An investigation into the controls on fracture tortuosity in rock sequences and the impact on fluid flow in the upper crust

Fractures are ubiquitous in geological sequences, and play an important role in the movement of fluids in the earth’s crust, particularly in fields such as hydrogeology, petroleum geology and volcanology. When predicting or analysing fluid flow, fractures are often simplified as a set of smooth parallel plates. In reality, they exhibit tortuosity on a number of scales: Fine-scale tortuosity, or roughness, is the product of the small-scale (µm – mm) irregularities in the fracture surface, whereas large-scale (> mm) tortuosity occurs as a result of anisotropy and heterogeneity within the host formation that leads to the formation of irregularities in the fracture surfaces. It is important to consider such tortuosity when analysing processes that rely on the movement (or hindrance) of fluids flowing through fractures in the subsurface. Such processes include fluid injection into granitic plutons for the extraction of heat in Engineered Geothermal Systems, or the injection of CO2 into reservoirs overlain by fine-grained mudrocks acting as seals in Carbon Capture and Storage projects. Although it is generally assumed that tortuosity is controlled by factors such as grain size, mineralogy and fracture mode, a systematic study of how these factors quantitatively affect tortuosity is currently lacking. Furthermore, in anisotropic rocks the fracture orientation with respect to any inherent anisotropy is also likely to affect tortuosity. In order to address this gap, we have induced fractures in a selection of different rock types (mudrocks, sandstones and carbonates) using the Brazil disk method, and imaged the fracture surfaces using both a digital optical microscope and X-ray Computed Tomography. Using these methods we are able to characterise both the fine-scale (roughness) and large-scale tortuosity. In order to understand the effect of fracture orientation on tortuosity we have also analysed fractures induced at different angles to bedding in samples of a highly anisotropic mudrock taken from South Wales, UK. Results indicate that fine-scale tortuosity is highly dependent on the fracture orientation with regards to the bedding plane, with fractures normal to bedding being rougher than those induced parallel to bedding. Finally, in order to measure the effect of tortuosity on fluid flow, we have carried out a series of core flooding experiments on a subset of fractured samples showing that fracture transmissivity decreases with increasing tortuosity

Modelling of long-term along-fault flow of CO2 from a natural reservoir

Geological sequestration of CO2 requires the presence of at least one competent seal above the storage reservoir to ensure containment of the stored CO2. Most of the considered storage sites are overlain by low-permeability evaporites or mudrocks that form competent seals in the absence of defects. Potential defects are formed by man-made well penetrations (necessary for exploration and appraisal, and injection) as well as (for mudrocks) natural or injection-induced fracture systems through the caprock. These defects need to be de-risked during site selection and characterisation. A European ACT-sponsored research consortium, DETECT, developed an integrated characterisation and risk assessment toolkit for natural fault/fracture pathways. In this paper we describe the DETECT experimental-modelling workflow, which aims to be predictive for fault-related leakage quantification, and its application to a field case example for validation. The workflow combines laboratory experiments to obtain single-fracture stress-sensitive permeabilities; single-fracture modelling for stress-sensitive relative permeabilities and capillary pressures; fracture network characterisation and modelling for the caprock(s); upscaling of properties and constitutive functions in fracture networks; and full compositional flow modelling at field scale. We focus the paper on the application of the workflow to the Green River Site in Utah. This is a rare case of leakage from a natural CO2 reservoir, where CO2 (dissolved or gaseous) migrates along two fault zones to the surface. This site provides a unique opportunity to understand CO2 leakage mechanisms and volumes along faults, because of its extensive characterisation including a large dataset of present-day CO2 surface flux measurements as well as historical records of CO2 leakage in the form of travertine mounds. When applied to this site, our methodology predicts leakage locations accurately and, within an order of magnitude, leakage rates correctly without extensive history matching. Subsequent history matching achieves accurate leak rate matches within a-priori uncertainty ranges for model input parameters

A systematic investigation of the intrinsic flow properties of fractures using a combined 3D printing and micro-computed tomography approach

Geological storage operations spanning energy, nuclear material and carbon dioxide (CO2) storage, require meticulous understanding of the integrity of geological seals over a range of temporal and spatial scales. Fluid-conductive fault and fracture systems in otherwise low-permeability rocks may threaten seal performance and compromise subsurface storage projects. The understanding of these systems is complicated by the occurrence of anisotropic aperture distribution caused by inherent surface roughness. Difficulties predicting fluid flow through fractures stems from our limited understanding of the fundamental controls on their intrinsic permeabilities, and the prevalence, severity and complexity of hydromechanical responses arising from the coupling of multiphase flow, pore pressure and effective stress. In this study, we systematically investigated the effect of surface roughness on the transport properties of 3D-printed (Acrylonitrile Butadiene Styrene resin) fracture surfaces with micrometre surface roughness distributions. We printed 11 separate fractures, 7 of which are synthetically generated self-affine surfaces encompassing a range of fractal dimensions (Df = 1.2 to 2.4) observed in nature. The remaining 4 are acquired from micrometre-scale surface scans from natural fractures within the Carmel mudrock, a caprock from a natural CO2 leakage site in Utah, USA. Fluid flow experiments using single (brine) and multiple fluids (decane and brine) are undertaken to investigate the fluid pathways and interactions between each phase across a range of effective stresses (5 to 25 bar). We investigate the interplay between multiphase flow dynamics, surface roughness and hydraulic aperture distribution to gain insight into the intrinsic transport properties of fractures with different origins of roughness. Experiments are performed and imaged using a micro-computed tomography scanner (EMCT; (Bultreys et al., 2016)), where the results can be used to further the understanding of the governing parameters influencing fracture transmissivity, while also constraining surface roughness inputs for single- and multiphase fracture flow models

Comparison of initial stress state and rock-failure risks for five prospective CO2 storage sites

We report initial assessments of the state of stress and the estimated conditions for rock failure at five prospective CO2 storage sites which are being considered in the ACT SHARP Project. This multinational project aims to improve understanding of stress history and reservoir pressure to enable improved quantification of CO2 storage containment risks. The goal is to improve the accuracy of subsurface CO2 storage containment risk management through the improvement and integration of subsurface stress models, rock mechanical data and seismicity observations. The case studies considered in this assessment are: • Norway – Horda/Smeaheia region; • UK – Southern North Sea, Bunter storage play; • Netherlands – Aramis site, Rotliegend pre-salt; • Denmark – Lisa Structure; • India – Bhagewala Heavy Oil Field, Rajasthan. These case studies have different levels of maturity of site development and data availability, which is useful for understanding what data is needed at different stages of a project. While detailed site characterisation and rock failure studies have been conducted for the Horda/Smeaheia region offshore Norway and for parts of the UK Southern North Sea (SNS) Bunter storage play, rock failure characterisation studies at the Aramis site and Lisa Structure are limited to regional studies. The Bhagewala Heavy Oil Field in India is the least mature of the case studies in terms of storage assessment

L'impatto della PEX sul regime fiscale delle operazioni straordinarie e l'opportunit\ue0 di un'imposizione sostitutiva nella cessione e nei conferimenti di azienda.

Outcrop fracture data sets can now be acquired with ever more accuracy using drone technology augmented by field observations. These models can be used to form realistic, deterministic models of fractured reservoirs. Fractured well test models are traditionally seen to be finite or infinite conductivity or double porosity - corresponding the fractures with or without matrix support. Using this simple field outcrop based geometrical model to generate typical well test responses for wells either intersecting fractures or well nearby fractures shows that such responses can occur in sequence as part as a diagnostic signature of naturally fractured reservoirs.Geoscience & EngineeringCivil Engineering and Geoscience