Coal Seam Gas Wells Research Articles

Rehabilitation challenges for the onshore coal seam gas sector in Australia

The development of conventional oil and gas reserves followed by valorisation of coal seam gas (CSG) reserves in Queensland has seen the installation of over 16,000 wells. These wells are accompanied by thousands of kilometres of gathering lines, compression and water treatment facilities and transmission pipelines. The major rehabilitation challenges for the industry result less from technical challenges but rather from the sheer scale of the task. At present, the CSG industry continues to grow, and few of the CSG wells (other than exploration and appraisal wells) have yet reached the end of their life. The US experience presents a cautionary tale for adequate financial provisioning to mitigate the risks of orphaned wells. Secondly, the rehabilitation of multiple small parcels of land such as drill pads presents logistical challenges. Strategies to aggregate parcels for relinquishment will be required if the industry is to avoid thousands of individual land parcel evaluations at the time of relinquishment. These two rehabilitation challenges will be explored, risks for the industry and community assessed, and a call made for industry and government to work collaboratively to ensure an orderly and responsible rehabilitation program becomes an integral part of ongoing operations.

Don’t forget your keys when trying to unlock the productivity of low-permeability coals

Low-permeability coal seam gas (CSG) wells have been the subject of laboratory research and modelling studies over the past decade, particularly focusing on the pressure-dependent permeability (PDP) behaviour of coals. These research efforts have progressed diagnostic methods to identify and quantify PDP and provide practical technologies to counter these effects. Firstly, machine learning methods based on drilling and historical well-test data can provide insight into the range of coal permeability during drilling. Next, the process of history-matching the after-closure pressures from a diagnostic fracture injection test (DFIT), using reservoir simulators, can determine best-fit values for fracture compressibility, a key parameter for reservoir models. Finally, these data, along with DFIT reservoir pressure and permeability data, can inform the decision-making process regarding the most applicable completion strategy and aid developmental planning. For areas where vertical or surface-to-inseam (SIS) wells have been unsuccessful, new hydraulic fracturing technologies have been developed to enhance the stimulated reservoir volume (SRV) in coals, using horizontal wells with multi-stage hydraulic fracturing in excess of 20 stages. Recent laboratory and modelling of micro-proppants has extended prior laboratory and modelling studies and provided insight into proppant transport, embedment, and screen-out behaviour. These well stimulation technologies can be co-applied in new or existing CSG fields and are suitable for areas where overlapping tenements limit conventional, steel-based completion strategies. In conclusion, this paper will bring the key findings of these studies together in a cohesive framework and provide the workflows to implement these technologies for better productivity in low-permeability coals.

Study on evaporation drainage of deep coal seam gas wells

Targeting the problem of a small amount of fluid accumulation in deep coal seam gas (CSG) wells during flowing production stage, the evaporation drainage method is proposed to discharge the liquid accumulation. Based on the Dalton evaporation model and wind speed function, a calculation model of evaporation drainage was established for deep CSG wells, which was verified by laboratory experiments. Taking a CSG well in the western Ordos Basin as an example to analyze the evaporation drainage capacity, the influence of temperature, daily gas production, bottomhole flowing pressure (BHFP), formation gas water saturation on the evaporation drainage capacity was investigated. The results show that the maximum evaporation water production is 2,533.8 kg/d at a bottomhole temperature of 80°C and a gas production rate of 30 × 103 m3/d. It is found that the temperature and pressure have a marked influence on the evaporation drainage. By improving the gas production and bottomhole temperature, and reducing the BHFP can effectively promote the evaporation drainage capacity. The initial moisture content of CSG in the reservoir are inversely proportional to the evaporation drainage capacity. By adjusting the BHFP and daily gas production, the evaporation drainage capacity can match the liquid production rate of the formation. Evaporation drainage can effectively extend the flowing production time of deep CSG wells and reduce the costs of production.

