Academic literature on the topic 'Ultra-deep coal seam'

Through field observation and theoretical study, we found that the Hanxing mining area has a typical ternary structure in coal mining under high water pressure of the aquifer. This ternary structure is the Ordovician limestone aquifer-aquiclude including thin limestones-coal seam. Although the aquiclude is considerably thick, there is still a great risk of water burst during mining under water pressure in the deep burial environment. Multidimensional characteristics of floor water inrush in deep mining are summarized in the paper, including water migration upwardly driven by the Ordovician confined water, the planar dispersion of the water inrush channel, the stepped increase of the water inrush intensity, the hysteretic effluent of the water inrush time and the exchange, and adsorption of the water quality. The water inrush mechanism is clarified that the permeability, dilatancy, fracturing, and ascending of the water from the Ordovician limestone aquifer form a planar and divergent flow through the transfer, storage, and transportation of thin limestone aquifers. The corresponding water inrush risk evaluation equation is also proposed. Based on the thickness of the aquiclude, the thickness of the failure zones, and the water inrush coefficient, the floor aquiclude is classified into five categories. While water inrush cannot be completely controlled by the traditional underground floor reinforcement with ultra-thick aquiclude or even zonal grouting, a comprehensive prevention and control concept of the four-dimensional floor water hazard in full time-space domain are proposed. A tridimensional prevention and control model of three-dimensional reticulated exploration, treatment, verification, and supplementation is presented. A full time domain technological quality control process of condition assessment, exploration, remediation, inspection, evaluation, monitoring, and reassurance is formed, and a water disaster prevention method with full time-space tridimensional network in deep coal mining is established. Case study in the Hanxing mining area demonstrates that the proposed methods are highly effective.

An alternative geomechanical reservoir boundary condition is proposed for ultra-deep coal seams of the Cooper Basin in central Australia. This new concept is embodied within the author’s ‘Expanding Reservoir Boundary (ERB) Theory’, which calls for a paradigm shift in gas extraction technology, diametrically opposed to current practices. As with shale, full-cycle, standalone commercial gas production from Cooper Basin ultra-deep coal seams requires a large stimulated reservoir volume (SRV) having high fracture surface area for gas desorption. This goal has not yet been achieved after 13 years of trials because, owing to the bipolar combination of shale-like reservoir properties and coal-like geomechanical properties, these poorly cleated, inertinitic coal seams exhibit ‘hybrid’ characteristics. Stimulation techniques adopted from other play types are incompatible with the highly unfavourable combination of nanoDarcy-scale permeability, ‘ductility’ and high stress. Nevertheless, gas flow potential counterintuitively increases with depth, contingent upon the creation of an effective SRV. Optimum reservoir conditions occur at depths beyond 9000 feet (2740 m), driven by dehydration, high gas content, gas oversaturation, overpressure and a rigid host rock framework. The physical response of ultra-deep coal seams and the surrounding host rock to pressure drawdown is inadequately characterised. It remains to be established how artificial fracture and coal fabric aperture width change due to the competition between desorption-induced coal matrix shrinkage and compaction caused by increasing effective stress. Studies by the author suggest that pressure arching may ultimately control gas extraction efficiency. Harnessing this geomechanical phenomenon could resolve the technical impasse that currently inhibits commercialisation. Pressure arching neutralises SRV compaction by deflecting stress to adjacent strata of greater integrity. These strata then function as an abutment for accommodating increased stress outside the SRV. This shielding effect allows producing ultra-deep coal seams to progressively de-stress and ‘self-fracture’ naturally, in an overall state of shrinkage-induced tensile failure. An ‘expanding reservoir boundary and decreasing confining stress’ condition is generated by the combined, mutually sustaining actions of coal matrix shrinkage and sympathetic pressure arch evolution. This causes the SRV to steadily increase in size and permeability. Cooper Basin ultra-deep coal seams may be effectively stimulated by harnessing this self-perpetuating, depth-resistant mechanism for creating permeability and surface area. The ultra-deep coal seams may be induced to pervasively ‘shatter’ or ‘self-fracture’ naturally during production, independent of ‘brittleness’, analogous to the manner in which shrinkage crack networks slowly form, in a state of intrinsic tension, within desiccating clay-rich surface sediment.

