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With the changing picture of global energy supplies and the shift toward the energy transition, it has never been more important to look for alternative sources of energy. Globally there are tens of thousands of abandoned oil fields with considerable reserves left behind. These have the potential to be reactivated to become an energy supply that is cleaner than conventional oil and gas. This can be achieved by the use of in situ combustion and the subsequent exploitation of the inherent increase in temperature and pressure to produce geothermal energy, allied to sequestration of the mixture of produced fluids. In situ combustion (ISC) has conventionally been used as an enhanced oil recovery technique, with a high failure rate that has been recently attributed to poor reservoir selection and project design. We suggest that the failure of many earlier ISC projects is due to insufficient appreciation of how the subsurface geology affects the process. With the use of computer numerical modelling, we aim to ascertain how the geometry and heterogeneity of the reservoir control the success of the process. Here we employ simple three-dimensional sector models to assess a variety of different petrophysical heterogeneities, within a set of different reservoir geometries, on the temperature, velocity, propagation stability and enthalpy rate. These models illustrate that the biggest impact on success of the ISC process for geothermal energy generation, as a function of temperature and enthalpy, is the location of the wells relative to the heterogeneities and the scale of heterogeneities. Metre-scale heterogeneities do not have a significant effect on this. Instead, the biggest contributor to the propagation stability and direction of the fire front is the presence of a large-scale (10 s to 100 s of metres) heterogeneities, such as channels, or the geometry of a tilted fault block; both have a strong control over the direction of the propagation, and therefore are important factors with regards to well placement. [ABSTRACT FROM AUTHOR]

Top seals and overburden above reservoirs at geological carbon capture and storage (CCS) sites can be major concerns when they are at risk of being mineralogically and texturally unstable in the presence of high-pressure CO2. Here we report on the pore systems, mineralogy, and surface area attributes of the Palaeocene Lista Shale, the caprock to the Mey Sandstone at the UK's planned East Mey CCS site. The core was logged, and then mineral quantification was undertaken with X-ray powder diffraction mineralogy, light optics and electron microscopy analyses. Laser particle size analysis was used for grain size determination. Porosity, pore throat diameter, surface area and pore body size were measured via mercury intrusion porosimetry and nitrogen adsorption analyses. The mudstone facies from the Lista Shale are dominated by smectite-rich matrix and silt-grade quartz, with small quantities of chlorite and sodic-plagioclase. Chlorite, sodic-plagioclase, and even smectite are known to be capable of reacting with, and potentially leading to mineral sequestration of CO2. The mean pore throat and pore body diameters are 17 and nearly 18 nm, respectively, showing that the Lista is mesoporous; the similarity of pore body and pore throat dimensions reveals a predominance of plate and slit pores. Gas adsorption analyses revealed that the overall pore structure is complex, with a high tortuosity of fluid movement through a complex clay-rich matrix (this equates to a mean fractal dimension D2 value of 2.67). Gas adsorption analyses have also shown that grain surfaces are moderately complex (rough) due to the dominance of clay aggregates (this equates to a mean fractal dimension D1 value of 2.56). D2 being higher than D1 suggests that there is a relatively low potential to physically store CO2 gas on grain surfaces. Conversely, the ability of the CO2 to react with minor quantities of chlorite and sodic plagioclase, or even with smectite, could lead to increasing surface area of the remaining shale minerals with newly exposed reactive silicates leading to further enhanced mineral trapping of the injected CO2. The restricted pore throat size linked to small grain size and poor sorting, and reflected by the high fractal D2 value, plus limited grain surface complexity, reflected by the low fractal D1 value, collectively suggest that mineral trapping of the injected CO2 would be relatively slow (on the order of 1000s of years) if CO2 penetrated the top seal. [ABSTRACT FROM AUTHOR]

Reservoir quality prediction in deeply buried reservoirs represents a complex challenge to geoscientists. In sandstones, reservoir quality is determined by the extent of compaction and cementation during burial. During compaction, porosity is lost through the rearrangement and fracture of rigid grains and the deformation of ductile grains. During cementation, porosity is predominantly lost through the growth of quartz cement, although carbonate and clay mineral growth can be locally important. The degree of quartz cementation is influenced by the surface area of quartz available for overgrowth nucleation and thermal history. Clay grain coats can significantly reduce the surface area of quartz available for overgrowth nucleation, preventing extensive cementation. Using a coupled-effect compaction and cementation model, we have forward-modelled porosity evolution of surface sediments from the modern Ravenglass Estuary under different maximum burial conditions, between 2000 and 5000 m depth, to aid the understanding of reservoir quality distribution in a marginal marine setting. Seven sand-dominated sub-depositional environments were subject to five burial models to assess porosity-preservation in sedimentary facies. Under relatively shallow burial conditions (<3000 m), modelled porosity is highest (34 to 36%) in medium to coarse-grained outer-estuary sediments due to moderate sorting and minimal fine-grained matrix material. Fine-grained tidal flat sediments (mixed flats) experience a higher degree of porosity loss due to elevated matrix volumes (20 to 31%). Sediments subjected to deep burial (>4000 m) experience a significant reduction in porosity due to extensive quartz cementation. Porosity is reduced to 1% in outer estuary sediments that lack grain-coating clays. However, in tidal flat sediments with continuous clay grain coats, porosity values of up to 30% are maintained due to quartz cement inhibition. The modelling approach powerfully emphasises the value of collecting quantitative data from modern analogue sedimentary environments to reveal how optimum reservoir quality is not always in the coarsest or cleanest clastic sediments. [ABSTRACT FROM AUTHOR]

Incised valley fills are complex as they correspond to multiple sea‐level cycles which makes interpretation and correlation of stratigraphic surfaces fraught with uncertainty. Despite numerous studies of the stratigraphy of incised valley fills, few have focused on extensive core coverage linked to high fidelity dating in a macro‐tidal, tide‐dominated setting. For this study nineteen sediment cores were drilled through the Holocene succession of the macro‐tidal Ravenglass Estuary in north‐west England, UK. A facies and stratigraphic model of the Ravenglass incised valley complex was constructed, to understand the lateral and vertical stacking patterns relative to the sea‐level changes. The Ravenglass Estuary formed in five main stages. First, incision by rivers (ca 11 500 to ca 10 500 yrs bp) cutting through the shelf during lowstand, which was a period of fluvial dominance. Secondly, a rapid transgression and landward migration of the shoreline (10 500 to 6000 yrs bp). Wave action was dominant, promoting spit formation. The third stage was a highstand at ca 6000 to ca 5000 yrs bp, creating maximum accommodation and the majority of backfilling. The spits narrowed the inlet and dampened wave action. The fourth stage was caused by a minor fall of sea level (ca 5000 to ca 226 yrs bp), which forced the system to shift basinward. The fifth and final stage (226 yrs bp to present) involved the backfilling of the River Irt, southward migration of the northerly (Drigg) spit and merging of the River Irt with the Rivers Esk and Mite. The final stage was synchronous with the development of the central basin. As an analogue for ancient and deeply buried sandstones, most of the estuarine sedimentation occurred after transgression, of which the coarsest and cleanest sands are found in the tidal inlet, on the foreshore and within in‐channel tidal bars. The best‐connected (up to 1 km) reservoir‐equivalent sands belong to the more stable channels. [ABSTRACT FROM AUTHOR]