Scale is the result of deposition of material normally in solution or created as a result of a chemical reaction. The scale problem is an important factor that can cause a reduction in the productivity and the economic viability of an oil or gas field. For example, Halite scaling has taken a major part of the uncertainty related to the Heron Cluster project, where the gas coming out of solution and the high salinity of the formation water were expected to result in Halite scaling in the production string. So the Halite deposition is an important issue to consider when developing a reservoir with high salinity formation water.

This project focuses on the process and the effects of Halite scaling in gas reservoirs. The research aims at identifying some of the processes involved in Halite deposition. The most relevant work that was found in the literature is the MEng project by Almehroqi (1998) from the Petroleum Engineering department and the file notes of J.K.Pucknell (1985) from the same department. Almehroqi (1998) gives an insight to the processes involved in the Halite deposition in gas reservoirs, but without taking into account the energy balance equations. J.K.Pucknell (1985) gives a more general overview of the different effects involved in any scale deposition. Other authors like Kuo & Closmann (1966) investigated the sulphur precipitation in gas wells and Dempsey (1979) MEng from the Petroleum Engineering department investigated the effect of pressure on the scale deposition and its effect on production.

The gas reservoirs with high salinity water formation like in the southern North Sea and Germany can be affected by the Halite deposition and see their productivity reduced due to a reduction in the permeability. Try to predict this phenomenon can allow us to simulate the best production plan that will reduce the Halite deposition and the cost of inhibition.

In the present work, the objective is to list all the equations related with the prediction of the Halite deposition, the permeability reduction and the effect in production. Thereafter write a computer program that will use these equations for a case of five components hydrocarbon gas with one electrolyte formation water (NaCl).

The first part of the project deals with the different effects of pressure and temperature on the both water and salt solubility in gas and liquid phase respectively. The equations and methods given in this section allow the estimation of the gas water content at any pressure and temperature and also the salt solubility in water. A major part is dedicated to the energy balance where the heat gain from the formation is considered in the final equation. The last part consists in developing a computer program for a radial homogeneous model. The different methods for the calculation of the gas mixture properties are listed in the appendices.

Some assumptions, though not necessary, have been taken in order to simplify the program. These simplifications are:

• Only one electrolyte component was considered,
• The energy balance equation was simplified, by assimilating the formation to an infinite solid with a constant initial temperature.
• The presence of water was not taken into account when calculating gas properties such as viscosity, specific heat and compressibility.
• The presence CO2, N2 and other souring components were not dealt with.
• The rates of precipitation and of crystal growth were not included, since they need to be based on experimental data. The assumption was that the supersaturation was completely deposited as scale over the time increment (one-week).
• The gas solubility and its effect on both water solubility in gas and salt solubility in water were not investigated.

The present work, form the basis for a more general program of increased accuracy (and complexity).