There is a vast, untapped gas resource in deep coal seams of the Cooper Basin, where extensive legacy gas infrastructure facilitates efficient access to markets. Proof-of-concept for the 5 million acre (20 000km2) Cooper Basin Deep Coal Gas (CBDCG) Play was demonstrated by Santos Limited in 2007 during the rise of shale gas. Commercial viability on a full-cycle, standalone basis is yet to be proven. If commercial reservoirs in nanoDarcy matrix permeability shale can be manufactured by engineers, why not in deep, dry, low-vitrinite, poorly cleated coal seams having comparable matrix permeability but higher gas content? Apart from gas being stored in a source rock reservoir format, there is little similarity to other unconventional plays. Without an analogue, development of an optimal reservoir stimulation technology must be undertaken from first principles, using deep coal-specific geotechnical and engineering assumptions. Results to date suggest that stimulation techniques for other unconventional reservoirs are unlikely to be transferable. A paradigm shift in extraction technology may be required, comparable to that devised for shale reservoirs. Recent collaborative studies between the South Australian Department of State Development, Geological Survey of Queensland and Geoscience Australia provide new insight into the hydrocarbon generative capacity of Cooper Basin coal seams. Sophisticated regional modelling relies upon a limited coal-specific raw dataset involving ~90 (5%) of the total 1900 wells penetrating Permian coal. Complex environmental overprints affecting resource concentration and gas flow capacity are not considered. Detailed resource estimation and the detection of anomalies such as sweet spots requires the incorporation of direct measurement. To increase granularity, the authors are conducting an independent, basin-wide review of underutilised open file data, not yet used for unconventional reservoir purposes. Reservoir parameters are quantified for seams thicker than 10feet (3m), primarily using mudlogs and electric logs. To date, ~3750 reservoir intersections are characterised in ~1000 wells. Some parameters relate to resource, others to extraction. A gas storage proxy is generated, not compromised by desorption lost gas corrections. A 2016 United States Geological Survey resource assessment, based on Geoscience Australia studies, suggests that the Play remains a world-class opportunity, despite being technology-stranded for the past 10years. Progress has been made in achieving small but incrementally economic flow rates from add-on hydraulic fracture stimulation treatments inside conventional gas fields. Nevertheless, a geology/technology impasse precludes full-cycle, standalone commercial production. A review of open file data and cross-industry literature suggests that the root cause is the inability of current techniques to generate the massive fracture network surface area essential for high gas flow. Coal ductility and high initial reservoir confining stress are interpreted to be responsible. Ultra-deep coal reservoirs, like shale reservoirs, must be artificially created by a large-scale stimulation event. Although coal seams fail the reservoir ‘brittleness test’ for shale reservoir stimulation practices, the authors conclude from recent studies that pervasive, mostly cemented or closed coal fabric planes of weakness may instead be reactivated on a large scale, to create a shale reservoir-like stimulated reservoir volume (SRV), by mechanisms which harness the reservoir stress reduction capacity of desorption-induced coal matrix shrinkage.

PurposeThis study aims to analyse the deep resource mining that causes high in situ stress, and the disturbance of tunnelling and mining which may induce large stress concentration, plastic deformation and rock strata compression deformation. The depth of deep resources, excavation rate and multilayered heterogeneity are critical factors of excavation disturbance in deep rock. However, at present, there are few engineering practices used in deep resource mining, and it is difficult to analyse the high in situ stress and dynamic three-dimensional (3D) excavation process in laboratory experiments. As a result, an understanding of the behaviours and mechanisms of the dynamic evolution of the stress field and plastic zone in deep tunnelling and mining surrounding rock is still lacking.Design/methodology/approachThis study introduced a 3D engineering-scale finite element model and analysed the scheme involved the elastoplastic constitutive and element deletion techniques, while considering the influence of the deep rock mass of the roadway excavation, coal seam mining-induced stress, plastic zone in the process of mining disturbance of the in situ stress state, excavation rate and layered rock mass properties at the depths of 500 m, 1,500 m and 2,500 m of several typical coal seams, and the tunnelling and excavation rates of 0.5 m/step, 1 m/step and 2 m/step. An engineering-scale numerical model of the layered rock and soil body in an actual mining area were also established.FindingsThe simulation results of the surrounding rock stress field, dynamic evolution and maximum value change of the plastic zone, large deformation and settlement of the layered rock mass are obtained. The numerical results indicate that the process of mining can be accelerated with the increase in the tunnelling and excavation rate, but the vertical concentrated stress induced by the surrounding rock intensifies with the increase in the excavation rate, which becomes a crucial factor affecting the instability of the surrounding rock. The deep rock mass is in the high in situ stress state, and the stress and plastic strain maxima of the surrounding rock induced by the tunnelling and mining processes increase sharply with the excavation depth. In ultra-deep conditions (depth of 2,500 m), the maximum vertical stress is quickly reached by the conventional tunnelling and mining process. Compared with the deep homogeneous rock mass model, the multilayered heterogeneous rock mass produces higher mining-induced stress and plastic strain in each layer during the entire process of tunnelling and mining, and each layer presents a squeeze and dislocation deformation.Originality/valueThe results of this study can provide a valuable reference for the dynamic evolution of stress and plastic deformation in roadway tunnelling and coal seam mining to investigate the mechanisms of in situ stress at typical depths, excavation rates, stress concentrations, plastic deformations and compression behaviours of multilayered heterogeneity.

