Modelling of hydraulic fracturing and its engineering application

The Hydraulic Fracturing process and its engineering applications have been studied and reported in this thesis. The Distinct Element Method (DEM) was adopted as the main and preferred numerical technique because of its distinctive features and advantages. This method allows the phenomenon to be modelled and viewed microscopically at the inter-particle level by conceptualising the rock mass as an assembly of discrete particles interacting with each other via contacts. This method allows for a more detailed and dynamic monitoring of the hydraulic fracturing process. Sequel to a detailed review on the study of the hydraulic fracturing phenomenon, the research was extended to investigate specific cases of applications of hydraulic fracturing in geo-mechanical and environmental problems. Examples of such cases include carbon dioxide injection and storage in a reservoir system, and the behaviour of naturally occurring faults subjected to hydrostatic fluid pressures. The key factors governing the geo-mechanical responses of porous media (rocks), including reservoir formations were identified and further examined to ascertain the following: the role and inter-relationship between operating and material/fluid variables such as injection flow rate, fluid pressure, and interstitial velocity; type and pattern of fracture propagation; influence of environmental conditions as well as the configuration of the well-reservoir system, amongst others. Because of broad similarities in enabling conditions, analyses and applications of the phenomenon were also extended to study the sand production process. However, since the emphasis of the study was on identifying and examining the controlling variables as well as establishing patterns of sanding production rates rather than the study of the cavitation process, investigations were conducted using a finite element procedure; moreover, the limit of computational capacity has prevented a large scale DEM model for such problems. Modelling results show that fracturing mode, pattern and intensity are highly dependent on operating and environmental conditions; the reservoir erosion processes also indicate likewise tendencies. The numerical modelling techniques adopted and results obtained facilitate an improved understanding of geo-mechanical mechanisms at sub-surface systems, and could be further improved for industrial applications, such as site evaluation and assessment of the efficiency of stimulation techniques

An Overview of Principles and Designs of Hydraulic Fracturing Experiments and an Inquiry into the Influence of Rock Permeability and Strength on Failure Mode

The relevance of hydraulic fracturing experiments in the analysis of subsurface flow mechanisms and interactions during fracking operations underpins past and current efforts towards designing and implementing more representative physical models. An overview has been presented that comprehensively discusses the key elements and design requirements for successful experimentations. In setting up a hydraulic fracturing experiment, it is imperative that, in line with the research objective, the physical model that includes the initial and boundary conditions, wellbore configuration, type of fracturing fluid and injection rate be a true representative of actual reservoir/underground flow environments. This investigation recognises the main elements that form the framework for effective laboratory scale experiments, which comprise the specimen, in‐situ stresses, pore pressure, fluid injection, duration, and visualisation and monitoring. Furthermore, an examination of the influence of rock properties on the characteristics of fracturing and failure of rocks subjected to wellbore conditions indicates a trend highly dependent on rock strength and permeability. Soft and highly permeable rocks tend to cause an inward collapse of the wellbore cavity. Cavity size is also shown to have a considerable effect on the failure process. Wellbore stability is inversely related to cavity size; larger cavities are found to be less stable

Production from Unconventional Petroleum Reservoirs: Précis of Stimulation Techniques and Fluid Systems

An overview of the different categories of unconventional oil and gas reservoirs, and corresponding stimulation techniques appropriate for them is examined. Three main groups of unconventional oil and gas formations are appraised: heavy oil, oil shale and tight reservoirs. The scope of stimulation methods applicable to heavy oil reservoirs is limited. This kind of formation contains characteristic high-viscous hydrocarbons and are produced majorly by cold production and thermal stimulation. On the other hand, a wider range of stimulation methods are successfully used to produce tight and oil shales formations. For oil shales, these include drilling horizontal wells as substitutes to vertical wells, hydraulic fracturing, surfactant treatment, water imbibition, thermal treatment and acidisation; whilst for tight formations, these include hydraulic fracturing, surfactant treatment, water imbibition, acidisation and the application of electro-kinetics. Fracturing fluid systems are integral to the implementation of most stimulation operations and are evaluated herein under the following groups: water-based, oil-based, foam-based and acid-based. The most commonly used fracturing fluids are water based, albeit there are several instances where other types of fluids or combination of fluids are more suitable based on factors such as formation sensitivity, costs, wettability, rock solubility, surface tension, capillarity, viscosity, density, rheology and reactivity

