H06263_Chap_06

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

Coalbed Methane: Principles and Practices

290 Form

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.


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.


292 Form

Fseation Evaluations, Logging June 2007

ig. 6.1Combination toolstring with the location of the different nsor measurement points.


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.


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.


June 2007


The natural gamma ray tool response in a typical coal.


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.


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.


298 Form

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.


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


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


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.


The microlog pad.


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


June 2007

Fig. 6.9the sameFormation Evaluations, Logging 303

Microlog response showing differences in permeability between coals in wellbore.


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).


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


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.


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.


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.


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.


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


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.


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.


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.


312 Form

Fig. 6.16partings.ation Evaluations, Logging June 2007

An electrical-image log in thin coals showing individual cleats and thin


June 2007


An electrical-Image log in a thick coal that is highly fractured.


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.


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


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.


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


318 Form

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.


June 2007

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


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.


June 2007

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,


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

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile () /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/Description > /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ > /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles false /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ]>> setdistillerparams> setpagedevice

H06263_Chap_06

Documents

wireline measurements

coal thickness

coal tonnage

coal industrymeans

wireline logging services

coal industry theses

mineral logging industry

btuireline logging