The chemical treatment of offshore oil and gas production
Authors: Holloway F.H. 
Published: December 2007

The realisation that the use of chemical can alleviate and in some cases totally remove, problems associated with oil and gas production has been recognised for many years. The classical approach to the design and application of chemicals works well in land based systems where there is ease of accessability and long residence times are available but in offshore situations, and particularly with the 'second generation' type of operation carried out in the North Sea, these factors play an important part in the successful development and application of chemicals,

Whilst wishing to concentrate on the status quo in the North Sea, it must not be forgotten that these fields are still young. A description of the likely trend in chemical requirements will be given, as exemplified by problems and solutions in other offshore locations.

THE RESOLUTION OF PRODUCED WATER IN OIL EMULSION

In a typical land based oilfield environment, crude being produced from individual wells is pipelined, may be many miles, into a central collecting station, where it is degassed, dehydrated and in some cases desalted into stock tank oil. It is usual for the residence time through the system from production manifold to pipeline to be large, may be several days. The resolution of emulsions and subsequent separation of the produced water takes place in three phase separators, of the gravity, heater place in three phase separators, of the gravity, heater or electrostatic type, followed by wash tanks, settling tanks and stock tanks before being pipelined.

North Sea production platform designs are, through necessity, totally different. Basically the residence time through the production train is very short, usually of the order of minutes, with wellheads located on the platform itself. In fields where the crude is pipelined, two basic systems operate or are planned. Either two phase separators (gas/oil) are used with no separation of water, such that relatively large quantities of water, in some cases as much as 20 per cent., are planned to be pipelined ashore; or three separators will be utilized (gas/oil/water) with subsequent dehydration of crude on the platform, resulting in low water content crude being pipelined.

On the marginal fields, or in fields where pipelines are not yet completed, crude is pumped via a short rigid and flexible line connected to a single buoy mooring (SBM) into a tanker. Because of the economics involved in shipping large quantities of water, it becomes very necessary to dehydrate the crude before loading, and as a consequence these platforms in the main make use of three phase separators.

The major obstacle to overcome in the resolution and separation of the produced water offshore is the very short time available to accomplish this task. It is the case at the present time that this separation cannot be achieved without the use of chemical demulsifiers.

Demulsifiers are surface-active agents which have properties which make them effective in disrupting the properties which make them effective in disrupting the effect of the natural emulsifiers present in the oil. Their initial action is at the water-oil interface. The 'skin' surrounding the tiny water droplets in the emulsion, prevents the water droplets from uniting and result in a prevents the water droplets from uniting and result in a stable emulsion. Once the demulsifier is at the water-oil interface, it is able to carry out its primary action of flocculation, A good demulsifier, concentrated at the surface of a water droplet, has a strong attraction for other droplets in the same condition. By this mechanism, large 'bunches' of drops are joined together and the oil takes on a very bright appearance since the small water droplets are no longer dispersed throughout the oil to diffuse the light.

The characteristics of the demulsifier to produce the joining of droplets does not disrupt the continuity of the emulsifier film but just adds to it. If the emulsifier has certain weaknesses, this flocculation force may be sufficient to cause complete resolution of the emulsion. However, in most cases further action is necessary for the water droplets to unite and become large enough and free enough to settle out. The action of uniting water drops is called coalescence.

A good demulsifier must not only be able to flocculate the water particles, but also it must be able to disrupt the films surrounding them and allow them to unite.

Since the particles are all close together because of flocculation, this coalescing process results in a very rapid growth of water drop size and rapid water separation.

In most crude oils, solids such as iron sulfide, silt, clay, drilling mud, paraffin, etc., complicate the demulsification process. They tend to collect at the interface and contribute significantly to emulsion stability. Often such solids are the primary stabilizing material and their removal is all that is necessary to achieve satisfactory treatment.

For removal from the interface, these solids can be dispersed in the oil or they can be water-wetted and removed with the water. If dispersed in the oil, the emulsion may be treated but the solids will still remain as a contaminate in the oil.