Uncertainty quantification of coal seam gas production prediction using Polynomial Chaos

A surrogate model approximates a computationally expensive solver. Polynomial Chaos is a method to construct surrogate models by summing combinations of carefully chosen polynomials. The polynomials are chosen to respect the probability distributions of the uncertain input variables (parameters); this allows for both uncertainty quantification and global sensitivity analysis. In this paper we apply these techniques to a commercial solver for the estimation of peak gas rate and cumulative gas extraction from a coal seam gas well. The polynomial expansion is shown to honour the underlying geophysics with low error when compared to a much more complex and computationally slower commercial solver. We make use of advanced numerical integration techniques to achieve this accuracy using relatively small amounts of training data

Heat flow, thermal structure and thermal evolution of the Parana Basin, southern Brazil.

I report 56 new heat flow measurements for the Parana Basin of southern Brazil, a large Phanerozoic basin covered with flood basalts in the Mesozoic. The heat flow increases from 40 mW m\sp{-2} in the central region of the basin (the region of thickest basalt cover) to 75 mW m\sp{-2} at the eastern basin margin. This pattern of heat flow can be explained by crustal and mantle structures associated with the generation and extrusion of flood basalts which divert deep mantle heat away from the central region into the thinner lithosphere underlying the basin margin and surrounding folded belts. A least squares inversion of the temperature and stratigraphic data yields best fitting geothermal gradients for representative formations and a best fit subsurface temperature field for the Parana Basin. This temperature field is in agreement with results of modeling of a purely conductive regime. Advective heat transport models suggest that the effects of basin scale subsurface fluid flow are likely to be small and if present would cause heat flow variation opposite to that observed. Numerical models of temperature histories in the sediments and of surface heat flow through time suggest that a basin-initiating thermal event would have been of little consequence to any but the earliest sediments. The history of the basin has been predominantly one of subsidence, interrupted by regional uplift. The last depositional event was the extrusion of flood basalts at 135-130 Ma over most of the basin surface, with associated intrusions in the sediments. The accompanying burial increased the temperatures within the sediments by up to 40\sp\circC. 'Illite crystallinity' data appear to reflect the combined effects of increasing temperature and time with burial. The I/S ratio of mixed-layer illite smectite measured on core samples taken in proximity to sill intrusions is compatible with the present-day temperatures controlled by the geothermal gradient and suggests that short-lived high temperatures associated with the igneous activity did not affect significantly the I/S ratio. A comparison of the thermal history of two sites of different subsidence/uplift histories confirms that the thermal maturation of the sediments is dominated by the long term subsidence history and is little affected by igneous activity.Ph.D.GeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103087/1/9303753.pdfDescription of 9303753.pdf : Restricted to UM users only

Atlas of geothermal resources in Europe

The geothermal resources of most European countries have been estimated and compiled in the recently published Atlas of Geothermal Resources in Europe, a companion volume to the Atlas of Geothermal Resources in the European Community, Austria and Switzerland. Publication of this Atlas comes at a time when the promotion of a sustainable and non-polluting energy is high on the agenda of local energy suppliers, municipal administrations and all European governments. The participating countries are: Albania, Austria, Belarus, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Netherlands, Poland, Portugal, Romania, Russia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Ukraine and the UK. A volumetric heat content model for porous reservoirs was the basis for calculating the resources, assuming that exploitation of the geothermal resources would take place in a doublet well system. The geothermal reservoirs are defined in a set of 4 maps, by depth, thickness, temperature and resources. The assessment methodology is simple and is based on a small number of parameters so that regions with very limited data coverage can also be evaluated. An example is given in this paper of the eastern North German Basin. The maps presented in the Atlas permit a first order evaluation of the geothermal potential in terms of technical and economic viability. This uniform procedure applied to all countries and regions allows comparisons and serves as a guide for setting priorities and planning geothermal development. This Atlas also helps in the search for appropriate partners for international cooperation in geothermal exploration in Europe

Spectral decomposition of the heterogeneous springbok sandstone and Walloon coal measures in the Surat Basin, Australia

