Introduction

The primary production phase of many of the North Sea reservoirs now being developed calls for pressure maintenance and flooding by the injection of seawater. Experimental work and preliminary experience with actual injection systems have highlighted problem areas which could be of great importance to the optimisation of injection systems, well location and design, and reservoir development planning. These problems areas extend over the whole process and are summarised schematically in Fig. 1.

  • Area one covers the sea water injection water itself, the characterisation of its suspended matter, its tendencies for corrosion and scale deposition, its bacterial content, and its compatibility with formation rock and formation water.

  • Area two covers the filtration process of the water before injection.

  • Area three covers other surface treatments of the water before injection.

  • Area four covers the zone of potential damage around the wellbore and the zone of potential damage in the formation.

AREA ONE
Sea water characterisation

Heriot-Watt interest in this problem area stems from work done in the microbiological field on the analysis of sea water for environmental studies and from work done on the characterisation of particles in gases. The microbiological research deserves a paper on its own and since moreover it is not strictly an engineering problem, it is not proposed to deal with it in detail here. problem, it is not proposed to deal with it in detail here. One of the most important problems in injection projects is to identify the microscopic particles which projects is to identify the microscopic particles which even in very small concentrations may cause plugging of the reservoir. In the characterisations of particulates we have identified in discussion with personnel from North Sea operating companies a need for an on-line instrument to give, ideally, a continuous measure of size, size distribution and concentration of suspended particles in sea water. The aim of the research project particles in sea water. The aim of the research project being carried out by Drs. Waldie and Boron of the University Department of Chemical and Process Engineering is to study the feasibility of using optical scattering techniques for this purpose with a view to developing a suitable working instrument.

Current instrumentation is based mainly on measurements of turbidity and onsize measurement by the Coulter principle. The first is an on-line measurement but does not give quantitative information on particle size, size distribution or concentration. The second yields quantitative information on particle size distribution but is not a direct on-line instrument, as an operator is involved and there is a resultant time-lag in obtaining size information. In addition the Coulter technique has a lower size limit of about 0.5 to 1 micron, whereas the presence and characteristics of much smaller particles may be of concern in reservoir pore blocking.

Optical scattering techniques whereby scattering data is inverted to give information on particle characteristics, have the potential advantages of on-line operation, of giving information on mean size, size distribution and concentration, and a lower size limit of 0.01 mu m or even smaller. However, both experimental and theoretical studies are needed to establish whether these advantages can all be realised in a practical instrument for injection water conditions. Use will be made of practical and theoretical experience gained over the past three years in a study of the analogous problem of characterising particles in gases by light scattering. Computer particles in gases by light scattering. Computer programmes have been developed for the inversion of programmes have been developed for the inversion of experimental scattered intensity data and are being used to explore theoretically the more promising scattering techniques.

The main aspects to be investigated are the sensitivity of scattering techniques to relative refractive index, mean particle size, size distribution, shape, and particle particle size, size distribution, shape, and particle concentration. These investigations would help resolve practical questions of instrument performance such as practical questions of instrument performance such as the relative detectability of marine organisms, inorganic sediments, filter aid, scale, and oil particles; upper and lower size limits; and concentration limits.

It will be essential to compare the performance of angular scattering techniques with results obtained from instruments in current use. Comparative measurements will therefore be made on an existing Coulter counter, and on a commercial turbidimeter. Electron microscopy measurements of particles separated on 'Millipore' type filters will also be made.

One of the aims of the project is to be able to test a prototype instrument on a platform on filtered injection water. Before that stage, measurements will be carried out in the laboratory on sea water filtered to various known solids contents, and on samples from production platforms.

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