Occurrence and behaviour of colloidal silica and silica-rich nanoparticles through stages of reverse osmosis treating coal seam gas associated water

Reverse Osmosis (RO) membrane filtration is a very common process for treating a wide range of groundwater types including produced water from coal seam gas (coalbed methane) wells. Mineral scaling limits water recovery for RO membranes and costs money in terms of treatment and downtime. Silica scaling can be particularly troublesome as it is often irreversible. Mitigating silica scaling requires an understanding of its occurrence, speciation mechanism and its interdependency with other operation factors. This study uses a range of techniques to show that silica colloids form during later stages of an RO process with very high recovery. This happens at silica concentrations above the solubility that would normally indicate high risk of silica scale. However, instead of scale, colloids preferentially formed which means the process can operate at high recoveries with RO performance maintained by regular cleaning cycles. The concentration of the colloidal silica through the RO stages was measured through the difference in total and dissolved silica. Once the existence was established with this technique, the particles were trapped and their size, morphology and composition were investigated with Scanning Electron Microscopy (SEM) in conjunction with Energy Dispersive X-Ray Spectroscopy (EDS). This revealed the particles to be predominantly silica with limited other elements involved.

Performance analysis of artificial lift systems deployed in natural gas wells: A time-series analytics approach

In Coal Seam Gas (CSG) production, Artificial Lift (AL) systems comprising various downhole pumps produce coal-fine-laden water. Over time, the accumulation of coal fines results in mechanical failures of such pumps, which leads to a loss in natural gas production. This problem is exacerbated by CSG operators having to manage thousands of wells in their day-to-day operations. The reason for having such an exuberant number of natural gas wells is due to the fact that CSG reservoirs have a short production cycle when compared to conventional reservoirs. Therefore, thousands of CSG wells must operate simultaneously to sustain natural gas volumes that satisfy domestic energy demand, and help meet contractual export obligations.To manage a large fleet of CSG wells, operators rely on Petroleum and Surveillance Engineers to mitigate production issues and advise timely corrective actions. Current methods of monitoring CSG wells involve observing real-time trends and alarms from Supervisory Control and Data Acquisition (SCADA) systems. However, managing a large fleet of wells with a monitoring-only approach can be detrimental to AL operations. Methods such as Exception Based Surveillance have been used to improve engineers' workload, but such methods are informative at best. In addition, they do not provide early indications of changed downhole pump performance. To improve how AL systems are monitored in CSG operation, we propose a novel well surveillance method that can help mitigate pump failure and provide engineers with insightful information to take corrective actions.In this work, we will present an innovative time-series analytics approach that examines the performance of AL systems in near real-time and helps Well Surveillance and Production Engineers make timely decisions that aid in mitigating pump failures. We used time-series data from 448 CSG wells to build the streaming analytics methodology that autonomously detects various performance states of downhole pumps during CSG production. Furthermore, we successfully detected the onset of AL systems failures, such as solids build-up, gas intake, high torque and pump degradation. We validated our solution on a separate set of 428 wells and were able to show reliable detection of detrimental events on live data from producing natural gas wells. This solution is currently deployed by 2 CSG operators where real-time notifications aid Petroleum and Well Surveillance Engineers with proactively managing AL systems across multiple CSG assets.

The complexity of identifying and quantifying natural and anthropogenic influences on surface movement in coal seam gas producing regions within the Surat Basin, Queensland

Coal seam gas (CSG) production can cause surface movement through the compaction of coal seams and adjacent geological units and may cause subtle changes in the topographic gradients that have been alleged to cause impacts on agriculture. Since surface movement can result from both natural and anthropogenic processes, the determination of which processes, and the magnitudes of their contributions, are important challenges in the management of impacts. Differential interferometric synthetic aperture radar (D-InSAR) is a proven remote sensing technique used to monitor large-scale surface movement via radar satellite imagery. It is currently used by major CSG producers to conduct monitoring over their tenements. As D-InSAR can only deduce total observed movement, integration with other datasets is required to deconvolve the influences within observations. This paper provides an overview of the range of processes that influence surface movement. A case study using a D-InSAR time-series dataset (2016–2022) reveals the surface movement in the Surat Basin. Velocity measurements show that surface movement in the vicinity of CSG wells ranges between −18 and +9 mm/year. Analysis of any correlations between landscape characteristics and surface movement is provided, along with preliminary findings on some key observations. This work aids in refining calculations on what proportion of surface movement may be attributable to gas extraction.