The Sensitivity of Micro—Macro Mechanical Behaviour of Sand to the Inter-Particle Properties

Sand is a particulate material but is treated as a continuum solid in some engineeringanalyses. This approach is proven to be acceptable when dealing with geotechnical structures,provided an adequate factor of safety is applied so that there is no risk of failure. However, thecontinuum approach does not account for the effect of interparticle forces on the micro–macrobehaviour of sand. Sand could be modelled as a particulate material using the discrete elementmethod (DEM), taking into account its discrete nature. This paper shows how the microscopiccontact properties between the idealised sand particles influence the macro-mechanical behaviour,highlighting the development of the fabric as the soil approaches failure. Thirty DEM biaxial testswere performed to study the sensitivity of the macro–micro mechanical properties of sand to theinter-particle properties of an idealised sand particle. The conditions of these simulations were thesame (e.g., particle size distribution, number of particles, porosity after radius enlargement, boundaryconditions, and rate of loading). The sensitivity of the pre-peak, peak, and post-peak behaviour ofthese simulations to the inter-particle properties of an idealised sand particle was studied. Two extraDEM biaxial tests under different confining pressures were performed to verify the cohesionlessnature of the synthetic material used for this study. Since a two-dimensional DEM is used for thisstudy, a detailed approach to interpret the results assuming either a plane strain or a plane stresssituation was discussed. This study highlighted the critical inter-particle properties and the rangeover which these influence macro-mechanical behaviour. The results show that Young’s modulus ismainly dependent on the normal contact stiffness, and peak stress and the angle of internal frictionare greatly dependent on the inter-particle coefficient of friction, while Poisson’s ratio and volumetricbehaviour of particulate sand are dictated mainly by shear contact stiffness. A set of relationshipswere established between inter-particle properties and macro-machinal parameters such as Young’smodulus, Poisson’s ratio, and angle of internal friction. The elastoplastic parameters obtained fromthese tests are qualitatively in agreement with the typical medium and dense sand behaviour

Investigating sand production phenomena: An appraisal of past and emerging laboratory experiments and analytical models

© 2021 The Authors. Published by MDPI. This is an open access article available under a Creative Commons licence. The published version can be accessed at the following link on the publisher’s website: https://doi.org/10.3390/geotechnics1020023This paper provides an in-depth review of research developments on a common phenomenon in oil and gas exploration: sand production. Due to its significant impact to reservoir productivity and production efficiency, sand production has been widely researched in recent years. This paper focused on the review of historical progress in experimental and analytical studies which helped to understand the nature of the sanding mechanism and identify conditions that favour the process. Collation of the experimental data and analytical solutions and formulations enabled the authors to comment on effectiveness and also limitations of the existing experimental protocols and analytical models. Sand production models were then grouped into categories based on initiation of sanding, rate and amount of sanding as well as the failure criterion incorporated in their formulation so that it will be more convenient for future researchers to identify and adopt an appropriate model for their own research. The review also confirms that there are still some aspects of sand production requiring further investigation, and maybe a hybrid approach combining experimental, analytical and numerical methods could be the best solution for future explorations.Published onlin

Evaluating the corollary of the interdependency of rock joint properties on subsurface fracturing

This is an accepted manuscript of an article published by Springer in Bulletin of Engineering Geology and the Environment on 14/08/2020, available online: https://doi.org/10.1007/s10064-020-01933-5 The accepted version of the publication may differ from the final published version.The characteristics of structural discontinuities in the subsurface environment often play a key role in the overall behaviour of such systems and their response to externally imposed conditions. Rock joints are one of such features that constitute the heterogeneity of rock masses. Akin to other forms of discontinuities, the characteristics of rock joints affect the performance of their parent rock masses, which are constituents of rock formations. The fracturing process is one of such key geo-mechanical phenomena that is inevitably influenced by pre-existing joints. A numerical technique implemented via a discrete element method (DEM) is herein adopted to evaluate two fundamental properties that control the shear and dilatancy responses of discontinuities. Though these properties are also assessed in isolation, their interdependency, which is a dominant factor, is investigated. As joint frictional resistance increases, it escalates the potential of the joint to attenuate the rate of fracture growth. On the other hand, an increase in joint dilatancy increases the intensity of fracturing. The impact of joint frictional resistance is more pronounced at high friction magnitudes, and in this range, the predominant influence of joint friction overwhelms any effect of joint dilatancy. Contrarily, at low joint frictional resistance, contributions from even a small magnitude of joint dilatancy increases the degree of fracturing. The inter-relationship between joint friction and dilatancy has influencing implications that govern the performance of rock masses. An inquiry into their combined contributions provides information prerequisite for a more accurate estimation and appraisal of fracture behaviour in underground systems.Published onlin

