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This thesis reports an experimental and theoretical study on filtration properties of water based drilling fluids under dynamic and static conditions. The tested muds cover Freshwater/Gypsum/Lignosulphonate mud and SeawaterlKCLlPolymer mud, bariteweighted and unweighted. The effects of the solid concentration, pressure and shear rate on the filter cake characteristics and the erodability were investigated. For static filtration experiments, all tests were conducted for two hours and the spurt loss, the filter cake thickness, the ratio of wet to dry cake mass and the cumulative filtrate volume against time were measured. For dynamic filtration experiments, however, only the spurt loss and the cumulative filtrate volume against time were measured and all tests were conducted for at least 8 hours. A general filtration equation was developed based on the cake filtration theory prevailing in the chemical engineering industry and it was utilised to obtain the modified classic static filtration equation and the dynamic filtration equation. The modified classic static filtration equation was then employed to fit the static filtration experimental data and the average specific static cake resistance and the effective filter medium resistance were calculated. The dynamic filtration equation showed a substantial agreement with the dynamic filtration experimental data. Using the static filter cake properties such as the ratio of wet to dry cake mass (m), the average specific dynamic cake resistance, the effective filter medium resistance and the dynamic filter cake erodability were calculated. In the study of the relationship between the static filtration data and the dynamic filtration data, an attempt of predicting the dynamic filtration data from the static filtration experimental data was conducted. Also, an attempt was carried out to predict the static filtration data and the dynamic filtration data in a sequential process. The experimental data suggests that a substantial difference exists between the specific resistances of static and dynamic filter cakes. No apparent distinction was found, however, between spurt loss and effective filter medium resistance. The reodability of dynamically deposited mud cake for Seawater/KCL/Polymer mud was found to be three fold for Freshwater/Gypsum/Lignosulphonate.

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During the tertiary stage of oil recovery, various chemicals and gases are injected into the reservoir. The success of these processes often depend on the ability of the injected chemicals/gases making contact with the residual hydrocarbon. The accessibility of the individual pore, where the hydrocarbon are trapped, to the injected fluid is therefore important. In many core flooding experiments which simulate such tertiary processes, the effluent profiles of the injected chemical often exhibit the capacitance effect of early breakthrough and long tails. These are mainly due to the fact that not all the pores are accessible to the displacing fluid. In this work, a mass transfer model, using physical meaningful parameters, has been set up to study the capacitance effect. The pore space are divided into a flowing fraction in which the bulk flow of the injected chemical takes place and a stagnant fraction where the chemical can only access by molecular diffusion. The significance and sensitivity of the five model parameters have been studied extensively using computer simulation. The extent of the mass transfer process is characterised by the different sets of family curves. A series of IPA/water miscible displacements using Clashach sandstone have been carried out to provide experimental data for model simulation. The assumption of the stagnant fraction in the form of dead end pores has been supported by the results of hexane/toluene displacements at connate water saturation. Some problems of history matching the experimental results by model simulation have been highlighted. This is mainly caused by the difficulty of establishing an analytical solution for the model equation and the need to optimise simultaneously the five model parameters. Various approach to overcome these problems have been successfully demonstrated in this work and further possible improvement has been identified. The source of numerical dispersion and the different corrective schemes proposed in various papers have been summarised and compared. One of these, the method of lines (MOL) has been used successfully in this work to minimise numerical dispersion. The understanding of the non-equilibrium capacitance effect in porous media is essential in order to interpret the production data and, in particular, laboratory core flooding results correctly. It is also important, for the mathematical model, to use parameters which are physically meaningful to the process itself. The work carried out in this research has provided a detailed study on this subject.

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In this thesis the taxation of North Sea oil is found to deter oilfield investment substantiaty - investment which it pursued would bring about Pareto benefits. Analysis of both the comparative static tax effects and the dynamics of taxation on oilfield development decisions reveals the nature of the deterrent, while a specially developed econometric model is used to quantify the impact of the tax structure and of tax changes on recoverable reserves. By detailing the mechanisms through which taxation distorts the allocation of investment resources away from North Sea developments, the thesis stresses that individual tax clauses tend not to perform the roles allotted to them and that tax changes alter the economics of oil projects in ways and in magnitudes that do not seem to be realised. Hence, not only is the nature of these effects misunderstood, but the extent oi these effects is much more damaging than commonly appreciated. These findings are reached by following the principles laid out in the early chapters which suggest that, if an improvement in the allocation of resources is to occur, laise beliets must be reclined. In particular, the inconsistencies between the targets and instruments of North Sea policy suggest there are false beliets about the need to stimulate development decisions and about the role of taxation - suggestions which are confirmed in the detailed research of the thesis. Having condemned existing North Sea oil tax policy, a practical, feasible and attainable alternative to the current tax system, that would not distort investment decisions so severely, is recommended.

Cyclone separators have been described in detail and, although substantial research has been performed on solid/gas devices, the use of cyclones for gas/liquid separation has been comparatively ignored; this is particularly true for higher concentrations of liquid and for degassing applications. Consequently no generic models are available which will predict separation efficiency or pressure drop for all designs of cyclone. A novel design of axial flow cyclone called WELLSEP was examined for the purpose of degassing. This design was not believed to be optimal and no design criteria or performance prediction models were available for it. An experimental programme was therefore produced and executed to investigate changes in geometry and the affect of fluid dynamics. Changes to the length, vortex finder and swirl generator were examined first and then one design was selected and tested over a number of liquid flow rates, Gas Void Fractions (GVFs) and liquid extractions. Data was collected from the experiments which assisted in the development of semi-empirical models for the prediction of pressure drop and separation efficiency. These models could be used in the design of WELLSEP. Geometric and fluid dynamics changes have both been shown to influence the performance of the tested cyclone. The principal conclusions that have been drawn from this research are: Of the tested designs, the design based upon a 30mm vortex finder diameter, settling chamber length of three times the diameter of the cyclone and a four start helix gave the optimum separation efficiency over the greatest range of conditions. The separation efficiency is affected by the superficial liquid velocity and the liquid extraction but not the GVF. The dimensionless pressure drop coefficient (Euler number) is a function of liquid extraction and GVF. It may also be a function of the superficial liquid velocity but it is unproven by this research.