Wireline logs are used to identify coal deposits in the subsurface, to determine seam thickness, and to measure or estimate various coal properties such as density, proximate analysis, and mechanical constants. First run to assess mineability of coal deposits, wireline logs have been adapted to coal gas exploration and development in recent years. Coal logs can be run in both open and cased holes filled with either liquid or air. More than a dozen wireline tools show response in coals, with the most useful logs for coal gas reservoir engineering being bulk density, gamma ray, and caliper logs. Identification of coal reservoirs with wireline logs is somewhat dependent on local coal properties, but for reservoir engineering purposes, coals generally exhibit densities less than 2.0 g/cm3 and a gamma ray response of less than 70 API units measured in an in-gauge hole. Additional logs sometimes run in coals include compensated neutron porosity, spectral density (photoelectric), sonic, spontaneous potential (SP), desorption pressure, resistivity, microlog, laterolog, formation imaging logs, carbon/oxygen, cement, and production logs. Typical log responses in a coal, along with reservoir engineering information derived from a particular log, are listed in table 3–5. Conversely, logs to determine specific reservoir properties are shown in table 3–6. Recommended openhole logs for coal wells are gamma ray, density, and caliper. Interpretation of any wireline log is, of course, strengthened with a mud log (also referred to as a surface log).
Table 3–5. Wireline log responses in coals
Log Purpose Open/cased hole Mud/air Response Units
bulk density coal identification net thickness
coal density proximate analysis mechanical properties
o m/a 0.70–1.80 g/cm3
gamma ray coal identification
proximate analysis mineralogy
o/c m/a 20–25 API
caliper hole size
washouts o m/a varies inches, cm
compensated neutron coal identification proximate analysis
gas content
o/c m >50 ×
spectral density (photoelectric) lithology
same as bulk density o m/a 0.18
sonic coal identification
coal rank o m 95–135 μsec/ft
spontaneous potential (SP) none o m varies mv
desorption pressure desorption pressure o water varies psi, MPa
resistivity coal identification o m 50–2,000 ohm-m
microlog permeability
cleating o m varies
varies ohm-m
ohm-m
Laterolog permeability
cleating o m varies
varies ohm-m
ohm-m
FMS cleat development o m varies ×
FMI cleat development
cleat orientation o m varies ×
carbon/oxygen coal identification
coal density o/c m/a varies ×
cement cement integrity c m varies ×
Source: Hollub, V., and Schafer, P. 1992; and Hall, R. V. 2002.
As with conventional logging tools, hole conditions affect tool response, and washout, especially in friable coals, complicates log interpretation. Running logs in holes filled with heavy muds or muds containing many additives can contribute to coal seam damage, especially in dry or permeable coals, which can imbibe significant liquid into the near-wellbore region during drilling, logging, and completion. The radius of investigation of various logging tools in coals is on the order of fractions of a meter, and wireline logs provide little, if any, information about coal permeability, porosity, and fluid saturations. Production logging in coal wells, similar to that in conventional wells, serves to quantify gas and water production from completed seams. However, it is compounded by the requirement that most coal wells require artificial lift.
Perhaps the most useful wireline log for coal gas reservoir engineering is the bulk density log, especially the high-resolution bulk density log, which is often used to identify coal, determine net coal thickness, and construct ash, density, and gas content correlations. The density tool pushes pads against the side of the wellbore, causing it to read erroneously low densities in washed-out sections. Confidence in a density log is higher when the caliper log indicates an in-gauge hole. Coal density, which is less than that of conventional reservoir rocks, is a function of coal rank and purity. In order of increasing rank, bulk density of lignites ranges from 0.7 to 1.5 g/cm3, subbituminous from 1.2 to 1.75 g/cm3, bituminous from 1.2 to 1.5 g/cm3, and anthracites from 1.4 to 1.8 g/cm3.113 The fraction of ash in a coal affects bulk density more strongly than does rank, precluding rank determination from a density log. Mullen correlated bulk density of north San Juan Basin coals with the volume fraction of ash.114 This data was then combined with proximate analysis data to estimate fixed carbon and volatile matter on a daf basis for rank determination. Modern algorithms for coal rank determination from wireline logs are more sophisticated, utilizing multiple logs, and are more accurate when driven with local, rather than general, coal correlations.
Table 3–6. Wireline logs for measuring specific reservoir properties
Property Wireline log
coal identification bulk density
spectral density gamma ray caliper carbon/oxygen sonic comp. neutron resisitivity
net thickness bulk density
spectral density comp. neutron
coal density bulk density
spectral density carbon/oxygen
proximate analysis bulk density
spectral density gamma ray comp. neutron
coal rank sonic
comp. neutron mechanical properties bulk density
spectral density
permeability microlog
FMS FMI
gas content density
gamma ray comp. neutron Source: Hollub, V., and Schafer, P. 1992: and Mavor, M. J., et al. 1994.
