A miniature needle probe was used to obtain thermal-conductivity measurements of quartz sand and a core specimen recovered from the Mallik reservoir.
Contents Index
Thermal conductivity of sediments within the gas-hydrate-bearing interval at the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well J.F. Wright1, F.M. Nixon1, S.R. Dallimore1, J. Henninges2, and M.M. Côté3 Wright, J.F., Nixon, F.M., Dallimore, S.R., Henninges, J., and Côté, M.M., 2005: Thermal conductivity of sediments within the gas-hydrate-bearing interval at the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well; in Scientific Results from the Mallik 2002 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada, (ed.) S.R. Dallimore and T.S. Collett; Geological Survey of Canada, Bulletin 585, 10 p.
Abstract: Three separate techniques were employed to investigate the thermal conductivity of unconsolidated sediments within the gas-hydrate-bearing reservoir at the Mallik gas hydrate production research site. A miniature needle probe was used to obtain thermal-conductivity measurements of quartz sand and a core specimen recovered from the Mallik reservoir. Measurements were obtained for various pore-water phases relevant to the Mallik geological setting, including pore occupancy by liquid water, gas hydrate, and ice. Measured thermal conductivities were compared to the predictions of an empirical model, which generates estimates of thermal conductivity based on the prescribed physical properties of sediments. Finally, calculations based on ground-temperature data and an estimate of the local geothermal heat flux identified distinct zonal variations in thermal conductivity within the broader gas-hydrate-bearing interval in the Mallik reservoir. These zonal variations are clearly related to variations in sediment character, rather than to the presence or absence of gas hydrate. Résumé : Nous avons employé trois méthodes différentes pour déterminer la conductivité thermique des sédiments non consolidés du réservoir d’hydrates de gaz qui a été mis à l’étude au site Mallik de recherche sur la production d’hydrates de gaz. Une sonde aiguille miniature a servi à effectuer des mesures de conductivité thermique sur du sable de quartz pur et sur un échantillon de carotte prélevé dans le réservoir Mallik. Ces mesures ont été obtenues pour diverses phases interstitielles pertinentes au cadre géologique du réservoir, dont l’eau liquide, les hydrates de gaz et la glace. Les conductivités thermiques mesurées ont été comparées à des prévisions obtenues au moyen d’un modèle empirique qui génère des estimations de la conductivité thermique à partir de spécifications des propriétés physiques des sédiments. Enfin, des calculs basés sur des données de température du sol et sur une estimation du flux géothermique local ont permis d’identifier des variations zonales distinctes de la conductivité thermique dans l’ensemble de l’intervalle renfermant des hydrates de gaz dans le réservoir Mallik. Il est clair que ces variations zonales sont reliées à des variations de la nature des sédiments plutôt qu’à la présence ou à l’absence d’hydrates de gaz.
1
Geological Survey of Canada, 9860 West Saanich Road, P.O. Box 6000, Sidney, British Columbia, Canada V8L 4B2 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany 3 15 Clarendon Avenue, Ottawa, Ontario, Canada K1Y 0P3 2
1
Contents GSC Bulletin 585
Index
INTRODUCTION A major achievement of the Mallik 2002 Gas Hydrate Production Research Well Program was the completion of an extended thermal-stimulation production test of an isolated gas-hydrate-bearing interval. As described by Hancock et al. (2005), the test consisted of controlled heating of a 13 m thick perforated zone, extending from 907 to 920 m, using a hot-brine circulating fluid. The methane gas released from the dissociated gas hydrate was separated from the circulating fluid at the surface and measured. This stimulation-response experiment was designed to provide critical data necessary to determine the production characteristics of the Mallik gas hydrate reservoir. These data serve as the basis for an intensive post-fieldwork analysis of the reservoir response to thermal forcing. A critical element of these investigations is the specification of appropriate values for a suite of physical and geothermal reservoir parameters critical for numerical modelling of reservoir behaviour. Where temperature forcing is used to drive gas hydrate dissociation, it is particularly important to have an adequate understanding of the thermal properties of the host sediments, in terms of the rate at which heat can be transmitted into the reservoir and the magnitude of the temperature change effected. In this paper, three distinct techniques are used to obtain estimates of the thermal conductivity of unconsolidated sediments occurring within the gas-hydrate-bearing interval at the Mallik site. A comparison of the results indicates a strong consistency between the estimates derived from the different methods, and suggests the practical utility of an empirical model for predicting the thermal conductivity of gas-hydrate-bearing sediments.
