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Title: Investigating the role of proppants in hydraulic fracturing of gas shales
Author: Bou Hamdan, Kamel F.
ISNI:       0000 0004 7652 6922
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2019
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It has been reported that different shale reservoirs behave differently to hydraulic fracturing. The reason behind this can be linked to their mechanical properties. Experiments, such as triaxial tests, can be used to measure different rock properties such as Young's modulus and Yield strength. Crush tests can be used to measure proppant's mechanical properties such as ultimate compressive strength. Conductivity cells are being used to find the interaction of proppant packs with the rock by measuring the permeability or conductivity changes at different loads. However, this method cannot provide direct observation of the rock and proppant interaction, nor measure the stresses and contact areas. Numerical methods are usually used for such studies but they can be time consuming and totally dependent on the boundary conditions used in the model. Also direct observations of the rock and proppant interaction have not been reported. The experimental limitations leads to failure in addressing the real interaction taking place, so based on these difficulties the key question of this study is: How does the relative stiffness of the rock and proppants affect the conductivity of open fractures? This was addressed by developing a theoretical model and a new experimental approach that can measure contact area by direct observation. A theoretical model was developed that can analyse the deformation behaviour of two flat surfaces pressed against multiple spheres making up a single layer. In reality, proppants are stiffer than shales (higher Young's modulus) and most of the deformation occurs on the rock surface. This allows to model the spheres as rigid bodies whereas the flat surfaces can deform elastically. This model uses the principles of superposition to adjust locations of the spheres. Effects of sphere sizes, arrangements, material properties (Young's modulus) and contact parameters on the deformed surface are analysed. A dimensionless transition stress, when exceeded, causes the deformed surface to make contact with smaller spheres, was used as a representation of the larger sphere's capability of withstanding crushing. The numerical simulation results showed good agreement with the theoretical predictions hence validating the applicability of this model. A new experimental method was introduced that uses ultrasonic waves to measure the contact area and stress due to the interaction of bodies. This experimental design allows a single or multiple objects e.g. spheres to be in contact with two parallel plates. It allows for different combinations of sphere-plate models to be tested out. In this study the following plates were used: polycarbonate, phenolic and two types of aluminium. Spheres and flat-ended cylinders, made up of: steel, glass or plastic, were used in contact with the plates. These were selected to simulate the relative elastic modulii ratio and the yield strength of a shale-proppant system. The experiment uses an ultrasonic transducer to scan a contact by operating in a pulse-echo mode. Reflected signals were recorded and analysed to assess the applicability of this method in different materials. Stresses were measured experimentally by two methods and were compared to Hertz theory for elastic deformations. Reflected waves at the outer boundary of contact were also recorded and correlated with vertical displacements through a numerical model from which the influential area could be observed. This parameter is important for contact of multiple spheres as it shows if the individual sphere-plane interaction can influence the neighbouring sphere contacts e.g. proppants in a fractured reservoir. Experimental results of stresses showed good agreement in polycarbonate and aluminium but not in phenolic where large attenuations was observed. Larger contact areas were observed for plates with large E/sY property suggesting pile-up deformation takes place. This was confirmed through numerical simulations. Correlation of reflection coefficient and displacements were in good agreement at different loads. The relation between both parameters differs with type of material. This could be due to different factors including incidence angle, Poisson's ratio, and attenuation coefficient especially the scattering effect. A linear relation was obtained between vertical displacement and reflection coefficient at the outer boundary for the sink-in behaviour. A new relation was proposed to compare deformations of different materials based on their mechanical properties, applied load and recorded reflection coefficients from a fixed distance of the contact boundary.
Supervisor: Syed, Amer ; Siddiq, Amir Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Shale gas ; Hydraulic fracturing ; Proppants