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Water Production Simulation Tool for Coal Seam Gas Operations in Bowen and Surat Basins
Abstract
A major challenge in managing coal seam gas (CSG) water is predicting how much water will be produced as the industry develops. Klohn Crippen Berger Ltd (KCB) was commissioned by the Queensland Department of Natural Resources and Mines (DNRM) to develop a tool to forecast where, when, and how much CSG water will be produced in the Surat and Southern Bowen Basins under various industry expansion scenarios. The Water Production Tool (WPT) uses the Theis equation, a non-equilibrium groundwater flow equation that accounts for the effect of pumping time on well yield in confined aquifers, to predict water production volumes. Where multiple pumping wells are in close proximity, the required pumping rates for coal seam depressurisation are reduced. Therefore, pumping effects are projected spatially to assess interference effects between wells. Water production is then up-scaled to the projected scale of the CSG industry within the Surat and Southern Bowen Basins. Two modifications were made to address the dual phase nature of CSG extraction (a transmissivity reduction factor) and the impact of weathering at the eastern margin of the Surat Basin (a 'near edge effect' factor). These factors modify relevant hydrogeological parameters in a consistent and defensible manner to more closely match observed water production. The WPT was verified by comparing results to equivalent Theis equation calculations and to type curves provided by CSG proponents. Calibration was achieved by varying the spatial distribution and intensity of the near edge effect factor in conjunction with a chi-squared analysis of residuals. The tool, which is now being maintained and further developed by the Centre for Water in the Minerals Industry (CWiMI), was built to balance numerical complexity against relative flexibility and simulation speed. The result is a tool that can produce statistically relevant results of prescribed scenarios to plan water handling and treatment requirements in discrete zones of operation.
... For bores with levels reduced by over 5 metres, the CSG industry is required to 'make good' by financial compensation or the provision of replacement water to affected landholders. CSW extraction is estimated at 100 GL/year over the life of the industry to 2060 (Strand, Reading, & Keir, 2013). Therefore, CSW extraction is nearly six times that currently extracted from the WCM for agricultural purposes (Office of Groundwater Impact Assessment, 2012, table 5.1, p. 40) Moreover, if CSW extraction from the WCM has any possible impact on other aquifers, then there may be further impacts for agriculture and the environment (Hillier, 2010). ...
... At present, it is estimated that cumulative water production will plateau at about 5000 GL after 2045. Following this, there is expected to be a rapid decline in water production as gas production peaks (Strand et al., 2013). The current expectation is that CSW water will cease to be produced after 2060. ...
... The current expectation is that CSW water will cease to be produced after 2060. Importantly, the relatively large initial volumes (approximately 150 GL/year) will decline dramatically after around 2040 (Strand et al., 2013). Thus, the major window of opportunity for producers to take advantage of CSW is estimated to be 25 years in duration. ...
The development of the coal seam gas industry in Australia has led to the extraction of large volumes of relatively salty groundwater. Once treated, this coal seam water (CSW) is being made available for irrigation and it is an opportunity for landholders to increase the level and reliability of their agricultural production. This paper reviews the use of CSW and approaches to sustainable agriculture. The review suggests that, barring technical and economic constraints, re-injection of treated CSW into stressed agricultural aquifers may provide more equitable access to water and broader economic benefits than current location-specific irrigation schemes.
A new solution of the unsteady confined radial flow equation subject to aquifer heterogeneity, as characterized by the statistics of the transmissivity, is presented. Simple applications to the direct and inverse problems useful to field hydro geologists are included. The results use the classical Thies solution as an initial condition and the method of decomposition, which does not require many of the restrictions of the traditional small perturbation methods. Given the mean and variance of the transmissivity, the direct problem aims at the prediction of the mean and the variance of the draw downs. Given the mean and the variance of the draw down, the indirect problem aims at the estimation of the mean and the variance of the transmissivity.