BACKGROUND Few data are available regarding the thermal conductivity of sediments containing gas hydrate. For ice-bearing earth materials, on the other hand, it is well understood that the comparatively high thermal conductivity of frozen versus unfrozen sediments is primarily due to the presence of ice (approx. 2.24 W•m-1•°C-1 at 0°C) rather than water (approx. 0.56 W•m-1•°C-1 at 0°C) within the sediment matrix (Williams and Smith, 1989). Although both gas hydrate and ice consist of similar crystalline lattices of hydrogen-bonded water molecules, the thermal conductivity of pure methane hydrate (approx. 0.50 W•m-1•°C-1; Sloan, 1998) is much closer to the accepted value for water than that of ice. Therefore, it is reasonable to expect little difference in the measured thermal conductivities of water-saturated versus gas-hydrate-saturated sediments. Indeed, this notion is supported by data compiled by Asher (1987) with respect to thermal conductivity measurements of Ottawa sand containing water, ice, or gas hydrate. In this example, the measured thermal conductivity of gas-hydrate-bearing sediments was about 2.7 W•m-1•°C-1, similar to the values obtained for water-saturated sediments at temperatures above 0°C. In contrast, the thermal conductivity was greatly increased in frozen sediments in which ice was present instead of gas hydrate, with the measured values for sand at 100% saturation being
2
greater than 5 W•m-1•°C-1. Very low thermal conductivities (approx. 0.3–0.4 W•m-1•°C-1) were measured for dry sediment in which pore spaces were occupied only by gas.
METHODOLOGY In this study, three distinct but complementary techniques were employed to obtain estimates of the thermal conductivity of unconsolidated sediments located within the broader gas-hydrate-bearing interval at the Mallik site: •
direct measurement of thermal conductivity
•
calculation of bulk/zonal thermal conductivities from geothermal-heat-flux measurements and local groundtemperature profiles
•
an empirical model for predicting thermal conductivity based on the physical and mineralogical properties of earth materials
Additional thermal-conductivity data from measurements on various calibration media and on generic quartz samples (both with and without gas hydrate) are also presented in order to demonstrate the reliability of the measurement techniques employed, and to facilitate comparison of the thermalconductivity estimates obtained by the different techniques.
Direct measurement of thermal conductivity Direct measurement of thermal conductivity was conducted in the laboratory using a small-diameter needle (line) probe (1.3 mm diameter by 6.0 mm long) embedded in the test medium, and powered by a Geotherm™ Thermal Properties Analyzer. In this application, power was applied to the needle probe for a period of approximately 3 to 5 minutes, at a rate of about 30 mW•cm-1. The thermal conductivity of the surrounding medium was determined from the transient temperature-time response at the probe, according to theoretical equations for a line source in a semi-infinite medium (following Manohar et al., 2000). A detailed description of line-probe theory and considerations for its practical utility are given in Von Herzen and Maxwell (1959). Needle-probe systems are generally reliable and well suited for measurement of the thermal conductivity of natural sediments; however, close attention must be given to such practical issues as the stability of the ambient temperature of the sample being measured, the nature of the contact between probe and sample, the possibility of induced convection in large pores or voids, and intensity of heating in systems susceptible to phase change. The latter is of particular concern for frozen sediments at temperatures near the melting point of ice and for sediments containing gas hydrate subject to dissociation. Prior to obtaining measurements of the thermal conductivity of the test sediments, the system was calibrated according to a series of measurements on materials of known thermal conductivity, specifically ice (approx. 2.24 W•m-1•°C-1 at 0°C) and water (approx. 0.59 W•m-1•°C-1 at 10°C). The measurements on ice were obtained after allowing water to freeze slowly around the thermal-conductivity probe in order
Contents Index to maximize the thermal contact between the probe and the ice. A dilute mixture of water and gelatin (approx. 6 g•L-1) provided a close approximation of the thermal conductivity of liquid water. This technique eliminates potential effects of convective heat transfer within the medium during the measurement, while ensuring optimum contact between the probe and the medium. Thermal-conductivity measurements on a quartz test sand (100% silica) and a section of gas-hydrate-bearing core recovered from the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well were conducted under controlled pressure-temperature conditions with the large-volume gas hydrate test reactor cell at the Geological Survey of Canada (GSC). The cell is equipped with precision thermistors and pressure transducers to facilitate continuous recording of pressure and temperature conditions throughout the testing period. A schematic diagram of the test-cell configuration is shown in Figure 1. Quartz-sand reference sample (100% silica) A specimen consisting of 100% quartz sand with grain size partitioned in the range 125 to 250 µm was prepared to serve as a reference datum for the comparison of published thermalconductivity data for Ottawa sand (Stoll and Bryan, 1979;
Gauge Thermistor T
P
H Heater
J.F. Wright et al.
