This is a facility that separates the well stream into the three phases, be it oil, water and gas. The purpose is to produce saleable products, or to produce a product that can be further refined or processed or even disposed of.
Typically each of the individual phases go on to be processed in an unique way depending on the chemical characteristics of the phase or depending on the limitations of the facility, be it on or offshore, space restrictions or environmental regulations.
Separation Process Overview
This process is key to the economics of the field. It allows for the valuable fluids from the well stream to be separated, purified, quantified and sold for financial return. If the streams are of no value either by its very nature, or because of in-sufficient quantities they will be further treated and brought to environmental standards and discharged to the environment. This would most likely apply to the water and gas streams.
The basic fluid flow for the facility is illustrated in the block diagram below. It starts with fluids from the wells being sent to the separator. At this stage the fluid is separated into the three possible fluids, oil, gas and water. The fluid stream of importance is the oil, this is treated and then sold. The water collected from the initial separation and then from the treatment of the oil is to be treated and then dis-charged to the sea. The gas is vented as it is not expected to be in economical values.
Because of the nature of produced fluid the higher the pressure of the initial separator the greater the chance of more liquid being obtained in the separator. However too high too may light components will stay in the liquid phase at the separator and be lost to the gas at tank. If the pressure is too low then not as many of these components will be stabilized into the liquid at the separator and will be lost to the gas phase.
Given the fact that this is small facility the pressure of the lowest stage could be set within the range of 25-50psig. This will allow for the oil and water to be dumped to their respective treatment systems. However if higher pressures are used then the amount of energy needed to send the oil to the onshore facilities would be less. A compromise between these two requirements would be to set the operating pressure between 50-100psig.
In all instances the water will have to be treated for impurities to an environmentally acceptable standard before being released to the sea. For this case the oil concentration limit for the disposed water will be 30mg/l in keeping with international standards.