Uninterrupted lift of gas, water, and fines in unconventional gas wells using foam-assisted artificial lift

Sustainable and clean production of natural gas is of paramount importance in transitioning to a low-emission economy. Gas production in an unconventional well such as a coal seam gas (CSG) relies on continuous dewatering, commonly performed through a progressive cavity pump (PCP). PCPs are susceptible to failure for different reasons, primarily the interference of solid particles. This results in interruptions, pump replacement, well cleaning and recommissioning which pose long downtimes (i.e., reduced well deliverability), work-over at substantial costs, along with environmental impacts such as increased methane emissions to the atmosphere.This paper aims to demonstrate and assess the performance and suitability of a sustainable dewatering technique, referred to as foam-assisted gas lift (FAGL) which offers uninterrupted dewatering, minimal downtime and minimised environmental impacts. FAGL is capable of maintaining the gas well deliverability by replacing the downhole pump with a gas compressor at the surface and an engineered gas-liquid-solid flow dynamics by injecting foamers and a small amount of surplus gas into the wellbore.The performance of FAGL is evaluated in 45 vertical CSG wells in Australia under different operating conditions and depths ranging between 400 and 1050 m and is compared with well data using PCPs. The flowing bottomhole pressure (FBHP) is calculated to assess the suitability of FAGL under different operating conditions. The results from this study show that FAGL outperforms PCPs in more than 50% of the cases under this study, offering on average 34% lower FBHP. The FAGL's capability in lifting solid particles is demonstrated using solids taken from gas wells in Queensland, showing a reliable and continuous lift of particles. In addition, chemical defoaming of the foam at the surface is demonstrated to avoid re-foaming in the downstream and mitigate the risk of contamination of the surrounding area.

A multi-stage screening approach to evaluate risks from inter-aquifer leakage associated with gas well and water bore integrity failure

Integrity failure of wells in gas resource developments poses a potential risk to groundwater resource quality and quantity by enhancing the connectivity and fluid migration between a gas reservoir and overlying aquifers, which may support groundwater dependent ecosystems or be used as a water source. This study assessed the potential for impact on groundwater resources from flow pathways created by well integrity failure in decommissioned coal seam gas (CSG) wells, deep water bores or gas wells repurposed as water bores and legacy coal exploration drillholes using an example based on a proposed CSG development near Narrabri, Australia. The study only considered the flow of water. A multi-stage screening method for likelihood and consequence assessment was used, which included (i) a semi-quantitative risk prioritisation of potential pathways for inter-aquifer leakage and development of metrics for impact assessment, (ii) assessment of the consequences of each of these pathways through an analytical model, and (iii) numerical groundwater modelling of single and multiple leaky wells. All three approaches indicated that increases in inter-aquifer leakage, drawdown in upper aquifers are not likely to be significant for high flow leaky gas wells (effective well conductivity, Kw <10−1 m/d) based on conditions reported in previous studies and for the case study. It is theoretically possible that extremely high flow leaky wells (Kw >102 m/d) could have an impact where aquitard conductivity is 10−4 m/d or less, or well failure density is higher than 1 well per km2, however Kw values for compromised gas wells of >1 m/d have not been identified in previous studies. The most extreme case tested, open legacy coal exploration drillholes or petroleum bores repurposed into water bores across an aquitard, should it occur, has the potential to deplete or contaminate groundwater resources in connected aquifers, the magnitude of which will depend on aquifer and production zone transmissivity. However, current Australian regulations for petroleum exploration make this an extremely unlikely case. More effort is required to determine Kw and failure rates of gas wells, water bores and exploration drillholes in Australian conditions to better quantify the potential risks associated with leaking infrastructure.

Concurrent 5. Presentation for: Multiple tracers for dis-connectivity of shallow aquifers, alluvium, and coal seam gas wells in the Great Artesian Basin

Presented on Tuesday 17 May: Session 5 The potential for connectivity between water supply aquifers and gas reservoirs raises community, government, and scientific concerns. Methane can occur naturally, making it difficult to determine whether water bore methane levels are being influenced by nearby gas operations. This poses a challenge in the Surat Basin, where coal seam gas production operates alongside groundwater using industries (including feedlots, agriculture, mines). Water and gas samples were taken from water bores and coal seam gas (CSG) wells in the Walloon Coal Measures and from overlying aquifers (nominally, the Springbok, Gubberamunda, Orallo, and Mooga sandstones) and the Condamine Alluvium, for stable isotopes of gases, groundwater and dissolved inorganic carbon, as well as strontium isotopes. Most of the sampled water bores had isotopic signatures distinct from CSG wells, though a minority from gassy Springbok Sandstone and Walloon Coal Measure water bores could not be distinguished from CSG wells. In those few cases, neither connectivity or dis-connectivity could be confirmed. Alluvium and shallow aquifer samples have higher R36Cl values distinct from the older CSG production waters, as is the case with most 14C measurements. Waters from the Condamine River indicate potential surface water connectivity with the alluvium. The use of multiple tracers has shown that groundwater in some aquifers can be differentiated from groundwater in the coal seam gas reservoir and hence are useful tools in identifying where groundwater connectivity occurs. Understanding this connectivity forms another line of evidence to improve impact prediction models on a regional scale as well as providing information on connectivity in local groundwater investigations. To access the presentation click the link on the right. To read the full paper click here