Table 1. Physical properties of the 100% quartzsand laboratory-test specimen. Property Texture class Dry density Porosity Gravimetric water content Gas hydrate saturation
Value Medium sand (125–250 µm) -3 1.56 g•cm 0.41 18% 0.70
Asher, 1987) and for subsequent measurements on the Mallik core specimen. The reference specimen also provides a simplified case for the application of an empirical model for predicting thermal conductivity based on specification of the physical properties and mineralogy of the sediment. The quartz sand was wetted to the desired water content and subsequently tamped, a little at a time, into a 7.5 cm diameter by 12 cm long stainless-steel sample holder. The physical properties of the 100% quartz-sand specimen are summarized in Table 1. A 1.5 mm diameter by 6 cm long needle probe was inserted along the radial axis of the test sediment. The test cell was hermetically sealed, purged, and charged with methane gas. Pressure and temperature conditions were manipulated within an environmental chamber to achieve the desired compositions of water and/or ice and/or gas hydrate prior to conducting individual thermal-conductivity measurements. Test conditions reflected those specified by Asher (1987) for a series of thermal-conductivity measurements on pure quartz sand and Ottawa sand, as summarized in Figure 2a. Tests included measurements of the thermal conductivity of 1) gas in sediment, 2) gas hydrate/water in sediment, 3) gas hydrate/ice in sediment, and 4) ice in sediment. Note that, for conditions 2 and 3, only residual amounts of water and ice, respectively, are considered to have been present, with the bulk of the pore space being occupied by gas hydrate. Mallik core specimen
Needle probe
Pressure P transducer Sample Free gas
T Valve
Figure 1. Schematic diagram of the large-volume gas hydrate reactor cell at the Geological Survey of Canada.
A gas-hydrate-bearing core specimen with a nominal diameter of 7.6 cm (3-in.), recovered from a depth of 929 m in the Mallik 5L-38 gas hydrate production research well, was preserved and shipped to the GSC laboratories in Ottawa for testing. Immediately upon recovery at the drill site, the core specimen was hermetically sealed in a 1 m long pressure vessel and pressurized with methane gas, and was maintained at approximately -10°C during transport and subsequent storage. It is clear that some significant proportion of the original gas hydrate content of the core specimen had dissociated during core recovery operations, specifically during core tripping and handling prior to accessing the inner core barrel (see Wright et al., 1999). It is also possible, however, that some additional gas hydrate may have formed within the sediment during the storage period between the time of recovery and the time of testing in the laboratory (Tulk et al., 1999). Working in a cold-room environment at ambient temperatures of about -10°C, the core specimen was extracted from the storage vessel and a series of subsamples taken to determine sediment texture class, bulk density, porosity, and water
3
Contents Index
GSC Bulletin 585
(a)
-1
5
4
3 Water in sediment Gas hydrate in sediment
2
Pure ice
1 Pure water
-10
-5
(b)
Ice in sediment
5
4
Gas hydrate/water in sediment
3
Water in sediment
(Pure ice)
2
Gas hydrate/ice in sediment
1
Gas in sediment
(Water-gel)
Pure propane hydrate
Gas in sediment
0
This study
6
Thermal conductivity (W•m-1•°C-1)
Ice in sediment
-1
Thermal conductivity (W•m •°C )
6
0
0
5
10
15
20
25
Temperature (°C) Asher (1987) Ottawa sand, 20-30 Mesh 80% water saturation 80% water saturation 0% water saturation 100% water saturation
Stoll and Bryan (1979) Ottawa sand, 20-30 Mesh and propane hydrate Propane hydrate
-10
-5
0
5
10
15
20
25
Temperature (°C) Quartz sand (100% saturation) Quartz sand (80% saturation) Quartz sand (dry) Mallik sand with gas hydrate Mallik sand with no gas hydrate Calibration data (ice; water-gel)
Figure 2. Thermal conductivity of sand containing gas, water, ice, and gas hydrate: a) data from Asher (1987) and Stoll and Bryan (1979); and b) this study, which measured thermal conductivity of pure quartz sand and a sand-dominated core specimen recovered from the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well. Arrows indicate the range of thermal-conductivity values that might reasonably be expected for the broader range of natural sediments, accounting for variability in sediment texture, mineralogy, and water content.