A review of the studies on CO₂–brine–rock interaction in geological storage process

© 2022 The Authors. Published by MDPI. This is an open access article available under a Creative Commons licence. The published version can be accessed at the following link on the publisher’s website: https://doi.org/10.3390/geosciences12040168CO2–brine–rock interaction impacts the behavior and efficiency of CO2 geological storage; a thorough understanding of these impacts is important. A lot of research in the past has considered the nature and impact of CO2–brine–rock interaction and much has been learned. Given that the solubility and rate of mineralization of CO2 in brine under reservoir conditions is slow, free and mobile, CO2 will be contained in the reservoir for a long time until the phase of CO2 evolves. A review of independent research indicates that the phase of CO2 affects the nature of CO2–brine–rock interaction. It is important to understand how different phases of CO2 that can be present in a reservoir affects CO2–brine–rock interaction. However, the impact of the phase of CO2 in a CO2–brine–rock interaction has not been given proper attention. This paper is a systematic review of relevant research on the impact of the phase of CO2 on the behavior and efficiency of CO2 geological storage, extending to long-term changes in CO2, brine, and rock properties; it articulates new knowledge on the effect of the phase of CO2 on CO2–brine–rock behavior in geosequestration sites and highlights areas for further development.Published onlin

Effect of CO₂ phase on pore geometry of saline reservoir rock

© 2022 The Authors. Published by Springer. This is an open access article available under a Creative Commons licence. The published version can be accessed at the following link on the publisher’s website: https://doi.org/10.1007/s00603-021-02658-xThe phase of CO₂ present in a saline reservoir influences the change of the pore geometry properties of reservoir rocks and consequently the transport and storage integrity of the reservoir. In this study, digital rock physics was used to evaluate pore geometry properties of rocks saturated with the different phaseCO₂-brine under reservoir conditions. The changes in the pore geometry properties due to the different phaseCO₂-brine-rock interaction were quantified. In addition to compression, CO₂-brine-rock interaction caused a further reduction in porosity by precipitation. Compared to the dry sample, the porosity of the gaseous CO₂-br sample was reduced the most, and was lower by 15% after saturation and compression. There was reduction in the pre-compression porosity after compression for all the samples, however, the reduction was highest in the gaseous CO₂-br-saturated sample (13%). The flatness of pore surfaces was reduced, and pores became less rounded after compression, especially in supercritical CO₂-br-saturated rock. The results from this research provide a valuable input to guide a robust simulation of CO₂ storage in reservoir rocks where different phases of CO₂ could be present.This study was funded by the Petroleum Technology Development Fund (PTDF), Nigeria.Published versio

Modelling of hydraulic fracturing process by coupled discrete element and fluid dynamic methods

A three-dimensional model is presented and used to reproduce the laboratory hydraulic fracturing test performed on a thick-walled hollow cylinder limestone sample. This work aims to investigate the implications of the fluid flow on the behaviour of the micro-structure of the rock sample, including the material strength, its elastic constants and the initialisation and propagation of fractures. The replication of the laboratory test conditions has been performed based on the coupled Discrete Element Method (DEM) and Computational Fluid Dynamics scheme. The numerical results are in good agreement with the experimental data, both qualitatively and quantitatively. The developed model closely validates the overall behaviour of the laboratory sample, providing a realistic overview of the cracking propagation towards total collapse as well as complying with Lame’s theory for thick-walled cylinders. This research aims to provide some insight into designing an accurate DEM model of a fracturing rock that can be used to predict its geo-mechanical behaviour during Enhanced Oil Recovery applications