May, 1991.
.J.: "Coalbed Methane Resource Evaluation from Wireline Logs in heastern San Juan Basin: A Case Study," paper SPE 18946 pre-t the Rocky Mountain Regional/Low Permeability Reservoirs Sym-Denver, CO, March 6-8 , 1989. .J.: "Cased Hole Coal Analysis in Producing Gas Wells in the San
sin" paper presented at the Coalbed Methane Symposium, Univer-abama/Tuscaloosa May 13-16, 1991.n Energy Services Chartbook, 1991.
P., Mullen, M.J., and Gegg, J.: Piceance MRIL paper..E., Weida, D., Mullen, M., and McDaniel, B.W.: "Matching Techni-
ions to the Lifecycle Phase is the Key to Developing a CBM Pros-per SPE 75684, presented at the SPE Gas Technology Symposium, Alberta, Canada, 30 April-2 May, 2002. Methane Play and Prospect Evaluations Using GeoGraphix Soft-ustomer White Paper published on \\http:www.geographix.com,
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Coalbed Methane: Principles and Practices
322 Formation Evaluations, Logging June 2007
Formation Evaluations, Logging6.1 Introduction6.2 Borehole Environment6.2.1 Downhole Environment6.2.2 Wireline Logging
6.3 Tool Measurement Response in Coal6.3.1 Natural Gamma Ray6.3.2 Spontaneous Potential6.3.3 Resistivity Measurements6.3.4 Micro-Resistivity Measurements6.3.5 Nuclear Measurements6.3.6 Acoustic Measurements6.3.7 Magnetic Resonance Measurements6.3.8 Electrical Imaging
6.4 Wireline Log Evaluation of CBM Wells6.4.1 Coal Identification6.4.2 Coal Tonnage6.4.3 Proximate Analysis6.4.4 Gas Content in Coal
6.5 Gas-In-Place Calculations6.6 Recovery Factor6.7 Drainage Area Calculations86.8 Coal Permeability/Cleating6.9 Natural Fracturing and Stress Orientation6.10 Mechanical Rock Properties in CBM Evaluation6.11 SummaryReferences
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