Methane is present in all coals, but a number of geological factors influence the potential economic concentration of gas. The key factors are (1) depositional environment, (2) tectonic and structural setting, (3) rank and gas generation, (4) gas content, (5) permeability, and (6) hydrogeology. Commercial coal seam gas production in Queensland has been entirely from the Permian coals of the Bowen Basin, but the Jurassic coals of the Surat and Clarence-Moreton basins are poised to deliver commercial gas volumes.Depositional environments range from fluvial to delta plain to paralic and marginal marine—coals in the Bowen Basin are laterally more continuous than those in the Surat and Clarence-Moreton basins. The tectonic and structural settings are important as they control the coal characteristics both in terms of deposition and burial history. The important coal seam gas seams were deposited in a foreland setting in the Bowen Basin and an intracratonic setting in the Surat and Clarence-Moreton basins. Both of these settings resulted in widespread coal deposition. The complex burial history of the Bowen Basin has resulted in a wide range of coal ranks and properties. Rank in the Bowen Basin coal seam gas fields varies from vitrinite reflectance of 0.55% to >1.1% Rv and from Rv 0.35-0.6% in the Surat and Clarence-Moreton basins in Queensland. High vitrinite coals provide optimal gas generation and cleat formation. The commercial gas fields and the prospective ones contain coals with >60% vitrinite.Gas generation in the Queensland basins is complex with isotopic studies indicating that biogenic gas, thermogenic gas and mixed gases are present. Biogenic processes occur at depths of up to a kilometre. Gas content is important, but lower gas contents can be economic if deliverability is good. Free gas is also present. Drilling and production techniques play an important role in making lower gas content coals viable. Since the Bowen and Surat basins are in a compressive regime, permeability becomes a defining parameter. Areas where the compression is offset by tensional forces provide the best chances for commercial coal seam gas production. Tensional setting such as anticline or structural hinges are important plays. Hydrodynamics control the production rate though water quality varies between the fields.
When a well is pumped or otherwise discharged, water‐levels in its neighborhood are lowered. Unless this lowering occurs instantaneously it represents a loss of storage, either by the un‐watering of a portion of the previously saturated sediments if the aquifer is nonartesian or by release of stored water by the compaction of the aquifer due to the lowered pressure if the aquifer is artesian. The mathematical theory of ground‐water hydraulics has been based, apparently entirely, on a postulate that equilibrium has been attained and therefore that water‐levels are no longer falling. In a great number of hydrologic problems, involving a well or pumping district near or in which water‐levels are falling, the current theory is therefore not strictly applicable. This paper investigates in part the nature and consequences of a mathematical theory that considers the motion of ground‐water before equilibrium is reached and, as a consequence, involves time as a variable.
A coupled fluid-flow and geomechanics model for simulating coalbed methane (CBM) recovery is presented. In the model, the fluid-flow process is simulated with a triple-porosity/dual-permeability model, and the coupling effects of effective stress and micro-pore swelling/shrinkage are modeled with the coupled fluid-flow and geomechanical deformation approach. The mathematical model is implemented with a finite volume method. First, a case without considering coupling between fluid-flow and geomechanics is simulated and compared with an existing simulator. The effects of coupled fluid-flow and geomechanics are then studied in detail with two illustrative examples. The first one is designed for testing the effective stress effect without micro-pore swelling/shrinkage effect, and the other for testing the coupling effects of the effective stress and micro-pore swelling/shrinkage on the methane production. The numerical results indicate that both the effective stress and the micro-pore shrinkage make a significant contribution to fluid-flow in CBM reservoir and to methane production. The methane production sensitivity to Young’s modulus and Langmuir sorption strain are investigated as well. Finally, we make a dynamic analysis of the coupling effects of fluid-flow process and geomechanics.
When water pressure is lowered to allow coalbed methane to desorb and flow to the well, three distinct flow regimes can be identified: (1) saturated water flow with no gas phase; (2) unsaturated water flow with an immobile gas phase; and (3) two-phase flow of gas and water. This paper derives a solution that includes the first two regimes in a single formula. The concept of parameter measurement windows is introduced and applied to analyze data from Glover Well 1 Drawdown Test 2 to obtain additional information on relative permeability and saturation curves. An increase in slope, which was initially attributed to other causes, is explained as a relative permeability effect.
The Middle Jurassic Walloon Subgroup is a prolific, low-rank coal seam gas (CSG) resource in the Surat Basin, Australia. This study applied a regionally consistent stratigraphic framework model to open-file gas desorption and coal property data from 69 cored CSG wells, to assess spatial trends in gas content and test the influence of large-scale geological heterogeneity on present-day gas distribution. Continued successful exploration and production, and possible future microbial regeneration of the Walloon CSG resource require an improved understanding of the controls on gas content distribution across the core region of production in the eastern Surat Basin.