content. The gas hydrate content of the core sample was estimated by relating the volume of gas released during dissociation to specimen porosity and water content. The physical properties of the Mallik core specimen are summarized in Table 2. Note that the quartz content was assumed to be 85%, based on average values determined from previous analysis of sand samples from the Mallik 2L-38 research well (Jenner et al., 1999). An approximately 10 cm long, full-diameter test specimen was sectioned from the larger core sample. The ends of the section were faced to achieve a cylindrical shape. The specimen was inserted into a stainless-steel sample holder specially designed to accept a full-diameter Mallik core section. The sample and holder were installed in the GSC’s large-volume gas hydrate test cell. A 1.5 mm diameter by 6 cm long needle probe was coated with an inert thermal-transfer paste and inserted into a pilot hole drilled along the radial axis of the core specimen. Previous GSC experience has shown that the thermal paste improves the thermal contact between the probe and the test specimen, and has produced good results for thermal-conductivity measurements of frozen, ice-bearing sediments. The test cell was hermetically sealed, purged, and charged with methane gas to stabilize the gas hydrate content of the specimen. Thermalconductivity measurements were subsequently conducted at
4
Table 2. Physical properties of a quartz-sand-dominated laboratory-test specimen remoulded from a Mallik core sample. Property Depth of recovery Texture class Dry Density Porosity Gravimetric water content Gas hydrate saturation
Value 929 m (approx.) Medium sand -3 1.75 g•cm 0.34 16% 55%
a series of pressure and temperature conditions corresponding to the presence of 1) gas hydrate/ice in sediment, 2) gas hydrate/water in sediment, and 3) water in sediment (as indicated by the circular symbols in Figure 2b).
Calculation of thermal conductivity from geothermal data An alternative technique for estimating the bulk thermal conductivity of sediments is based on the influence of variations in geology on the local geothermal gradient, and can be
Contents Index applied where suitable information about in situ ground temperatures and the local/regional geothermal heat flux is available (Blackwell, 1983; Ponzini et al., 1989). In theory, the average thermal conductivity of a specified sediment layer can be calculated directly, according to the formula K = Qg / Gg, where K is the thermal conductivity of the substrate, Qg is the local geothermal heat flux, and Gg is the regional geothermal gradient. Utilization of this technique is supported by regional geothermal data specific to the immediate area of the Mallik well site. Majorowicz et al. (1996) conducted a regional assessment of geothermal heat flux data from deep borehole measurements taken at industry exploration wells in the Mackenzie Delta, establishing a geothermal heat flux at the Mallik site of approximately 0.055 W•m-2. Distributed temperature sensing (DTS) cables installed in the three Mallik wells drilled in 2002 provide quasicontinuous measurement of formation temperatures to an accuracy of ±0.3°C (Henninges et al., 2005). The ground-temperature profile at the Mallik site was determined from DTS observations at the Mallik 3L-38 research well (located about 40 m from the main production well) in November 2002, approximately 9 months after completion of drilling operations. Time-series analyses of these data suggest that the thermal disturbance caused by drilling and productiontest operations had largely dissipated at the time of measurement. Indeed, close scrutiny of the record indicates no apparent local temperature disturbance of significance to this analysis. The data indicate that the average geothermal gradient within the broader gas-hydrate-bearing interval in the Mallik reservoir (i.e. between 990 and 1107 m in depth) is 0.0287°C•m-1. Substituting the regional heat flux and measured geothermal gradient into the above equation yields an average thermalconductivity value of 1.92 W•m-1•°C-1 within the Mallik gas hydrate reservoir. The bulk thermal conductivity of each of the individual sedimentary units identified in the Mallik well was determined based on the regional geothermal-heat flux and the average geothermal gradient across the appropriate depth interval.
Empirical model for estimating thermal conductivity Although both geothermal calculations and direct measurement techniques can provide good estimates of the thermal conductivity of natural sediments, they are of limited utility for most field applications and/or regional studies because they rely on expensive field operations for the collection of core specimens and/or the measurement of in situ ground temperatures. Alternatively, a suitable empirical model can be used to predict the thermal conductivity of sediments in cases where the local geology is well known and/or available well-log data can provide basic information about the physical properties of sediments. A number of empirical models have been developed for estimating the thermal conductivity of earth materials (in both unfrozen and frozen state), based on
J.F. Wright et al.
