Many non-chemical treatment devices claim to control or reduce scaling by produced fluids. Obtaining controlled test data in the field is very difficult. Some controlled laboratory testing procedures for calcium carbonate and calcium sulphate dihydrate (gypsum) scaling are presented, together with sample results for treatment of supersaturated brines passed through a 13,500 Gauss field from an electromagnet, and a pair of dummy and magnetic tools for downhole application. Neither treatment had a significant effect on the amount of scale deposited, the morphology of the scale, and the turbidity of the mixed brines, over a wide range of supersaturations in the carbonate scaling system chosen. In a more limited study of the calcium sulphate system, significant differences were found between treatments with the magnetic and non-magnetic tools. However, in one experiment the magnetic tool increased the amount of deposit, while in a second identical one it decreased it. It is suggested that this inconsistency may be linked to random scale nucleation processes rather than any effect of an applied magnetic field.
Production of oil and gas is often accompanied by increasing amounts of water, either connate water alone, or mixed with water that has been injected into the reservoir to maintain pressure. Depressurisation of the produced fluid, or reaction between incompatible waters in the wellbore or near-wellbore regions can lead rapidly to the formation of CaCO 3 or (Ba,Sr)SO 4 scales that reduce production rates, and in extreme cases can lead to abandonment of wells as a result of scaling up of production tubing. Antiscaling chemicals are therefore introduced into the formation by a squeeze process to provide a reservoir of chemical that desorbs or dissolves to provide the necessary protection. The costs of this intervention, the uncertainties and delays associated with the performance and monitoring of the treatment, and the possibility of unforeseen changes in the scaling potential of the produced water, mean that purely physical methods of scale control, such as by a magnetic field, provided that their performance can be guaranteed, are very attractive. Such methods are even more attractive for subsea completions and in marginal fields where the cost of provision of conventional scale treatment facilities may be unjustifiable.
Although there are many reported instances of successful application of physical methods of scale control, there are many others where the technique has been reported to be unsuccessful. Reported successful applications usually refer to industrial situations in which the level of control and monitoring of process conditions is poor, meaning that no proper controlled comparison of behaviour with and without the application of the technique can be made.
Because of the potential advantages of using magnets to reduce or eliminate scale deposition, the initial objective of the programme of work was to provide definitive experimental evidence for the success or otherwise of the technique. If successful, investigations would be continued to establish the mechanism with a view to optimising the treatment parameters for at least one potential application.
The calcium bicarbonate-calcium carbonate system was chosen because carbonate deposits are frequently encountered during oil production, and the majority of literature references on the use of magnetic fields to control scaling refer to this system.
Outline of previous work
The literature on scale control by magnetic fields is extensive, as demonstrated in a recent review paper f~). Many of the systems investigated refer to conventional cooling water, boiler water or