The density used to define coal, typically referred to as the pay cutoff density, influences not only seam thickness and average seam density but also volumetric gas in place calculations. Schopf’s definition of coal as a rock that is at least 50% by weight and 70% by volume carbonaceous material was combined with densities of organic material, ash, and water in chapter 2 to calculate coal densities.115 Maximum density of bituminous and subbituminous coals were calculated to be 1.63 and 1.34 g/cm3, respectively. While these densities are often suitable for identification of mineable coals, poorer quality coals with higher densities are often integral components of coal gas reservoirs. As noted in chapter 2, many operators currently employ a density cutoff of 2.0 g/cm3, which results in ash fractions of bituminous and subbituminous coals of 75% and 84%, respectively.
Use of a higher pay cutoff density results in thicker coal seams, higher average seam densities, and lower in-situ gas content, all of which affect volumetric gas in place calculations. The effect of density cutoff on these parameters and the resulting volumetric gas in place calculated for the Ferron coals in the Drunkard’s Wash Field of the Uinta Basin was investigated by Lamarre and Pratt.116 They concluded that increasing the pay cutoff monotonically increased gas in place. Opening up the pay cutoff from 1.75 g/cm3 to 2.55 g/cm3 more than doubled original gas in place. Increases in calculated gas in place volumes due to higher density cutoffs are more dramatic in dirty, shaley coals, reflective of swamps subject to frequent flooding or volcanic ashfalls. Such increases in calculated gas in place volumes are also seen in coal deposits encased in thick, shaley rinds, indicative of marginal swamp environments over relatively long geologic periods. Rapid changes in geological conditions conducive to coal swamp formation and growth results in sharp density contrasts between coal and bounding beds. Higher density cutoffs in such coals weakly affect coal seam thickness, average density, and gas in place.
Vertical resolution of the bulk density tool is on the order of 0.5 ft (0.15 m), while that of some coal mining density tools can be as small as 1 in. (0.025 m). Coal partings less than 0.5 ft in thickness, readily apparent in cores, are smoothed out in the density log curve. Comparison of density logs with coal cores from the same interval provides insight into the fine-scale vertical heterogeneity of a seam. Density tool resolution also affects definition of net coal thickness and yields erroneously high average seam densities in thin coals. In plays such as
the Cherokee and Illinois basins, where the coals are present as multiple, thin beds, many operators are currently completing coal seams as thin as 2 ft. In such plays, differences in tool calibration, logging speed, and log analysis can result in different interpretations of seam characteristics and the gas resource they hold.
Natural radioactivity in a formation is measured by the gamma ray tool. Gamma ray log response in a coal can be either low or high, depending on local conditions. Clean coals contain few radioactive minerals and consequently have gamma ray signatures less than 70 API units, sometimes as low as 25 API units, similar to that of sandstones and less than that of shales.117 In contrast some coals, such the Ferron coals in the Drunkard’s Wash Unit of the Uinta Basin, exhibit high gamma ray responses. Carbon converts soluble uranium salts to an insoluble form, and uranium precipitated from groundwater flowing through the Ferron coals clearly shows on spectral gamma ray logs. Clay and shale laminations in coals can often be identified by their high gamma ray responses compared to clean coal. Thus, gamma ray logs are useful for understanding coal seam vertical heterogeneity, for correlation of seams between wells, and for estimation of the ash fraction in proximate analysis correlations. Washouts suppress gamma ray response, and thus confidence in this log is increased if a caliper log is also available. A gamma ray log can also be run in cased holes, making it useful in perforating and production logging operations, similar to conventional reservoirs.
Neutron induced gamma ray logs can be used to identify and quantify a variety of minerals such as sulfur, calcium, and silicon. The presence of sulfur, usually pyritic, indicates a marine influence during deposition and initial burial. Calcium, especially calcium carbonate, is an indicator of cleat mineralization. Mineral concentrations, estimated from these elemental concentrations using correlations based on local conditions, are used to determine coal ash and mineral matter content.
The caliper log measures hole diameter and is more useful as a quality check on other wireline logs, such as density and gamma ray logs, than as a direct indicator of coal properties. Running this log helps prevent mistaken identification of coals from the erroneously low formation density and gamma ray responses seen across severe washouts. Washout on a caliper log is sometimes taken as an indication of permeability when in fact it is more of an indication of friable coal.
