Saikat Mazumder

Differential swelling and permeability change of coal in response to CO2 injection for ECBM

Abstract The matrix volume of coal swells when CO 2 /CH 4 adsorb on the coal structure. In coalbed gas reservoirs, matrix swelling could cause the fracture aperture width to decrease, causing a considerable reduction in permeability. On a unit concentration basis, CO 2 causes greater degree of coal matrix swelling compared to CH 4 . Much of this difference is attributable to the differing sorption capacity that coal has towards carbon dioxide and methane. This condition in a coal reservoir would lead to differential swelling. Differential swelling will have consequences in terms of porosity/permeability loss, with serious implication for the performance and implementation of carbon sequestration projects. Coal can be understood as a macromolecular cross-linked polymeric structure. An experimental effort has been made to measure the differential swelling effect of CO 2 /CH 4 on this macromolecular structure and to theoretically translate that effect in terms of porosity and permeability. A unique feature of this work is that, real time permeability measurements were done to see the true effect of differential strain from CH 4 saturated coal core flooding experiments.

Capillary Pressure and Wettability Behavior of CO 2 Sequestration in Coal at Elevated Pressures

Summary Enhanced coalbed-methane (ECBM) recovery combines recovery of methane (CH4) from coal seams with storage of carbon dioxide (CO2). The efficiency of ECBM recovery depends on the CO2 transfer rate between the macrocleats, via the microcleats to the coal matrix. Diffusive transport of CO2 in the small cleats is enhanced when the coal is CO2-wet. Indeed, for water-wet conditions, the small fracture system is filled with water and the rate of CO2 sorption and CH4 desorption is affected by slow diffusion of CO2. This work investigates the wetting behavior of coal using capillary pressures between CO2 and water, measured continuously as a function of water saturation at in-situ conditions. To facilitate the interpretation of the coal measurements, we also obtain capillary pressure curves for unconsolidated-sand samples. For medium- and high-rank coal, the primary drainage capillary pressure curves show a water-wet behavior. Secondary forcedimbibition experiments show that the medium-rank coal becomes CO2-wet as the CO2 pressure increases. High-rank coal is CO2-wet during primary imbibition. The imbibition behavior is in agreement with contact-angle measurements. Hence, we conclude that imbibition tests provide the practically relevant data to evaluate the wetting properties of coal.

Calibrated Mechanical Earth Models Answer Questions on Hydraulic Fracture Containment and Wellbore Stability in Some of the CSG Wells in the Bowen Basin

The Coal Seam Gas (CSG) industry is currently focused on the large-scale development of a number of liquefied natural gas (LNG) projects in Queensland, Australia. Arrow Energy is one of four proponents undertaking to develop such a project, with a focus on the Sural and Bowen Basins. Arrow Energy also supplies gas to power stations, mineral refining facilities and ammonium nitrate plants in north Queensland. They are committed to maintaining the supply to these domestic customers and to the growing international gas market through the LNG project at Gladstone through its CSG projects. The wells discussed in this paper are situated within Arrow Energys tenements in the northern Bowen Basin in Queensland. Australia. The targeted Coal Measures in the region usually occur at depths between 230m to 500m below the surface. A number of wells have been drilled in these coal measures and completed either by open-hole, horizontal drilling or, less frequently, hydraulic fracturing. In order to maximize the productivity, and thus success, of these projects, there is a distinct need to understand the borehole stability of horizontal wells, as well the potential for hydraulic fracture containment in the target formations. A rigorous one-dimensional mechanical earth model (ID MEM) was constructed utilizing: • A complete suite of wireline logs, processed and interpreted for the purpose of construction of the model; • Rock mechanical laboratory tests; • Closure pressures obtained from Diagnostic Fracture Injection Tests (DFIT): and, • Drilling-induced borehole failures when drilling the well. The model reveals the variation of maximum horizontal stress direction, the coal properties in the various layers and the contrast in stresses between the coal seams and inter-burden formations. This is one of the first times that a ID MEM has been constructed for CSG application utilizing data acquired specifically for the construction of such a model. This paper presents the workflow utilized in the construction of the MEM for a coal scam gas project and the results obtained. The particular focus is around the following: • Data acquisition strategy (Logs that were acquired for the 1D MEM) • Rock mechanical tests performed at the laboratory, the results obtained and how these results were utilized in calibrating the ID MEM • Closure pressure estimates derived from well tests (DFiTs). and how these results were utilized in calibrating the minimum horizontal stress. • Borehole image log interpretation, characterized drilling induced features (break out and drilling induced fractures) that yield the direction of maximum horizontal stress and calibration characteristics for calibrating the wellbore stability model. • Predominant strike of fractures in the coal from advanced sonic tool.

