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Title: Impact of drilling fluids on geomechanical stability of wellbore
Author: Mfanga, Dhelda Reginald
ISNI:       0000 0004 7432 0322
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2018
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The reliable assessment of the geomechanical stability of the wellbore depends on the accurate determination of the stresses and strength of the rockmass around the wellbore region. When the wellbore is created, usually drilling fluids are used to facilitate the drilling process and gather subsurface information of the reservoir and the overburden. These fluids are complex in nature and are known to exhibit both elastic and viscous behaviour (viscoelasticity). The knowledge on how these fluids affect the stress regime and mechanical properties of the rockmass specifically near the wellbore region is limited. Therefore, the work described in this thesis aimed at investigating the influence of drilling fluid's viscoelastic behaviour on the stability of the rockmass near the wellbore region during drilling operations. To investigate the full near wellbore mechanics, the area around the wellbore was divided into three zones: 1. drilling fluid zone 2. filter cake zone 3. rockmass zone. In each zone, a theoretical framework to analyse stability of the wellbore using experimentally measured mechanical properties given in terms of viscoelastic properties (elastic and viscous moduli) was developed. The underlying hypothesis which governed this framework was that, under certain conditions the elasticity of drilling fluids will have the ability to carry some the stresses imposed by the rockmass during deformation. Standard experimental methods and procedures were designed to investigate quantitatively the viscoelastic properties of drilling fluids and drilling fluid filter cakes. The methods include steady shear rheology and dynamic mechanical tests which were conducted at different experimental conditions. Bentonite suspensions at different concentration were used as model drilling fluids. Effect of adding different rheological additives, which were Barite, NaCl, Lignin and Polyanionic Cellulose to bentonite suspensions were also studied. After obtained the viscoelastic data, a theoretical model which incorporate the elasticity of drilling on the stress distribution around the wellbore was developed. The model considered the isotropic and anisotropic horizontal in-situ stresses separately in the case of strong formation and weak formations. Results from isotropic model did not show any influence of the elasticity of drilling fluid on the wellbore stresses in both weak and strong formations. For the anisotropic model, significant changes of wellbore stresses due to the influence of the elasticity of drilling fluid were observed for all the stress regimes considered in weak formations. The influence of viscoelasticity of drilling fluid on mechanical properties of consolidated and unconsolidated rocks were also investigated experimentally through triaxial tests. Specimens were prepared and saturated with drilling fluid of different elasticity and then tested for strain controlled triaxial test until failure. Results obtained from the consolidated samples did not evident conclude the influence of elasticity of drilling fluid on their mechanical properties. For unconsolidated samples, the effect of elasticity of drilling fluid were shown by higher undrained Young's moduli and undrained shear strengths. From laboratory testing to theoretical modelling, a major conclusion drawn from this study is that, the elasticity of drilling fluid has a significant impact on the redistribution of stresses around the wellbore especially in weak formations. It has proved to reduce and increase magnitude of the tangential and axial stresses in compression and tension zones respectively as well as increasing the magnitude of the radial stresses around the wellbore, hence create more stable conditions around the wellbore.
Supervisor: Syed, Amer ; Tanino, Yukie ; Healey, David Sponsor: Aberdeen Institute of Energy ; European Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Drilling muds ; Boring ; Strains and stresses