Search

Abstract: The Huntly coalfield has significant coal deposits that contain biogenically-sourced methane. The coals are subbituminous in rank and Eocene in age and have been previously characterised with relatively low to moderate measured gas (CH4) contents (2–4 m3/ton). The CO2 holding capacity is relatively high (18.0 m3/ton) compared with that of CH4 (2.6 m3/ton) and N2 (0.7 m3/ton) at the same pressure (4 MPa; all as received basis). The geothermal gradient is also quite high at 55 °C/km. A study has been conducted which simulates enhancement of methane recovery (ECBM) from these deposits using a new version of the TOUGH2 (version 2) reservoir simulator (ECBM-TOUGH2) that can handle non-isothermal, multi-phase flows of mixtures of water, CH4, CO2 and N2. The initial phase of the simulation is CH4 production for the first 5 years of the field history. The model indicates that methane production can be significantly improved (from less than 80% recovery to nearly 90%) through injection of CO2. However, although an increase in the rate of CO2 injection increases the amount of CO2 sequestered, the methane recovery (because of earlier breakthrough with increasing injection rate) decreases. Modeling of pure N2 injection produced little enhanced CH4 production. The injection of a hypothetical flue gas mixture (CO2 and N2) also produced little increase in CH4 production. This is related to the low adsorption capacity of the Huntly coal to N2 which results in almost instantaneous breakthrough into the production well. [Copyright &y& Elsevier]

Abstract: Secondary biogenic gas content can be related to textural characteristics in Eocene age subbituminous coals from the Huntly coalfield, New Zealand. However, the relationships between the two major coal seams in the basin are considerably different despite their close stratigraphic proximity (less than 25 m). In this study, 163 coal samples were collected and desorbed from eight drill holes. Gas adsorption capacity and proximate analyses were conducted as well as macroscopic logging for coal type and vitrain banding characteristics. Vitrain bands were quantitatively point counted and the longest dimension of the shortest axis measured. Three coal types were recognized: bright luster non-banded, bright moderately banded and bright highly banded. Vitrain band thickness, converted to the phi (−log2) scale, was found to increase across the coal types with the thickest bands being associated with the most banded coal type. Overall, when normalized by seam and location, the dataset reveals a relationship between coal type and gas content with the non-banded coal type having the highest gas contents and conversely, the coal types with the most vitrain bands having the lowest gas contents. However, when the seams are considered separately, it can be seen that in the stratigraphically higher Renown coal seam, gas has an indirect association with increasing band thickness, in agreement with the overall trend, while the stratigraphically lower Kupakupa coal seam appears to have a direct relationship. Interestingly the Renown seam, which has a greater percentage of non-banded material, generally has a greater methane adsorption capacity as well as a greater gas content compared to the Kupakupa seam. It is believed these differences are related to macroscopic texture and that the differing proportions of the coal types between the two seams has a fundamental effect on microporosity, ultimately controlling the available surface area for gas adsorption. [Copyright &y& Elsevier]

Abstract: Samples of subbituminous coal from the Huntly Coalfield, New Zealand, were analyzed for methane adsorption at a variety of moisture contents and temperatures. Density and moisture determinations as part of the procedure were used to experimentally confirm that the packing density of water adsorbed to the coal''s surface is equivalent to that of normal liquid water. Moisture content was found to be critical in evaluating the storage capacity of these coals. Low moisture was associated with elevated adsorption isotherms. The relationship between moisture content and gas adsorption is non-linear. Temperature over the range we tested was found to play only a small role in storage capacity for this coal. A series of algorithms were developed to correct for moisture and temperature variations between samples. Testing of the algorithms by using an additional sample indicated that they are close to predicting experimental data but that the constants still require further refinement. These algorithms only apply to this coalfield and are not intended to be widely applied. In addition, it was found that methane adsorption isotherms determined on samples collected after desorption were elevated in comparison to those determined on samples which were collected immediately after drilling. [Copyright &y& Elsevier]

