EnergyBC

Construction and Drilling

Construction and Drilling - (For details see UKOOA Offshore Site and SFU 2004)

New technologies have significantly reduced the cost of drilling offshore wells, however the price tag is still $30 to $100 million dollars or more. That is many times the cost of drilling a well onshore. There are two basic types of drilling rigs - fixed platform rigs and mobile rigs. Fixed platform rigs are installed on large offshore platforms and remain in place for many years. Mobile rigs comprise two types: jack-up rigs used in shallow water less than 100 meters deep and semi-submersible rigs used in deeper waters down to 1000 meters or more. In very deep waters, specialized drilling ships are used. Jack-up rigs have lattice legs which are lowered to the sea floor before the floating section carrying the derrick is raised above the sea surface. Semi-submersible rigs float at all times, but when in position for drilling are anchored and ballasted to float lower in the water with their pontoons below wave-level. Some semi-submersible rigs and most drill ships have 'dynamic-positioning' propellers that allow them to drill in very deep water. Drill ships were used to drill wells off Nova Scotia in 2002 and 2003 at water depths of more than 1,600 meters.

In offshore drilling, the first step is to put down a wide-diameter conductor pipe into the seabed to guide the drilling and contain the drilling fluid. Drilling fluid (also called "mud"), is mainly water-based and gets pumped continuously down the drillstring while drilling is taking place. It lubricates the drilling tools, washes up rock cuttings and most importantly, balances the pressure of fluids in the rock formations below to prevent blowouts. On semi-submersible rigs a conductor pipe is drilled into the seabed, but on production platforms a pile-driver may be used. As drilling continues, completed sections of the well are cased with steel pipe cemented into place. A blowout preventer is attached to the top of the casing. This is a stack of hydraulic rams which can close off the well instantly if back pressure (a kick) develops from invading oil, gas or water.

Drilling grinds up the rock into cuttings which are brought to the surface by the drilling mud. The drilling mud is passed over a shale shaker which sieves out the cuttings (see photo to the left). In exploration drilling, the cuttings are taken for examination by a geologist known as a "mudlogger" who is watching for oil and gas. Oil entrapped in the mud is detected by its fluorescence in ultraviolet light. Gas is extracted from the mud in a gas trap and sent under vacuum to a gas detector and analyzer. An increase in the amount triggers an alarm to alert the mudlogger and the drilling superintendent.

If oil and gas resources discovered during the exploration stage appear to be economically viable, a developer may decide to exploit the resource. Development entails constructing marine and onshore infrastructure (such as drilling rigs and submarine pipelines), and also preparing on- and offshore sites for activity. Pile driving, dredging, construction and pipeline emplacement may be required, and further surveying and well-drilling may be conducted. The number of wells drilled depends on the size of the reserve and the drilling strategy of the developer. Gas is typically transported from offshore sites via subsea pipelines, while oil and gas condensates are typically transported via tanker. In some cases, liquid natural gas may be transported via tanker. Onshore, development may involve construction of supply bases, roads, helicopter landing pads, pipelines, processing plants, employee housing and other infrastructure. Subsidiary facilities may include petrochemical plants and refineries.

Logging and Coring

Another primary objective in exploratory drilling is to take long core samples of each of the rock layers in and around prospective targets. These samples are then analyzed to accurately determine the production potential of a particular well. Special logging devices are also lowered into the well to record data on rock thickness, density, porosity (spaces between rock grains), permeability, pressure and any fluids that might be present (oil, water, gas). If tests indicate that the conditions are favourable and the well is capable of producing commercially viable quantities of hydrocarbons, then it is usually capped for production once the proper facilities are in place. If it is not capable of commercial production, it is capped closed and called a dry hole.

Example of a well log from a drill core.

Some credit from above text to Review of Offshore Oil and Gas Development by Simon Fraser University, 2004 and UKOOA - The UK's Offshore Oil and Gas Web Site, 2004