Concurrent 6. Presentation for: Effects of rapid gas decompression on coal seam gas swellables

Presented on Tuesday 17 May: Session 6 Thousands of non-stimulated coal seam gas (CSG) wells in Queensland’s Surat Basin rely on swellable packers as the first line of defence against interburden solids production and poor well run life. This paper is aimed at understanding some of the impacts of long-term well operations on swellable performance from rapid changes in downhole pressure. For the first time, rapid gas decompression (RGD) effects on CSG swellables were experimented on in a quantitative manner as an analogue to underbalanced workover and pump trip conditions. RGD has the potential to break down swellables due to rapid release of high-pressure methane diffused in the rubber matrix resulting in a flow path for interburden solids. Five commonly available swellables from the CSG market were lab-tested for rapid decompression with methane at operational conditions. Coupon samples were swollen to representative conditions and placed in an autoclave under high-pressure methane, then rapidly decompressed in cycles. Results of this study showed relatively low levels of physical degradation under test conditions but shrinkage effects varied between products largely grouped into material properties, confirmed with separate ambient shrinkage test. As such, the focus on swellable placement geometry remains paramount. To access the presentation click the link on the right. To read the full paper click here

Concurrent 18. Presentation for: Simplicity and digitalisation – using GIS to reduce design and operation costs in gas gathering systems

Presented on Wednesday 18 May: Session 18 With the drive to continually reduce the cost of connecting new Coal Seam Gas (CSG) wells, Worley developed new methodologies using commercial off-the-shelf software, and custom development, to automate engineering and improve operational efficiency for pipelines and gathering systems. One of the recent developments is an automated and data-driven approach to produce Gathering Network Diagrams (GNDs), an alternative to conventional Piping &amp; Instrumentation Diagrams (P&amp;IDs). On a recent project, Worley implemented a GND workflow capable of showing ~40 wells per A3 deliverable in conformance with intelligent drafting standards. This reduced ~2000 as-built P&amp;IDs to ~200 GNDs (and associated standard details) with a significant improvement in quality and usability. Prior to generation, a site walkdown was conducted to address mismatches between as-built P&amp;IDs, Geographic Information System (GIS) and what was actually installed at site. Our client now has a suite of GNDs that match the installed asset and are produced from the corporate GIS database, essentially their Digital Twin. Worley then focused on integrating flow assurance software with GIS allowing data-driven execution of future design packages to be incorporated into the master GNDs. We successfully integrated with PipeSIM and GAP, allowing for seamless transfer of hydraulic inputs from GIS to the modelling software. Engineers can now synchronise the confirmed process design back to GIS without producing red-lined and/or CAD-drafted deliverables which greatly improves efficiency and reduces human error. This paper demonstrates how digital automation can be applied to the CSG industry to significantly reduce cost and schedule whilst enabling data-driven operational efficiencies. To access the presentation click the link on the right. To read the full paper click here

Multiple tracers for dis-connectivity of shallow aquifers, alluvium, and coal seam gas wells in the Great Artesian Basin

The potential for connectivity between water supply aquifers and gas reservoirs raises community, government, and scientific concerns. Methane can occur naturally, making it difficult to determine whether water bore methane levels are being influenced by nearby gas operations. This poses a challenge in the Surat Basin, where coal seam gas production operates alongside groundwater using industries (including feedlots, agriculture, mines). Water and gas samples were taken from water bores and coal seam gas (CSG) wells in the Walloon Coal Measures and from overlying aquifers (nominally, the Springbok, Gubberamunda, Orallo, and Mooga sandstones) and the Condamine Alluvium, for stable isotopes of gases, groundwater and dissolved inorganic carbon, as well as strontium isotopes. Most of the sampled water bores had isotopic signatures distinct from CSG wells, though a minority from gassy Springbok Sandstone and Walloon Coal Measure water bores could not be distinguished from CSG wells. In those few cases, neither connectivity or dis-connectivity could be confirmed. Alluvium and shallow aquifer samples have higher R36Cl values distinct from the older CSG production waters, as is the case with most 14C measurements. Waters from the Condamine River indicate potential surface water connectivity with the alluvium. The use of multiple tracers has shown that groundwater in some aquifers can be differentiated from groundwater in the coal seam gas reservoir and hence are useful tools in identifying where groundwater connectivity occurs. Understanding this connectivity forms another line of evidence to improve impact prediction models on a regional scale as well as providing information on connectivity in local groundwater investigations.