Coal Seam Gas (CSG) is a significant resource; it makes up 90% of the gas production in Queensland, Australia, most of it from the Surat Basin Walloon Coal Measures, (WCM). However, coal connectivity and distribution are poorly understood resulting in production and modelling challenges. Simplistic, homogeneous models cannot adequately explain volumes or flow rates of gas and water. Initially, the focus of the industry was on optimizing drilling, often without the aid of seismic data, relying on correlations based on wireline logs and cores alone. Those correlations were often based on lithology, and connected sand with sand and coal with coal. Those models seem to overestimate flow between wells and volumes of hydrocarbons or water (Cardwell, 2018). Here, log interpretation is integrated with 3D seismic using a sequence stratigraphic framework. Special attention is placed on the architecture of the WCM revealed by seismic. Seismic mapping of individual coal seams in the Springbok Sandstone or the WCM is difficult on a field scale. Most coals are below seismic resolution, yet have a strong influence on seismic amplitudes. Four methods of spectral decomposition are compared and evaluated on how well they predict known geology at many well locations throughout the 3D seismic survey. Sequence stratigraphy applied to core and log interpretation integrated with seismic produces an improved view of coal distribution and geometries. Coals are mostly thin and discontinuous and segregated by channels cutting through them that appear to be meandering through flood plains. Coal forms along channels and sands are likely to be present in point bars. Therefore, sands or coals identified in wireline logs are not likely to be connected to a large sand sheet, but are rather individual channels or sand bodies likely to be hydraulically isolated. In the next phase, it will be investigated how results of spectral decomposition can be used inform geostatistical algorithms for modelling to produce more representative static geological models

Modelling the Contribution of Individual Seams to Coal Seam Gas Production

In coal seam gas (CSG) fields, where single wells tap multiple seams, it is likely that some of the individual seams hardly contribute to gas recovery. This study aims to examine the contribution of individual seams to the total gas and water production considering that each seam may have different properties and dimensions. A sensitivity analysis using reservoir simulation investigates the effects of individual seam properties on production profiles. A radial model simulates the production of a single CSG well consisting of a stack of 2 seams with a range of properties for permeability, thickness, seam extent, initial reservoir pressure, compressibility and porosity. The stress-dependency of permeability obeys the Palmer and Mansoori (1998) model. A coefficient (α) relates seam radius, thickness, porosity, compressibility, permeability, and initial pressure. It is used to aid interpretation of the sensitivity study. Finally, a case study is modelled with 5 seams of different thicknesses and depths obtained from a producing well. The range in properties represents conditions found in the Walloon Coal Measures of the Surat Basin, relevant to the Australian CSG industry. The sequence in which peak of gas production rate of each seam is achieved can be estimated using α. For αtop/αbottom > 1, the bottom seam peaks first but achieves lower gas recovery than the top seam. For αtop/ αbottom 27, the contrast between maximum rate and time to peak increases and the top seam’s contribution is significantly reduced in early production time. A more realistic case based on a section of an actual Surat Basin well with 5 seams confirmed that when the αtop/αbottom of seams of greater permeability-thickness (kh) is higher than 27, gas recovery decreases. Even with higher total kh, seams with α ratio = 100 produced less gas than seams with αtop/αbottom = 10. An increasing α ratio is associated with inhibition of less permeable seams and reduced overall well productivity

Modeling the contribution of individual coal seams on commingled gas production

In coal-seam-gas (CSG) fields, where single wells tap multiple seams, it is likely that some of the individual seams hardly contribute to gas recovery. This study aims to examine the contribution of individual seams to the total gas and water production considering that each seam can have different properties and dimensions. A sensitivity analysis using reservoir simulation investigates the effects of individual seam properties on production profiles.A radial model simulates the production of a single CSG well consisting of a stack of two seams with a range of properties for permeability, thickness, seam extent, initial reservoir pressure, coal compressibility and porosity. The stress dependency of permeability obeys the Palmer and Mansoori (1998) model. A time coefficient (a) relates seam radius, viscosity, porosity, fracture compressibility, and permeability. It is used to aid interpretation of the sensitivity study. Finally, two hypothetical simulation scenarios with five seams of different thicknesses and depths obtained from producing wells are explored. The range in properties represents conditions found in the Walloon Coal Measures (WCM) of the Surat Basin, relevant to the Australian CSG industry.Each seam in the stack achieves its peak production rate at different times, and this can be estimated using\ua0a. Seams with lower\ua0a\ua0reach the peak gas rate earlier than those with higher\ua0a-coefficient. The distinct behavior of gas-production profiles depends on the combination of individual seam properties and multiseam interaction. At a\ua0aratio\ua0> 1 (i.e.,\ua0atop/abottom\ua0> 1), the bottom seam peaks first but achieves lower gas recovery than the top seam. An increasing\ua0aratio\ua0is associated with the inhibition of less-permeable seams and reduced overall well productivity. For\ua0aratio\ua0< 1, the top seam experiences fast depletion and total gas-production rates decrease drastically. This outcome is confirmed by a more realistic scenario with a higher number of coal layers. Poor combination of seams leads to severe production inhibition of some coal reservoirs and possible wellbore crossflow. The contrast of the seam-lateral extent in the stack and fracture compressibility play an important role in well productivity in the commingled operation of a stack of coal seams. Unfortunately, the lateral extent of individual coal seams is difficult to estimate and poorly known and, therefore, represents a major uncertainty in gas-production prognosis. The\ua0aratio\ua0analysis is a useful tool to gain understanding of modeled well productivity from commingled CSG reservoirs

