14.3.5 DIRECTIONAL DRILLING FOR OIL
Wells do not have to be drilled as a straight perpendicular bore. By directional drilling they can be drilled in any direction with angles up to ninety degrees. This is enabled by Motor Drilling whereby the bit is powered by a down-hole motor independently of the drill string which therefore does not have to rotate; and by the use of short-bend tubes with which the angle can be built up. Thus they can be extended from the perpendicular in any direction by up to 10km.
Directional drilling from multiple wells on a single site is now undertaken as a matter of routine in order to minimise the surface 'footprint' of the development phase of an oil field.
14.3.6 OIL PRODUCTION
Where commercial quantities of oil are struck, the exploration well becomes a production well. Here the Production String, being a steel tube of about 12cm diameter, is let down into the well, as shown in figure 7. The end of the tube is perforated to allow any gas, the oil and the water to flow into it. The productive zone of the well is sealed off from the rest of the well by a rubber Packer.
The well head is attached to a Flow Line, which allows the oil and any water and gas (associated with the oil) to flow to a Flow Station. The flow may be assisted on its way by either submersible pumps in the well bore and/or by pump stations along the flowline route.
The flow station receives a mixture of oil, gas and water, so that processing involves three basic steps. These are, in the processing order:
- Gas Processing - whereby the gas (methane, ethane, butane and/or propane) is either flared off, processed (see section 14.6 below) or re-injected into the sandstone sponge.
- Oil Processing - whereby the Crude Oil is separated from the water and the gas, desalted, stabilised and then pumped along the Pipeline to the Oil Terminal.
- Water Processing - whereby the water is re-injected into the sandstone or passed into the local water system.
Re-injection of gas and/or water into the sandstone sponge may be necessary to maintain or enhance the natural pressure in the reservoir, thereby recovering the maximum recovery of hydrocarbons.
What are the Financial and Engineering Problems Associated with Re-Injecting Associated Gas? (Answer given by a senior oil industry geologist.)
In order to separate gas from the liquids produced, the fluids are feed into one or more production separators where the pressure is reduced in stages to allow the gas to evolve. The fluid stages may be near atmospheric. To utilise the produced associated gas, it must be re-compressed for distribution via pipelines as sales gas and/or returned to the reservoir for re-injection, at higher pressure than the original pressure. Compression for re-injection is an expensive option therefore, and not be commercially viable.
Equally important is the shape and distribution of the reservoir. A simple dome with a thick reservoir containing a natural gas cap is an obvious and attractive candidate for gas re-injection. A complex trap containing thinly interbedded reservoir zones with multiple fault compartments is not a likely candidate, as gas re-injection would, most likely, destroy oil production.