Simplicity and digitalisation – using GIS to reduce design and operation costs in gas gathering systems

With the drive to continually reduce the cost of connecting new Coal Seam Gas (CSG) wells, Worley developed new methodologies using commercial off-the-shelf software, and custom development, to automate engineering and improve operational efficiency for pipelines and gathering systems. One of the recent developments is an automated and data-driven approach to produce Gathering Network Diagrams (GNDs), an alternative to conventional Piping &amp; Instrumentation Diagrams (P&amp;IDs). On a recent project, Worley implemented a GND workflow capable of showing ~40 wells per A3 deliverable in conformance with intelligent drafting standards. This reduced ~2000 as-built P&amp;IDs to ~200 GNDs (and associated standard details) with a significant improvement in quality and usability. Prior to generation, a site walkdown was conducted to address mismatches between as-built P&amp;IDs, Geographic Information System (GIS) and what was actually installed at site. Our client now has a suite of GNDs that match the installed asset and are produced from the corporate GIS database, essentially their Digital Twin. Worley then focused on integrating flow assurance software with GIS allowing data-driven execution of future design packages to be incorporated into the master GNDs. We successfully integrated with PipeSIM and GAP, allowing for seamless transfer of hydraulic inputs from GIS to the modelling software. Engineers can now synchronise the confirmed process design back to GIS without producing red-lined and/or CAD-drafted deliverables which greatly improves efficiency and reduces human error. This paper demonstrates how digital automation can be applied to the CSG industry to significantly reduce cost and schedule whilst enabling data-driven operational efficiencies.

Effects of rapid gas decompression on coal seam gas swellables

Thousands of non-stimulated coal seam gas (CSG) wells in Queensland’s Surat Basin rely on swellable packers as the first line of defence against interburden solids production and poor well run life. This paper is aimed at understanding some of the impacts of long-term well operations on swellable performance from rapid changes in downhole pressure. For the first time, rapid gas decompression (RGD) effects on CSG swellables were experimented on in a quantitative manner as an analogue to underbalanced workover and pump trip conditions. RGD has the potential to break down swellables due to rapid release of high-pressure methane diffused in the rubber matrix resulting in a flow path for interburden solids. Five commonly available swellables from the CSG market were lab-tested for rapid decompression with methane at operational conditions. Coupon samples were swollen to representative conditions and placed in an autoclave under high-pressure methane, then rapidly decompressed in cycles. Results of this study showed relatively low levels of physical degradation under test conditions but shrinkage effects varied between products largely grouped into material properties, confirmed with separate ambient shrinkage test. As such, the focus on swellable placement geometry remains paramount.

Modeling and Economic Analyses of Graded Particle Injections in Conjunction with Hydraulic Fracturing of Coal Seam Gas Reservoirs

Summary Hydraulic fractures can enhance well productivity from stress-sensitive naturally fractured reservoirs, such as coalbed methane or coal seam gas (CSG) reservoirs. Graded proppant injection (GPI) has been proposed to enhance long-term, far-field interconnectivity between the created hydraulic and short-term, enhanced natural fracture permeability, resulting from fracture fluid leakoff and lowered net effective stress. This novel study shows how applying GPI with hydraulic fracturing treatments resulting in an increased stimulated reservoir volume (SRV) can enhance well productivity and improve CSG well economics. A commercially available reservoir model and history-matched hydraulically fractured coal seam case are used to evaluate well performance differences between a hydraulic fractured reservoir and one including GPI application. A dual-porosity system and the Palmer and Mansoori model are used to simulate initial and long-term permeability accounting for reservoir depletion (i.e., increased net effective stress and matrix shrinkage). A previously validated case study is used to describe the post-embedment benefits of GPI based on the porosity model and history-matched reservoir properties. A net present value (NPV) can then be calculated for each scenario, based on the production differences and typical Australian CSG costs. Our results show that permeability enhancement is achieved beyond the hydraulically fractured region for all post-GPI stimulation cases. An optimal SRV can be found relative to permeability that maximizes the incremental NPV from GPI application. The next most significant parameters after permeability that influence the economic outcomes are fracture porosity and coal compressibility. A larger SRV yields higher cumulative gas production over 30 years with up to 7.2 times increase over gas production without GPI. This study substantially increases our understanding of how to model and understand the benefits of GPI application along with hydraulic fracturing to increase the SRV in CSG wells.