Comment on “Numerical investigation of the potential contamination of a shallow aquifer in producing coalbed methane” by Xianbo Su, Fengde Zhou, and Stephen Tyson

This commentary addresses “Numerical investigation of the potential contamination of a shallow aquifer in producing coalbed methane” by Xianbo Su, Fengde Zhou, and Stephen Tyson. We think the models used in the simulations described in the paper are unrealistic, even as a conceptual worst case scenario. Concerns regarding how the results of these simulations are interpreted and portrayed, and in particular how they are related to previous works are discussed in detail. We believe the original paper uses language which could be misleading, and possibly alarmist, and we suggest references cited in the original paper may have been misinterpreted

Impact of injection temperature on CO2\ua0storage in the Surat Basin, Eastern Australia

In CO2\ua0storage projects, CO2\ua0usually enters the target reservoir at a lower temperature than that of the surrounding rock and its density is increased. The injection temperature affects how much CO2\ua0can be stored. In this work we investigate the impact of heat exchange during CO2\ua0injection into the Surat Basin, Australia, using integrated reservoir modelling. We evaluate the aquifer storage and sealing capacities, as well as pressure build up and CO2\ua0plume migration.Flow simulations of CO2\ua0injection into the Precipice Sandstone were conducted with injection temperature from 40 to 80°C. The modelling domain consists of the reservoir sandstone, an overlying transition zone (muddy sandstone) and above it, the ultimate seal. The distribution of porosity, permeability and capillary pressure is heterogeneous. Heat exchange between rock and fluids was enabled in the commercial simulator to evaluate changes in fluid properties due to wellbore cooling. The initial temperature was set to 80°C. The injector’s wellbore pressure drop is modelled honouring a constant well head pressure of 15,000 kPa. The maximum allowed bottom-hole pressure is 90% of a thermally reduced fracturing pressure.The viscosity of water and CO2\ua0increases during cooling of the near wellbore zone; thus, pressure build up grows faster in the case of lower injection temperatures. Although the bottom-hole pressure becomes higher, injection rate is constrained by well head pressure. Heat exchange also increases the density and saturation of CO2\ua0at the plume edge, which causes a sharper and faster advancing front. Higher pressure in the reservoir forces fluids to migrate to the transition zone, which also reduces its temperature. CO2\ua0flows preferentially through the lowest capillary pressure channels and is able to permeate slightly into the transition zone. These physical conditions at the bottom of the well (lower temperature and higher pressure) lead to a denser CO2\ua0plume and a greater mass is stored in the reservoir each year.This work analyses non-isothermal injection of CO2\ua0into an aquifer using integrated reservoir modelling. It illustrates how reservoir cooling may increase the rate of CO2\ua0storage and slight migration to the transition zone

Comparison of flow dynamics of air-water flows with foam flows in vertical pipes

This paper focuses on investigating the dynamic behaviour of foam flows, including flow regimes, their transitions and pressure profiles in relation to foam characteristics (foam holdup and wetness) at different surfactant concentrations (0–500 ppm). The experiments cover a wide range of gas and water flowrates in a 4.3 m long vertical acrylic pipe with 44 mm internal diameter. Flow regimes and their underlying mechanisms are characterised through Power Spectral Density analysis of the associated pressure fluctuation signals, collected at 100 Hz frequency. A flow map is developed for foam flow in a vertical pipe based on the gas and liquid Webber numbers, representing different flow regimes. The results from this study reveal that, even at small concentrations, the surfactant attenuates flow fluctuations, as observed in the pressure data, resulting a relatively uniform flow compared with air-water flows. The results also indicate the promoting effect of surfactant on the flow transition from churn to annular flow at much lower gas velocities compared to air-water flows. The flow transition from slug to churn and churn to annular in foam flows is shown to be associated with an increase in foam wetness. The lowering effect of surfactant on the pressure gradient is most pronounced at lower gas rates. Increasing the gas rate in the foam flow encourages bubble breakup, leading to the formation of a denser foam, composed of smaller bubbles carrying more liquid in the lamella of the foam network. This therefore leads to a higher pressure gradient at higher gas rates