Abstract: Four coal cores, one from the Huntly (Eocene), two from the Ohai (Cretaceous) and one from the Greymouth (Cretaceous) coalfields, were sampled and analysed in terms of gas content and coal properties. The coals vary in rank from subbituminous B–A (Huntly) to subbituminous C–A (Ohai), and high volatile A bituminous (Greymouth). Average gas contents were 1.60 m3/t (s =0.2) in the Huntly core, 4.80 m3/t (s =0.8) in the Ohai cores, and 2.39 m3/t (s =0.8) in the Greymouth core. The Ohai core not only contained more gas but also had the highest saturation (75%) compared with the Huntly (33%) and Greymouth (45%) cores. Carbon isotopes indicate that the Ohai gas is more mature, containing higher δ 13C isotopes values than either the Huntly or Greymouth gas samples. This may indicate that the gas was derived from a mixed biogenic and thermogenic source. The Huntly and Greymouth gases appear to be derived solely from a secondary biogenic (by CO2 reduction) source. Although the data set is limited, preliminary analysis indicates that ash yield is the dominant control on gas volume in all samples where the ash yield was above 10%. Below 10%, the amount of gas variation is unrelated to ash yield. Although organic content has some influence on gas volume, associations are basin and/or rank dependent. In the Huntly core total gas content and structured vitrinite increase together. Although this relationship does not appear for the other core data for the Ohai SC3 core, lost gas and fusinite are associated whereas gelovitrinite (unstructured vitrinite) correlates positively with residual gas for the Greymouth data. [Copyright &y& Elsevier]

The article presents a study that determines the concentration of dissolve organic carbon (DOC) and the specific ultraviolet absorbance of 193 samples of water collected from groundwater, porewater, drainage ditches, and streams at peatlands in New Zealand. Results from the study shows that there was a wide range in DOC concentration in peatlands where there appeared to be large amount of iron. Also, DOC concentrations were large in ombrotrophic, or rain-fed peatlands.

Coal seams within the Waikato Coal Measures can vary significantly (>50%) in thickness over short lateral distances (<500 m). Normal faulting during coal-measure deposition could produce the observed variations in seam thicknesses, but the timing of faulting has not been determined precisely. To constrain the timing of faulting, and its potential impact on coal seam geometry, we have analysed displacements for nine normal faults striking both NNW to N and NNE to NE. Displacements were derived from drillholes, mine workings, and seismic reflection lines. The data indicate that normal faulting occurred after peat accumulation in the coal measures, in some cases perhaps commencing in the latest Eocene to early Oligocene (i.e., c. 33-36 Ma). Normal faulting had little or no impact on the accumulation of peat in the Waikato Coal Measures. Variations in coal-seam thickness were controlled by basement paleotopography and fluctuations in sediment supply. [ABSTRACT FROM AUTHOR]

Abstract: Previous studies of mineral matter in low ash yield, low rank coals have suggested that much of the inorganic material is present as organically bound elements rather than as discrete minerals. This study investigates the location and occurrence of this inorganic material to the angstrom level using non-invasive small angle scattering techniques. Microstructural analysis conducted on matrix and vitrain samples, collected from the subbituminous coals of the Huntly coalfield, found that the inorganic material is located in the 12.5Å< r <125Å size range. This was consistent for all samples, irrespective of seam or location. As there was greater influence on X-ray scattering curves seen in the vitrain samples than the matrix samples, it is suggested that the inorganic material is located within the structured vitrinite tissues, likely originating from the original plant material rather than being precipitated post deposition. The fitting of microstructural models to the scattering curves suggests that the inorganic material exists as inorganic shells or coatings. [Copyright &y& Elsevier]

Abstract: Small angle scattering techniques (SAXS and SANS) have been used to investigate the microstructural properties of the subbituminous coals (R max 0.42–0.45%) from the Huntly Coalfield, New Zealand. Samples were collected from the two thick (>5 m) coal seams in the coalfield and have been analysed for methane and carbon dioxide sorption capacity, petrography, pore size distribution, specific surface area and porosity. Specific surface area (SSA) available for carbon dioxide adsorption, extrapolated to a probe size of 4 Å, ranged from 1.25×106 cm−1 to 4.26×106 cm−1 with total porosity varying from 16% to 25%. Porosity was found to be predominantly composed of microporosity, which contributed the majority of the available SSA. Although considerable variation was seen between samples, the results fit well with published rank trends. Gas holding capacity at the reservoir pressure (approximately 4 MPa) ranged from 2.63 to 4.18 m3/t for methane on a dry, ash-free basis (daf) and from 22.00 to 23.72 m3/t daf for carbon dioxide. The resulting ratio of CO2:CH4 ranged from 5.7 to 8.6, with an average of 6.7:1. Holding capacities for both methane and carbon dioxide on a dry ash free basis (daf) were found to be correlated with sample microporosity. However, holding capacities for the two gases on an as analysed (aa) basis (that is including mineral matter and moisture), showed no such correlation. Carbon dioxide (aa) does show a negative correlation with both specific surface area and microporosity. As the coals have low inorganic matter content, the reversal is thought to be related to moisture which is likely concentrated in the pore size range 12.5–125 Å. Methane holding capacity, both daf and aa, correlates with macroporosity, thus suggesting that the holding capacity of micropores is diminished by the presence of moisture in the pores. [Copyright &y& Elsevier]