A machine learning-based predictive model for real-time monitoring of flowing bottom-hole pressure of gas wells

Prediction of the flowing bottom hole pressure (FBHP) of gas–water two-phase flows is of great importance in optimising the production of gas and reducing down-time in unconventional gas wells. Unlike the case for conventional gas wells, prediction of FBHP for unconventional gas wells, particularly coal seam gas (CSG) wells, has not been studied.Monitoring of FBHP typically is done using a downhole pressure sensor placed close to the bottom of the well. Replacing a failed pressure sensor or recalibration of a pressure gauge, which is required frequently throughout the life of a well for reliable measurement of FBHP, requires interruption of the gas production at a high cost. A low-cost and reliable model for continuous prediction of FBHP, would enable smooth operation of CSG wells in the event a pressure gauge fails, without interrupting the well production.This work presents predictive models for real-time and reliable prediction of FBHP using surface and subsurface data, acquired from 91 CSG wells in Australia over 5–19 month production periods. Two sets of models are developed; one for specific wells using data from that individual well and another one for a group of wells. Three different modelling approaches, multiple linear regression, linear mixed-effects and gradient boosting regression tree (XGBoost) are implemented. The XGBoost modelling outcomes show promising results with the best mean absolute percentage error (MAPE) of 10% and 11.7% for specific well models and multiple well models, respectively.

Stabilization of clay-rich interburdens using silica nanoparticles

Detachment of clay particles from clay-rich interburdens is the main source of solid production in coal seam gas wells (CSG), costing the Australian operators millions of dollars every year. Several techniques have been proposed to reduce the solid production and consequent pump failure in already constructed open hole/slotted liner wells. Amongst all remedial techniques for these existing problematic wells, chemical stabilization seems to be the most practical method. The high cost of chemical additives, their environmental issues, potential low effectiveness and technical difficulties for their placement downhole have, however, hindered their wide application. In particular, the long-term effect of current chemical additives on interburden stability and their influence on coal seam matrix gas diffusion need further investigation. In this study, the application of colloidal silica nanoparticle solution (silica SOL) as a long-term chemical stabilization method for clay-rich interburden in CSG wells was investigated. A series of comprehensive experiments were performed to investigate the effect of the silica SOL on stability of pure compacted dry smectite clays and clay-rich sandstone samples in stationary and turbulent conditions (resembling the wellbore condition during gas production) using a uniquely designed experimental setup. Additionally the effect of silica SOL on coal and its gas adsorption was analyzed. The results of this study demonstrated that the silica SOL is a suitable candidate for long-term chemical stabilization of clay minerals as a cost-efficient and environmentally friendly additive. Its non-reactivity with coal and its inert response on gas sorption is another significant benefit of the proposed chemical solution.

Big Data and Machine Learning Optimize Operational Performance and Drill-Bit Design

