In 2009, QGC (a BG Group business) first planned to produce coal seam gas (CSG) in the Surat Basin as feedstock for a new Liquefied Natural Gas (LNG) plant. Subsurface models and associated field development plans were generated to underpin the investment case for the Queensland Curtis LNG (QCLNG) Project. This paper discusses history matching experience from QGC's Surat Basin reservoir simulation models, the challenges involved, and how these challenges have been overcome. Similar to conventional reservoirs, full-field numerical simulation is necessary to accurately account for well interference and broader reservoir connectivity. Simulationalso fully integrates modelled 3D static property variations and honours the physics of multi-phase flow through porous media. CSG reservoirs possess unique characteristics which differ from conventional reservoirs and tend to increase the challenge of history matching. Some of these challenges include the physics of diffusive flow from the matrix into fractures that combines with relative permeability to create a more complex multi-phase flow problem. Another challenge is the requirement to de-water the coal before gas production commences and the associated stress dependency of coal properties. At the same time, the nature of CSG developments makes conventional history-matching approaches impractical given the large number of wells and the need for quick turnaround and fast-cycle decision making, especially during development ramp-up. These demands are set against a lean business environment where cost efficiency is paramount. The properties of Walloon Subgroup (WSG) coals in the Surat Basin are unique compared to other CSG basin plays worldwide. The WSG is characterised by low to moderate coal rank, highly interbedded seams distributed through an extremely low permeability inter-burden, and highly variable coal permeability throughout the basin. Some of these differentiating properties of the WSG make history matching these reservoirs very challenging. This paper presents guidelines for overcoming these challenges and history-matching CSG production in a dynamic simulation model of the Surat basin WSG. Handling of uncertainty is discussed to consider a range of possible history-matches for 345 wells with 9 years of production history. The history matching guidelines that have been developed are enabling a faster turnaround of model predictions, capturing key uncertainty parameters and informing field development decision-making.
This paper discusses a set of laboratory experiments on coal core samples, which seek to understand the changes in coal fracture permeability with varying confining stress, pore pressure and methane adsorption. Currently, no measurements exist to support QGC'sSurat Basin Coal Seam Gas (CSG) development, and therefore themagnitude of change in permeability relative to initial conditions is unknown. The set of experiments described here use a flow apparatus to measure the permeability of coal core samples as a single variable is changed. The ability to control the laboratory test to separate out the different variablesis an improvement on inferring coal permeability variations from well test data, which isaffected by multiple unknown variables at the same time. Four separate experiments have been carried out. The first investigated the impact of higher burial pressures on permeability, a four-fold reduction in permeability was measured with confining pressure increase from 150-750psi. The second replicated the reduction in pore pressure during dewatering the intention was to measure ‘stress-dependent permeability’, a halving of permeability was observed with pore pressure reduction from 220-30psi. The final two experiments used methane gas to investigate the effect of coal shrinkage with changing pore pressure. Under constant stress conditionspermeability halved and under constant volume conditions permeability saw a significant fifteen-fold reduction. Focus on the resulting magnitude of permeability change is more important at the current stage of this study than the absolute permeability change. The magnitude of permeability change can be used assensitivity in numerical simulation studies toassess the impact of pressure dependency of coal permeability and therefore better characterise coal reservoir behaviour and, consequently, improve the accuracy of any forecast derived from the reservoir model.
This paper discusses the use of distributed temperature sensing (DTS) in QGC's Surat Basin wells, and how this new technology has been used to improve reservoir characterisation through enhanced resolution of zonal productivity and flowing contributions.In stacked reservoirs with zones of varying quality, it is vitally important to allocate production volumes to flowing zones in order to adequately characterise the reservoir and create a representative dynamic simulation model that can accurately match reservoir productivity and depletion.For conventional oil and gas production, subsurface allocation of flowing contributions can be achieved through the use of production logging tools, which provide a measurement of volumetric flow rate and fluid density along the completed well length. The nature of Coal Seam Gas (CSG) well completions reduces the options available for allocation of flow to zones. Also, the producing characteristics of a CSG well can make this zonal allocation more complicated. Firstly, the lower zones of QGC's Surat wells are subject to higher back-pressure due to the column of water sat in the annulus, rising above the pump location. Secondly, the de-watering and subsequent de-sorption periods complicate matters, where initially 100% water flows from a zone followed by a gradually increasing gas-water-ratio: the rate of transition from water to gas production is expected to be different for each producing zone. Additionally, due to the stress dependency of coal fracture permeability during the de-watering phase, and later the impact of gas de-sorption on fracture permeability, different zones might be expected to show larger or smaller changes in productivity with time: to understand this would require repeatable measurements through the production life of wells.In this work, DTS technology has been proven, for the first time in QGC's Surat CSG wells, as a means of achieving subsurface allocation of well test kh. This is important for the justification of permanent installations that will allow ongoing zonal allocation of produced volumes. The impact of this methodology on history match quality and reservoir characterisation accuracy is discussed. It is suggested that these new workflows could prove very beneficial in the characterisation of CSG reservoirs.