Abstract Sustainability of an underground longwall operation is highly dependent on stability of the pillars during the panels extractions to ensure the continuous serviceability of gateroads. In Chinese underground longwall mining, the gateroads are typically driven as a single roadway with a “long gateroad pillar” which is different to a common practice where a gateroad consists of a number of pillars known as chain pillars. Such a unique practice has been proven to be more economical with maximum recovery while the safety remains at its highest level. In this study, based on the data obtained from Tongxin coal mine in China, the mechanical stability of the driven long gateroad pillar was investigated. The focus was on two nearby longwall top coal caving panels and their impacts on the mechanical behaviour of long gateroad pillar during the longwall retreat. To do so, initially an area of the gateroad was selected for the field instrumentation and data collection. These included vertical stress measurement and depth of damage assessment within the gateroad pillar and the longwall panel. Hence, the convergence level of gateroad was quantified to be used for the numerical modelling and assessing the performance of the designed long gateroad pillar based on the finite difference modelling technique using FLAC3D. Double-yield and strain-softening ubiquitous-joint constitutive models were used to simulate goaf material and strata, respectively. Finally, an extensive sensitivity analysis was conducted to compare the mechanical behaviour of a range of wide and narrow long gateroad pillars. It was concluded that the 50 m wide pillar is an ideal dimension for the future panels of Tongxin coal mine to achieve the maximum productivity and safety. Article highlights The validated numerical model was used to study on stability of “long gateroad pillar”. Premature yielding of the 38 m pillar would lead to severe geotechnical issues. Deep roof blasting of goaf edge or 50 m pillar can improve pillar conditions