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202243, “Use of Big Data and Machine Learning To Optimize Operational Performance and Drill-Bit Design,” by Simon Cornel, SPE, Baker Hughes, and Gonzalo Vazquez, Senex Energy, prepared for the 2020 SPE Asia Pacific Oil and Gas Conference and Exhibition, originally scheduled to be held in Perth, Australia, 20–22 October. The paper has not been peer reviewed. Time savings and bit longevity are major challenges in coal-seam gas (CSG) unconventional fields onshore Queensland. Maximizing rate of penetration (ROP) on the basis of optimal drilling parameters was the key to tackling these issues. A formal process for optimizing performance was developed, with a focus on optimizing polycrystalline diamond compact (PDC) bit design and drilling hydraulics and developing a drillers’ road map. As a result, ROP increased from 50 to 150 m/h. Time savings of more than 150 hours for the drilling campaign was achieved. Background The drilling campaign encompassed two areas for development. Field A is a CSG acreage 45 km southwest, and Field E 30 km northeast, of Wandoan, Queensland. Eighty vertical wells would be drilled across the two fields. This would provide the opportunity to trial the data-mapping concept to increase ROP and fine-tune bit designs to improve longevity. In addition, the rig-sensor data set provided the basis for creating a machine-learning model to help improve drilling parameters during the production-drilling phase. Drilling Challenges The three primary challenges to be addressed included the following: - Economics for unconventional fields, specifically delivering time savings in order to lower the average cost per well - Control vibrations to avoid twistoffs and bottomhole-assembly fatigue - Consistent performance The benchmark for the initial project was an on-bottom ROP of 80 m/h. Therefore, the target for this campaign was to achieve an average on-bottom ROP of over 100 m/h and drilling-time savings of 1 hour per well. Methodology Bit optimization typically has been an iterative process, taking bit-record information to make assumptions about bit performance before providing recommendations for the next well. A more-detailed approach may include the use of log data (i.e., gamma ray and sonic) to estimate rock strength and overlay the depth-based well data to identify key incidents that affected performance (both positive and negative) so such incidents can be mitigated in future wells. This approach, while exhaustive, takes time to complete and is open to subjective interpretation of the engineer performing the analysis. When drilling CSG wells, typical well duration is 3–5 days. With short turn-around times between wells to analyze the data, the goal in this study was to develop an automated approach using real-time data gathered from the rig and present it to the field and rig crew in a visualized method. A two-step approach was devised in which data from the rig was used, followed by a real-time model.

Numerical investigation of the effects of proppant embedment on fracture permeability and well production in Queensland coal seam gas reservoirs

This paper introduces the development of a new predictive model in support of proppant injection in naturally fractured coal seam gas (CSG) reservoirs. In the proposed model, the finite element method (FEM) is used for the prediction of proppant embedment and elastoplastic deformation of the coal. The lattice Boltzmann method (LBM) is applied to the modelling of fluid flow through propped fractures, in which the modified partially saturated method (MPSM) is implemented to characterise the fluid–solid interactions. Permeability diagrams of the fractures are then generated as functions of particle packing ratio and effective stress. Finally, these results are incorporated into a radial Darcy flow analytical solution to predict the productivity index of the CSG wells under various proppant injection pressures and cleat compressibilities. The developed model is applied to selected coal samples from the Bowen and Surat Basins in Queensland, Australia. Modelling results indicate that proppant injection leads to increased fracture permeabilities and enhanced well productivity indexes. The elastoplastic deformation results in smaller permeability increase and less production enhancement when compared to the traditional linear elastic models. Although focused on Australian coals, the developed workflow can be broadly applicable to the assessment of potential stimulation efficacy in other unconventional reservoirs. In addition, a better understanding and implementation of the proppant injection scheme can effectively improve the post-fracturing performance, particularly in low-permeability coal intervals, which benefits the CSG industry.

Estimating porosity and permeability in the Springbok Sandstone, Surat Basin (Queensland), using new wireline log-based workflow

The Upper Jurassic Walloon Coal Measures of the Surat Basin (Queensland) host some of the most prominent coal seam gas (CSG) resources in Australia. The Walloon Coal Measures are directly overlain by the Springbok Sandstone formation, historically referred to as a regional aquifer. An increasing number of studies and industry models suggest relatively limited hydraulic connectivity within the formation and between it and the underlying coal measures, due to extreme lithological heterogeneity. Accurate evaluation of the permeability, as well as lateral and vertical continuity of the lithological units within the Springbok Sandstone, is critical in reservoir models that form the basis of reasonable aquifer protection practices and impact prediction. This study presents a wireline log-based workflow applied to identify permeable zones within the Springbok Sandstone in 31 CSG wells across the Surat Basin that allows robust estimations of porosities and Klinkenberg permeabilities. The workflow primarily utilises spontaneous potential, density, neutron and resistivity logs, and was developed by integrating current industry practices implemented by operators on a local scale to identify risk (permeable) zones in the vicinity of targeted coal seams. The results of this case study indicate that permeable zones within the interval are volumetrically minor (on average 25% N/G) and likely isolated, with Klinkenberg permeabilities rarely exceeding 10–20mD. This evidence for low hydraulic connectivity, as well as significant local variations in the character of the Springbok Sandstone, suggests that the definition of the formation as a regional, continuous aquifer and the way it is modelled needs to be revised.