correlations of laboratory measurements with specifications of the physical and mineralogical properties of sediments. Farouki (1981) provided detailed descriptions of several of the most popular of these methods, and concluded that Johansen’s (1975) empirical equations produce good results for a wide variety of conditions. In this study, Johansen’s (1975) equations are employed to generate estimates of thermal conductivity based on the specification of sediment texture, quartz content, dry bulk density, water saturation, and unfrozen water content. Although Johansen’s empirical model is normally used to generate predictions of the thermal conductivity of frozen and/or unfrozen sediments, the model has been adapted to simulate the physical and thermal characteristics of gas-hydrate-bearing sediments, based on the assumption that the thermal conductivity of water and hydrate are essentially equal (i.e. approx. 0.56 W•m-1•°C-1). The model can also simulate cases in which both ice and gas hydrate are present at temperatures below 0°C, through adjustment of the specified unfrozen water content to include both gas hydrate and the remaining liquid-water fraction. Although this situation does not occur within the major gas-hydrate-bearing interval of the Mallik reservoir, it may be relevant to the investigation of possible intrapermafrost gas hydrate at shallower reservoir depths. Sets of model parameter values were established that represent the characteristics of the test sediments for which the measured thermal conductivities presented in Figure 2b were obtained (i.e. dry sediment, moist sediment with or without gas hydrate, ice in sediment, and frozen sediment containing ice with or without gas hydrate). Johansen’s (1975) empirical equations were employed to generate thermal-conductivity estimates for both the generic quartz sand and the Mallik core specimen, and for each of the six sedimentary units identified in Figure 3, according to the physical parameters summarized in Tables 1, 2, and 3, respectively. For modelling purposes, the physical and mineralogical properties of sediments within the individual sedimentary units were assumed uniform at the macro scale (i.e. over depth intervals of tens of metres). This assumption is supported by the observation of a generally uniform temperature profile within each unit. Average bulk density and sediment porosity were determined from well-log data presented by Collett et al. (2005). Dry density was determined on the basis of the porosity estimate and an assumed particle density of 2.65 kg•m-3 (note that bulk density was subsequently backcalculated as a check against the log value). Pore saturation levels were assumed to be 100% in all cases. For the sake of convenience, the distribution of fine versus coarse materials within each unit was characterized according to the USDA soil-texture classification, based on the relative dominance of sand, silt, and clay as indicated in the sedimentological description (Medioli et al., 2005). This surrogate designation is important only with regard to the specification of the quartz content for each sediment class, which was tied to the USDA soil-texture class following Peters-Lidard et al. (1998).
5
Contents
Sedimentary unit Gas hydrate saturation (Sh)
Index
GSC Bulletin 585
Temperature (°C) 5
6
7
8
9
10
11
12
13
900
0
100
910 920
K=2.78
SAND with interbedded GRAVEL
1
SILT
2
SAND with interbedded SILT
3
SILT with interbedded SAND
4a
SILT with interbedded CLAY & COAL
4b
930 940
K=1.49
950 960 970
K=2.16
980 990 1000
Depth (m)
1010 1020
Lignite (K=1.05) K=2.00
1030 1040 1050 1060
K=1.42
1070 1080 1090 1100 1110
K=2.35
SAND
5
1120 1130 1140
1150
SILT with interbedded SAND (K=2.07)
6
1160 1170
Figure 3. Bulk thermal conductivity (K, in W•m-1•°C-1) of sedimentary units within the Mallik gas hydrate reservoir, determined through analysis of local geothermal data. Average geothermal gradients in successive units vary according to the thermal conductivity of the sediments. Note the lack of dependence on the presence or absence of gas hydrate.
6
Contents Index
J.F. Wright et al.