This thesis reveals atypical dynamic reservoir behaviour within Cooper Basin ultra-deep coal seams during gas production that calls for a paradigm shift in gas extraction technology, diametrically opposed to the evolutionary path of current drilling, wellbore completion, and reservoir stimulation practices. An anomalous geomechanical reservoir boundary condition is detected that is, by definition, mostly restricted to ultra-deep coal seams. The discovery has resulted in the formulation of a new coal seam reservoir concept - “Expanding Reservoir Boundary Theory”. Ultra-deep Permian coal seams of the Cooper Basin in central Australia represent a nascent thermogenic source rock reservoir play. Proof-of-concept gas flow occurred in 2007. The vast (100+ Tscf) potential resource is comparable in commercial significance, and technical challenge, to the shale gas plays of North America. As with shale, full-cycle, standalone commercial gas production from Cooper Basin ultra-deep coal seams requires a large, complex, permeable “stimulated reservoir volume” (SRV) domain having high fracture / fabric face surface area for gas desorption. This goal has not yet been achieved after 13 years of trials because, owing to the bipolar combination of coal-like geomechanical properties and shale-like reservoir properties, these poorly cleated, inertinitic coal seams exhibit “hybrid” characteristics. This is problematic for achieving effective reservoir stimulation, and poses the greatest immediate challenge. Stimulation techniques adopted from other play types are incompatible with the highly unfavourable combination of nanoDarcy-scale permeability, “ductility”, and high stress. The Cooper Basin Deep Coal Gas (CBDCG) Play commences 6,000 feet (1,830 metres) below the “commercial permeability depth limit” for most shallow coal seam gas (CSG) reservoirs but this does not reduce gas flow potential. Shale gas industry technologies have, in principle, eliminated the requirement for naturally occurring coal fabric permeability. Optimum reservoir conditions occur at depths beyond 9,000 feet (2,740 metres), driven by very low water saturation, high gas content, gas oversaturation, overpressure, rigid host rock strata, and high deviatoric stress. The limited literature does not yet adequately characterise the physical response of ultra-deep coal seams, and the surrounding host rock strata, to production pressure drawdown. It remains to be established how artificial fracture and coal fabric aperture width change as a consequence of the dynamic, diametric competition between gas desorption-induced coal matrix shrinkage and the omnipresent tendency for reservoir compaction caused by increasing production pressure drawdown-induced effective stress. This technical impasse, inhibiting commercialisation, is addressed by analysing the atypical flowback behaviour of hydraulically fracture stimulated coal seams within a dedicated vertical wellbore at 9,500 feet (2,900 metres). High-resolution, non-classical flowback analysis is performed on the pure dataset of Australia’s first ultra-deep coal gas well. Wellhead and fracture network pressures are recorded continuously for 8 1/2 years, at a 10-minute sample interval, while flowing to atmosphere. Natural flowback behaviour is analogous to that of a mechanical gas plunger artificial lift system. A low but gradually increasing quasi-steady state base gas flow, free of produced formation water, is overprinted by a non-steady state, cyclical pressure signature that is diagnostic of dynamic reservoir behaviour during gas production. A total of 114 high-rate, “geyser-like” gas surge events, gradually increasing in duration from 2 hours to 2 weeks, and in reservoir equivalent volume from 360 to 20,000 rcf (10 to 570 rcm), suggest the gas headspace compartment of a “down-hole void space domain” is steadily increasing in size. The gas surge events result from intermittent release of fracture network gas, hydrostatically compressed by flowback fluid slowly accumulating within the wellbore. A production “history match” for the gas surge event pressure profile is obtained by designing, fabricating, operating, and data logging a computer-controlled hydraulic apparatus within The University of Adelaide’s experimental wellbore, at a depth of 230 feet (70 metres). This physically simulates open-ended flowing manometer-like hydrodynamic behaviour of the wellbore-reservoir system. A postulated geological trigger mechanism for surge initiation is tested and validated; “wellbore hydrostatic back-pressure and reservoir stress-dependent leak-off”. Time-lapse pressure transient analysis (PTA) is performed on three extended wellbore pressure build-up tests, lasting 157, 259, and 295 days respectively. Increasing permeability is recognised within coal fabric surrounding the initial fracture network SRV domain. Time-lapse rate transient analysis (RTA) performed on the first two subsequent wellbore pressure “blow-down to atmosphere” (BDTA) gas flow rate decline profiles indicates that hydraulic fracture flow conductivity increased during the intervening 327-day flowback period. Interpreted dilation of hydraulic fracture apertures is supported by a 60% increase in the initial BDTA gas flow rates, from 7.5 to 12.0 MMscfd (212.4 to 340.0 Mscmd). Cooper Basin ultra-deep coal gas reservoirs behave differently to other deep, thermogenic source rock reservoirs, and require a paradigm shift in reservoir stimulation technology that does not rely exclusively upon hydraulic fracture stimulation and the “brittleness factor”. Pressure arching may fill this role by neutralising the omnipresent tendency for reservoir compaction caused by increasing production pressure drawdown-induced effective stress. The combined, mutually sustaining actions of desorption-induced coal matrix shrinkage and sympathetic pressure arch “stress shield” evolution generate an “expanding reservoir boundary and decreasing confining stress” condition that allows producing ultra-deep coal seams, and adjacent strata indirectly (which may include other reservoir types), to progressively de-stress and “self-fracture” in an overall state of endogenous tensile failure. As with underground coal mine excavations, pressure arching will deflect maximum stress vectors around the dilating “dispersed coal fabric void space” domain of a growing fracture network SRV domain that has developed reduced bulk structural integrity, and reduced bulk compressive strength, compared to the surrounding native coal seam and host rock strata. Size and effectiveness of pressure arching increases with depth. Cooper Basin ultra-deep coal seams, and adjacent “non-coal” reservoirs indirectly, may be effectively stimulated to flow gas on a large scale by harnessing this self-perpetuating, depth-resistant mechanism for creating coal fracture / fabric permeability and surface area for gas desorption. They may be induced to pervasively “shatter”, or “self-fracture”, naturally during gas production, independent of the lack of “brittleness”, analogous to the manner in which shrinkage crack networks slowly form, in a state of intrinsic, endogenous tension, within desiccating clay-rich surface sediment. Full-cycle, standalone commercial gas production is considered likely to occur when “Expanding Reservoir Boundary Theory” is applied, so as to replicate the very large, complex fracture network SRV domain of commercial shale gas reservoirs.
Thesis (Ph.D.) -- University of Adelaide, Australian School of Petroleum, 2020

The P-25 platform is located at 575 m water depth in Campos Basin, offshore Brazil. In the time of its conversion (1996), its process plant was designed to use cold water streaming from the Antartic continent to cool its equipment, accessories and compressing gas plant for exportation. The uptake riser installed in 1997 is 330 m long, 24″ OD pipe, hanging from the pontoon and the cold water flowing through it enters a sea chest located next to the support. This fully rigid riser was recently asked to be replaced. This paper presents the stages of the reanalysis of this compliant structure under conditions of low cost, so that the platform continues to suck cold water. Because of the presence of the uptake riser, the platform does not have a cooling plant that would else request area and weight, two important items in an arrangement, besides the high cost involved. Within the scope of the analyses, the replacement of the material (originally steel) with offloading hoses is tried out, in search for weight savings and lower cost. These hoses were reinforced with steel cables and their behavior was checked. Some tests were performed to verify the mechanical strength of this material and vibrations by VIV that occur in this structure. Monitoring systems were designed to check all forces and displacements during the referred installation. These actions will consolidate the technology for Petrobras leading to another riser system option for production in ultra deep waters.