Detailed Characterization of a Multilayered Coalbed Methane Field Using High-Resolution Sequence Stratigraphy: Examples from the Surat Basin in Australia

The Walloon Coal Measure (WCM) in the Surat Basin in Australia consists of coal-rich mire and a fine-grained meandering fluvial system. The main gas producing targets of WCM are numerous thin coal plies within six coal members with frequent pinching outs, splitting and merging. The geology is stratigraphically complex making correlations of individual coal plies difficult. Consequently, previous geological studies have been mostly based on coal members instead of individual coal plies resulting in inadequate description of the heterogeneity of the coal deposit. To remedy this situation, we proposed a workflow using high-resolution sequence stratigraphy to build an isochronic stratigraphy framework of sublayers and coal plies by utilizing all available data from cores and logs. The key methodology was to identify single fining-upwards cycles with coal, clay or siltstone at the top and sandstone at the base. Then similarity analysis on the cycles was used to identify aggradation, progradation or retrogradation of fluvial facies sequence between adjacent wells. Log density cutoff was used to identify coal, shaly coal, shale, sandstone and siltstone from the whole Walloon fluvial system. Reservoir parameters including gas, ash, moisture content, density, and permeability versus depth were correlated taking into consideration depth shift, regional core data and lithology in different members. All of the above were integrated into a ply-based geomodel which was used to identify highly concentrated, overlapping, continuous plies that are potential sweet pots for field development. Our intent is to provide analogue information and understanding for the coal seam distribution in the green field development of the Surat Basin. This methodology was applied to WCM to perform division and correlation of 20 sub-layers and 125 single plies with thickness ranging from 0.3-1.4 m. Coal distribution area versus thickness relationship was generated to analyze the variogram range used for some key properties, especially density and net-to-gross, and to investigate the impact of coal continuity on well spacing. Five micro-facies in fluvial system were used to describe the distribution of coal properties, impact of coal architecture and heterogeneity. Several potential sweet spots for field development were identified. With proper upscaling, this high-resolution ply-based model can be used in reservoir simulation to forecast production and calculate estimated ultimate recovery (EUR). This methodology has been applied to three coalbed methane (CBM) fields in the Surat Basin in Australia. It is novel in applying high-resolution sequence stratigraphy used in geomodel building of convention oil and gas reservoirs to CBM characterization. It has resulted in a better understanding of the complex depositional character of the WCM and consequently more accurate determination of potential sweet spots, production forecast and EUR calculation.

Surface Well Test Of Methane Gas Saturation In CSG Lateral Completions During Dewatering Using Raman Spectroscopy

This paper describes application of an established coal seam gas (CSG) testing technology to the reservoir and production conditions prevalent in the Bowen basin, Queensland, Australia. The technology involves using Raman spectroscopy to measure gas partial pressure in single phase fluids produced under pressure from dewatering coal seam gas wells. The technique’s efficacy has been described extensively in technical literature. Prior tests have been conducted on permeable coals that produce several hundred BWPD. Applying the technology to tight coals in the Bowen basin is not considered straightforward. Introduction Arrow Energy is a leading coal seam gas (CSG) company with five domestic gas supply operations, ownership of one gasfired power station, interest in two others and plans to safely produce and export liquefied natural gas (LNG) through a world class plant on Curtis Island off Gladstone, Queensland, Australia. Arrow Energy is continuously striving to implement a number of core strategic business and technical initiatives, one being to evaluate and harness new innovative technologies, techniques and best practices directed at minimising field development costs. This drove Arrow Energy to commission a multi-well field trial of a spectroscopy-based technology, which provides a low cost, rapid option for establishing critical desorption pressure (CDP) across a CSG field, and important information regarding gas saturation change over time and gas drainage progress for CSG operators with tenements overlaying underground coal mine resource targets. In the absence of viable production logging tools for being able to back allocate production from individual coal seams, this could prove to be a game changer in terms of how CSG companies work in close association with overlapping tenure holders. Background Efficiently developing gas from coal seams can be a significant challenge for coal seam gas lease holders. This is because heterogeneity of gas pressures and gas content in coal seams can be extremely high, both laterally across a field and vertically between seams. Coal seam heterogeneities exist at essentially every level of resolution of inspection, ranging from microscopic changes in a coal’s sorptivity for gas to “mesoscopic” variations in ash content, cleating and fracturing, and “macroscopic” changes in coal seam continuity, gas genesis and overburden burial history across a CSG field. Existing ex-situ techniques for measuring gas content of coals require collection and laboratory analysis of numerous core samples, at sufficient density and spatial distribution, to capture the complex, distributed characteristics of the coal seam being evaluated. In some cases however, the sampling program is too sparse, culminating with inaccurate gas distribution models. In some cases, the analyses are complicated by changes to the samples that may occur during collection. Furthermore, gas content of coals is typically measured using the direct method analysis (DMA) on freshly cut cores, or more recently developed fast desorption techniques. A problem with these techniques is that overall results can be influenced by artifacts of the test apparatus and procedures used, by core sample type, by sample collection methodology and by the analysis conditions. Even if all these factors are precisely controlled, the accuracy of derived in-situ gas content values obtained using either technique can still be compromised through significant errors in Q1 values, which can only be modeled, not measured. Compounding this inherent error is the fact that core desorption is a destructive testing method that cannot be completed twice on the same sample. This precludes methods for validating measurements, with no possibility therefore of assigning error bars to core desorption data.