Table 3. Assumed physical parameters and predicted thermal conductivity of the various sedimentary units in the Mallik reservoir. USDA texture class (surrogate) Unit 1 Sand 2 Silt 3 Sandy loam 4a Loam 4b Silty clay loam 5 Sand 6 Loam 1 2 3 4
1
Descrip. Sand-silt-Claycoal S s S with s s with S s with C & c S s with S
2
Quartz content (%) 85 10 60 40 10 85 40
Dry density -3 (kg•m ) 1720 1908 1775 1881 1855 1775 1987
Porosity (%) 35 28 33 29 30 33 25
3
Water content (wt %) 20.3 14.7 18.6 15.4 16.7 18.6 12.6
Thermal 4 conductivity -1 -1 (W•m •°C ) 2.708 1.55 2.27 2.04 1.51 2.35 2.19
From sedimentological descriptions (Medioli et al., 2005) Based on information from Peters-Lidard et al. (1998) Estimated average from well-log data (Collett et al., 2004) Predicted values determined using Johansen’s (1975) empirical equations
RESULTS AND DISCUSSION Measured thermal conductivity of laboratorytest specimens The thermal-conductivity values obtained by direct measurement of the generic quartz sand and the recovered Mallik core specimen are presented in Figure 2b, together with calibration data obtained by direct measurement of the thermal conductivity of water (water-gel) and ice. In general, the measured thermal conductivities of the two test sediments compare favourably with measurements made by Asher (1987) on similar sediments, with the data following a similar pattern of clustering and separation. At temperatures above 0°C, however, the data from this study indicate slightly higher thermalconductivity values for gas-hydrate-bearing sand, compared to the same sediment when only water is present. At temperatures below 0°C, thermal conductivities of gas-hydrate-bearing sand units are generally higher (approx. 3.00 W•m-1•°C-1) than those presented by Asher (1987), likely due to the presence of ice as well as gas hydrate (see data for Mallik sand and pure quartz sand at 80% water saturation). Furthermore, the measured thermal conductivity of frozen quartz sand at 100% water saturation (>5.00 W•m-1•°C-1) is considerably higher than that of sand at 80% water saturation (approx. 4.00 W•m-1•°C-1). This observation is significant, as it indicates a broader range of thermal-conductivity values for ‘ice in sediment’ and reduces the apparent gap between the values for this category and those of the ‘hydrate in sediment’ category, as presented in Asher (1987). In fact, the grouping of the data presented in Figure 2a and b into the discrete boxes shown is misleading, describing a more limited range of thermal-conductivity values for each category than should be expected. The authors have therefore added a series of arrows to accompany the data groupings in Figure 2b, to present a more realistic range of thermal-conductivity values, representing a wider range of sediment textures (i.e. including silt and clay) and water contents. It is reasonable to conclude that the thermal conductivity of gas-hydrate-bearing sediments in the frozen/unfrozen state is dependent on the relative proportions of gas hydrate/ice
and gas hydrate/water, respectively. This situation is not adequately acknowledged in Asher’s (1987) data as presented in Figure 2a, given that the amount of ice or water that might have been present during the thermal-conductivity measurement of ‘gas hydrate in sediment’ is unknown. In the present study, continuous recording of temperature data during the experiments revealed only a very small latent-heat signature during freezing and thawing of the gas-hydrate-bearing test specimens. Thus, the authors are confident that only a small fraction of the pore water was present in non–gas hydrate form during thermal-conductivity measurement of ‘gas hydrate/ice in sediment’ and ‘gas hydrate/water in sediment’, as presented in Figure 2b. Direct measurement of the thermal conductivity of both the generic quartz and Mallik sand specimens, however, reveal logical patterns that can be explained based on the understanding of the influences of different pore-water phases, water contents, and the presence/absence of gas hydrate.
Bulk thermal conductivity of Mallik sedimentary units as calculated from geothermal gradients The results of geothermal calculations to obtain estimates of the bulk thermal conductivity of Mallik sedimentary units are presented in Figure 3. The Mallik reservoir is known to consist of many individual sediment layers containing comparatively high concentrations of gas hydrate, modest gas hydrate amounts, or little or no gas hydrate (Jenner et al., 1999; Dallimore and Collett, 1999). Ground-temperature data indicate a number of distinct transitions in the geothermal gradient (i.e. changes in the slope of temperature versus depth), defining at least six discrete zones, each typically several tens of metres in thickness. These transitions correspond closely to the boundaries of the individual sedimentary units (designated as units 1 through 6) described by Medioli et al. (2005) during characterization of core samples recovered from the Mallik 5L-38 well. It is obvious that the average geothermal gradient within a given sedimentary unit is distinct from that of neighbouring zones, and displays a general internal uniformity. A notable
7
Contents GSC Bulletin 585
Index
exception occurs in unit 4 (1005–1085 m), within which a distinct change in the geothermal gradient is evident at about 1040 m. The average in situ thermal conductivity of the individual sedimentary units was calculated according to the local geothermal heat flux (0.055 W•m-2) and the observed geothermal gradients within each interval. Geothermal gradients were determined from linear regression of temperature versus depth data for each interval (or sedimentary unit). The calculated bulk thermal conductivities of the individual sedimentary units ranged from a low of 1.42 W•m-1•°C-1 in silt-dominated sediments between about 1040 and 1085 m (unit 4b, mean Sh