The compensated neutron log (CNL), used to measure porosity in conventional reservoirs, shows erroneously high porosity in coals, making it useful for identifying coals and determining net thickness. It is also used in some log analysis algorithms for rank determination and estimation of gas content. The CNL can be run in both open and cased holes, but like many other wireline logs run in coals, the tool is affected by washouts.
The spectral density tool, similar to the bulk density tool, provides lithology identification but employs the photoelectric effect. The photoelectric absorption index of coal is an order of magnitude less than those of sandstone and shale, providing a second means of coal identification. In addition, it is often used in conjunction with other logs in correlations to determine proximate analysis.
The sonic log can be used to identify coals and determine coal rank. Sonic travel times in coals fall between those of sandstones and shales. Due to similar travel times, discrimination between anthracite and shales is sometimes difficult. While this log is useful in assessing mineability and quality of a coal, it has seen limited application in coal gas reservoir engineering.
The spontaneous potential (SP) log response to coals is weak and inconsistent. Some coals exhibit marked SP deflection, making discrimination with a porous, wet sandstone difficult. Although SP deflection can often be qualitatively correlated with permeability development in conventional reservoirs, such interpretations are rarely possible in coal deposits.
The fraction of the coal void volume occupied by free gas is termed gas saturation, similar to conventional reservoirs. Unfortunately, the gas volume sorbed onto the surface of the coal matrix expressed as a fraction of the theoretical maximum sorbed gas at reservoir pressure and temperature is also termed gas saturation. A coal that holds this theoretical maximum sorbed gas is said to be saturated. Coals that contain less than the theoretical maximum are said to be undersaturated. Undersaturated coals are commercially challenging, as a project must bear the expenses of producing and disposing of large volumes of water prior to any gas production and revenue from gas sales. The pressure at which gas desorbs from a coal is termed desorption pressure, and comparison of this pressure against initial reservoir pressure provides an indication of undersaturation. Partial pressure of methane dissolved in coal formation water can be obtained via Raman spectroscopy.118 Physical chemistry
arguments that this pressure equals methane desorption pressure allow computation of coalbed gas content if a sorption isotherm has been measured for this coal. Knowledge of coal seam pressure permits calculation of saturated coalbed gas content from that same isotherm. Division of coalbed gas content calculated from methane partial pressure by that calculated from reservoir pressure yields gas saturation of the coal matrix.
Difficulties with running this log include optical clarity of the wellbore fluid and ensuring that it is representative of the native reservoir water.
Resistivity of coal is high compared to other lithologies, ranging from 50 to 2,000 ohms, and is more a function of impurities and water salinity than rank.119 In washed-out holes, a resistivity log can be used in conjunction with density and gamma ray logs to increase confidence in identification of coal seams but is of limited use in determination of coal properties. In frontier areas, resistivity logs from logging suites run in old wells drilled to deeper, conventional targets can be used to identify coals. Vertical resolution of resistivity tools, especially older tools, is on the order of 1 ft (0.3 m), precluding accurate seam thickness measurements. Mavor et al.
noted that separation of shallow, medium, and deep resistivity traces were useful indicators of coal permeability development but gave no details.120
Cleat detection with wireline logs is difficult. Coal mineralogy based on neutron induced gamma ray logs has been used to infer cleat development, with lower percentages of a given mineral being interpreted as indicative of a well-cleated coal and higher percentages indicative of poorly cleated coals.121 This method is complicated by well-cleated coals with mineralized cleats or high ash contents. Johnston and Scholes combined gamma ray, density, neutron, and resistivity logs to determine coal cleating and maceral groups in Fruitland coal in the San Juan Basin.122 Multiple assumptions and local log responses limit applicability of this method to other coals. The spectral density log is sensitive to barite, and a high photoelectric response in a coal drilled with mud containing barite is sometimes interpreted as an indicator of coal cleat development and, hence, coal permeability. The photoelectric absorption index of barite is nearly three orders of magnitude greater than coal (266 versus 0.3).
As a result, the photoelectric trace of the spectral density log run in wells drilled with muds containing barite additives is useful for identifying thick mudcake, and by inference, well-cleated, highly permeable coals. Mullen noted that reliable identification of cleating in north San Juan Basin coals with this method requires mud weights of 11.5 lb/gal or higher, increasing the risk of drilling damage to the coal seams.123
The microlog, which measures mudcake and formation resistivities, can show significant separation in some well-cleated coals but should be used with caution, as the response can vary from basin to basin and seam to seam.124 The degree of separation of the resistivity traces is influenced by mud additives and viscosity and hole washouts, as well as coal cleat aperture and intensity. Thus, this tool is limited to qualitative indications of coal permeability at best. Correlation of microlog response with well productivity on a local basis can predict productivity of new wells, but quantification of permeability with this log remains elusive.