This paper presents elements of an extensive single well modelling (SWM) programme to improve confidence in the parameters used to condition the full-field reservoir simulation model. The impacts of local grid refinement (LGR), fine vs upscaled layering, and a range of static and dynamic properties have been investigated. Values for some fundamental input parameters for coal seam gas (CSG) reservoir simulation, such as coal fracture porosity and relative permeability, are extremely difficult to constrain by laboratory test results and are typically derived from history matching studies. Also, the stress and shrinkage dependent permeability effects exhibited by many coal cleat systems can be very difficult to characterise and other parameter variations could be invoked to produce similar trends in simulated water and gas production and flowing bottom-hole pressure (FBHP) behaviour. Many of QGC's CSG well production histories exhibit some very large pressure drawdowns, and obtaining a good history match to water and gas production histories in addition to shut-in bottom hole pressure (SIBHP) and flowing bottom hole pressure can be problematic for many of these wells. Simulation input parameters such as permeability, porosity and cell to cell transmissibility have been investigated using a wide range of values. Up to now, it has generally been possible to match water and gas rates and either SIBHP or FBHP, but not both pressures. Relative permeability and coal gas saturation have been the most effective parameters used to obtain an acceptable history match (HM) in recent modelling efforts. The full-field modelling assumptions for these parameters have now been validated and informed by the SWM work reported in this paper, and the range of uncertainty narrowed for these critical parameters. For the wells exhibiting the highest drawdowns, initially a very good match was obtained using a very ‘aggressive’ suite of relative permeability parameters and an irreducible water saturation of around 90%. This relies on an assumption of very low or zero critical gas saturation and extremely rapid reductions in water relative permeability as gas begins to desorb from the coal matrix and enter the coal fractures. The validity of these assumptions has been tested by considering wellbore modelling effects, to reducethe reliance on aggressive relative permeability parameters to achieve a history match. The full-field model (FFM) gridding assumptions have also been validated by this work.
This paper discusses the use of permanent Distributed Temperature Sensing (DTS) using optical fibres for flow zone allocation in Coal Seam Gas (CSG) production wells. In a commingled well the allocation of total well production to the multiple stacked reservoir units is extremely important. Understanding which zones are most productive can influence future development decisions and operational practices. Additionally, the subsurface reservoir model can be calibrated and verified from the data, improving the accuracy of any forecast derived from the model. At QGC, well completion design has focused on simplicity to drive efficiency in the execution of the ca. 2,500 wells drilled so far. These open hole completions have been run with a pre-perforated liner and no external packers to separate formations. This adds uncertainty to the interpretation of results from conventional wireline production logging tools (PLTs) run inside the pre-perforated liner. To perform a conventional PLT, the down hole pump for dewatering must be pulled, requiring a rig and making flow allocation data acquired by PLT very costly. DTS is seen as a viable alternative to PLTs, with the measurement fibre run permanently on the outside of the casing or tubing. It can remain in the well during pump operation and gather data throughout the life of the well. Typically, at QGC, a recording is made every 6 hours and is transmitted to a cloud based database for immediate visualisation. Since a temporary single day DTS installation was trialled in 2014, QGC has run 11 permanent fibres across its Surat CSG development. This has yielded valuable information on production allocation and depletion trends. There have also been a number of challenges encountered. These include interpretation of dual phase flow, and identifying the correct geothermal gradient for interpretation. The advantages and disadvantages of casing conveyed versus tubing conveyed DTS installations are also discussed.
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