An idealized model has been developed for the purpose of studying the characteristic behavior of a permeable medium which contains regions which contribute significantly to the pore volume of the system hut contribute negligibly to the flow capacity; e.g., a naturally fractured or vugular reservoir. Unsteady-state flow in this model reservoir has been investigated analytically. The pressure build-up performance has been examined in some detail; and, a technique for analyzing the build-up data to evaluate the desired parameters has been suggested. The use of this approach in the interpretation of field data has been discussed. As a result of this study, the following general conclusions can be drawn:Two parameters are sufficient to characterize the deviation of the behavior of a medium with "double porosity" from that of a homogeneously porous medium.These parameters can be evaluated by the proper analysis of pressure build-up data obtained from adequately designed tests.Since the build-up curve associated with this type of porous system is similar to that obtained from a stratified reservoir, an unambiguous interpretation is not possible without additional information.Differencing methods which utilize pressure data from the final stages of a build-up test should be used with extreme caution.
Introduction
In order to plan a sound exploitation program or a successful secondary-recovery project, sufficient reliable information concerning the nature of the reservoir-fluid system must be available. Since it is evident that an adequate description of the reservoir rock is necessary if this condition is to be fulfilled, the present investigation was undertaken for the purpose of improving the fluid-flow characterization, based on normally available data, of a particular porous medium.
DISCUSSION OF THE PROBLEM
For many years it was widely assumed that, for the purpose of making engineering studies, two parameters were sufficient to describe the single-phase flow properties of a producing formation, i.e., the absolute permeability and the effective porosity. It later became evident that the concept of directional permeability was of more than academic interest; consequently, the degree of permeability anisotropy and the orientation of the principal axes of permeability were accepted as basic parameters governing reservoir performance. More recently, it was recognized that at least one additional parameter was required to depict the behavior of a porous system containing regions which contributed significantly to the pore volume but contributed negligibly to the flow capacity. Microscopically, these regions could be "dead-end" or "storage" pores or, macroscopically, they could be discrete volumes of low-permeability matrix rock combined with natural fissures in a reservoir. It is obvious that some provision for the inclusion of all the indicated parameters, as well as their spatial variations, must be made if a truly useful, conceptual model of a reservoir is to be developed. A dichotomy of the internal voids of reservoir rocks has been suggested. These two classes of porosity can be described as follows:Primary porosity is intergranular and controlled by deposition and lithification. It is highly interconnected and usually can be correlated with permeability since it is largely dependent on the geometry, size distribution and spatial distribution of the grains. The void systems of sands, sandstones and oolitic limestones are typical of this type.Secondary porosity is foramenular and is controlled by fracturing, jointing and/or solution in circulating water although it may be modified by infilling as a result of precipitation.
It is not highly interconnected and usually cannot be correlated with permeability. Solution channels or vugular voids developed during weathering or burial in sedimentary basins are indigenous to carbonate rocks such as limestones or dolomites. Joints or fissures which occur in massive, extensive formations composed of shale, siltstone, schist, limestone or dolomite are generally vertical, and they are ascribed to tensional failure during mechanical deformation (the permeability associated with this type of void system is often anisotropic).
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Relative permeability to gas and water for 2-phase flow coalbed methane (CBM) reservoirs has long been known to exhibit a strong control on (gas and water) production profile characteristics. Despite its important control on both primary and enhanced recovery of CBM for coal seams that have not been fully dewatered, relative permeability in coal has received little attention in the literature in the past decade. There are few published laboratory-derived curves; these studies and their resulting data represent a small subset of the commercial CBM reservoirs and do not allow for a systematic investigation of the physical controls on relative permeability curve shape. Other methods for estimation of relative permeability curves include derivation from simulation history-matching, and production data analysis. Both of these methods will yield pseudo relative permeability curves whose shapes could be affected by several dynamic CBM reservoir and operating characteristics.
The purpose of the current work is to perform a systematic investigation of the controls on CBM relative permeability curve shape, including non-static fracture permeability and porosity, multi-layer effects and transient flow. To derive the relative permeability curves, effective permeability to gas and water are obtained from flow equations using a modified version of the approach described by Clarkson et al. (2007). Simulated cases are analyzed so that derived and input curves may be compared allowing for investigation of CBM reservoir properties on curve-shape. One set of relative permeability curves that were input into the simulator were derived from pore-scale modeling. Field cases from two basins are also examined and controls on derived relative permeability curve shape inferred. The results of this work should be useful for future CBM development and greenhouse gas sequestration studies, and it is hoped that it will spark additional research of this critical CBM flow property.
Underground Water Impact Report for the Surat Cumulative Management Area
QWC, 2012. Underground Water Impact Report for the Surat Cumulative Management Area. Brisbane:
Queensland Water Commission.
Preliminary Assessment of Cumulative Drawdown Impacts in the Surat Basin Associated with the Coal Seam Industry
- Usq
Usq, 2011. Preliminary Assessment of Cumulative Drawdown Impacts in the Surat Basin Associated with the
Coal Seam Industry.