A method for qualitative assessment of coal cleating in San Juan Basin coals was developed by Mullen based on a microlog index defined as125
(R – Rm)
I = ———–—
(Rc – Rm)
where
I = microlog index,
Rm = mud resistivity corrected to bottomhole temperature, Rc = cutoff resistivity, typically 30 × mud resistivity, Rm, and
R = average microlog resistivity.
Average microlog resistivity is defined by
(Rml – Rmn)
R = ————–—
2
where
Rml = microlateral resistivity, and Rmn = micronormal resistivity.
This microlog index, conveniently independent of mud resistivity and ranging from 0 to 1, was used to characterize the degree of cleating in selected wells. Well-developed cleats were defined to be those intervals with a microlog index less than 0.3. Moderately developed cleats had a microlog index between 0.3 and 0.6, and poorly cleated sections had microlog index values greater than 0.6. Comparison of cleat development in a half dozen wells with subsequent performance allowed producibility prediction of newly drilled wells.
Cleat development in Fruitland coals in the north San Juan Basin was evaluated by Hoyer using the Dual- Laterolog.126 Although this tool cannot resolve cleat aperture width or spacing, the conductivity differential between shallow and deep laterologs permitted estimation of seam productivity. Cleat development in a single seam, similar to the work done by Mullen, should be possible with this tool but was not reported by Hoyer.
Response from the Dual-Laterolog was employed by Aguilera to determine cleat spacing and aperture and, by extension, cleat porosity and permeability.127 Application to a San Juan Basin coal well indicated the method needed additional refinement but could be a reasonable predictor of well productivity after initial logging results were correlated with subsequent well performance.
Schlumberger’s Formation Microscanner (FMS) and Formation Micro Imager (FMI) have been employed to characterize coal cleating with various degrees of success. Hollub and Schafer reported orientation of Warrior Basin coal cleats determined by the FMS tool and oriented cores were in good agreement.128 The FMI tool can be used to determine cleat orientation and, because cleat geometries are often at the resolution limits of the tool, qualitative cleat development. Characterization of coals in the Alberta plains with the FMI is discussed by Schlumberger, where this tool was also used to identify a fault in a coal seam.129 The tool can be run in both the static and dynamic mode, with the preferred mode being determined by experience in a given area.
Popular cased-hole logs for coal gas reservoir engineering are the carbon/oxygen, gamma ray, compensated neutron, and cement bond logs. Carbon content of a formation and the fluids it holds is measured by the carbon/
oxygen log. Virtually all of the carbon in conventional reservoirs occurs as hydrocarbon fluids in the pores, allowing determination of fluid saturations from this log. Coals, composed primarily of a carbonaceous matrix with small, water-filled porosities, gives a distinctive signature on a carbon/oxygen log that can be correlated with coal density. This log can be run in both open and cased holes, permitting identification of behind-pipe coals in wells drilled to deeper conventional targets. Sensitive to washouts, interpretations from a carbon/oxygen log are strengthened when a caliper log is available.
Gamma ray and compensated neutron logs are employed in cased holes to identify coals and determine net coal thickness. The pulsed neutron log can also be used to identify coals but requires determination of the pulsed neutron ratio cutoff to determine coals, sometimes on a well-by-well basis but certainly on a regional basis. All three of these logs are sensitive to washouts behind pipe.
Cement bond logs are employed in coal wells, as in conventional wells, to assess integrity of cement. Often required by regulations, these logs provide little information about the coal reservoir, including the degree of cement damage or infiltration.
Production logs are difficult to run in coal wells because these wells often require artificial lift. Once a coal well is dewatered and the pump removed, little incentive exists for running production logs, as most of the flow is simply single-phase gas. Hollub and Schafer discuss use of various production logging tools, including downhole video.130
While wireline logs are fundamental for characterization of coal reservoirs, currently they cannot determine coal permeability, porosity, or water saturation. Several tools, including the microlog, Dual-Laterolog, and Formation Microscanner, can often be utilized to estimate well productivity, especially on a local basis when log response can be calibrated against subsequent gas and water production. Effective permeabilities to gas and water in a coal seam are best determined from well tests or performance analysis.
Coal is primarily a solid, organic matrix with little void space. This small void space, divided between matrix porosity and fractures, is often only a few percent of the coal seam volume. The matrix porosity typically contains