|BRITAIN'S OFFSHORE OIL & GAS
How Much Oil and Gas?
When deciding whether to develop a field, a company must estimate how much oil and gas will be recovered and how easily they will be produced. Although the volume of oil and gas in place can be estimated from the volume of the reservoir, its porosity, and the amount of oil or gas in the pore spaces, only a proportion of this amount will be recovered. This proportion is the recovery factor, and is determined by various factors such as reservoir dimensions, pressure, the nature of the hydrocarbon, and the development plan.
Pressure is the driving force in oil and gas production. Reservoir drive is powered by the difference in pressures within the reservoir and the well (F82), which can be thought of as a column of low surface pressure let into the highly pressured reservoir. If permeability is good and the reservoir fluids flow easily, oil, gas and water will be driven by natural depletion into the well and up to the surface. Expansion of the gas cap and water drives oil towards the well bore. Gas and water occupy the space vacated by the oil. In reservoirs with insufficient natural drive energy, water or gas is injected to maintain the reservoir pressure.
The proportion of oil that can be recovered from a reservoir is dependent on the ease with which oil in the pore spaces can be replaced by other fluids like water or gas. Tests on reservoir rock in the laboratory indicate the fraction of the original oil in place that can be recovered. Viscous oil is difficult to displace by less viscous fluids such as water or gas as the displacing fluids tend to channel their way towards the wells, leaving a lot of oil in the reservoir. The quoted recovery factor for most North Sea fields is about 35 percent, but may be as low as 9 percent where the oil is very viscous, or perhaps as high as 70 percent where reservoir properties are exceptionally good and the oil of low viscosity. The recovery factor in gasfields is much higher, figures of over 85 percent being quoted for most.
Each oil and gas reservoir is a unique system of rocks and fluids that must be understood before production is planned. Petroleum engineers use all the available data to develop a mathematical model of the reservoir. Computer simulations of different production techniques are tried on this reservoir engineering model to predict reservoir behaviour during production, and select the most effective method of recovery. For example, if too few production wells are drilled water may 'cusp' or channel towards the wells, leaving large areas of the reservoir upswept.
Factors, such as construction requirements, cost inflation and future oil prices must also be considered when deciding whether to develop an oil or gas field. When a company is satisfied with the plans for development and production, they must be approved by the Government, which monitors all aspects of offshore development.