Exploration drilling

Exploration drilling

Once a promising geological structure has been identified, the only way to confirm the presence of hydrocarbons and the thickness and internal pressure of a reservoir is to drill exploratory boreholes. All wells that are drilled to discover hydrocarbons are called 'exploration' wells, commonly known by drillers as 'wildcats'. The location of a drill site depends on the characteristics of the underlying geological formations. It is generally possible to balance environmental protection criteria with logistical needs, and the need for efficient drilling.

For land-based operations a pad is constructed at the chosen site to accommodate drilling equipment and support services. A pad for a single exploration well occupies between 4000—15 000 m2. The type of pad construction depends on terrain, soil conditions and seasonal constraints. Operations over water can be conducted using a variety of self-contained mobile offshore drilling units (MODUs), the choice of which depends on the depth of water, seabed conditions and prevailing meteorological conditions, particularly wind speed, wave height and current speed. Mobile rigs commonly used offshore include jack-ups, semi submersibles and drill ships, whilst in shallow protected waters barges may be used.

Land-based drilling rigs and support equipment are normally split into modules to make them easier to move. Drilling rigs may be moved by land, air or water depending on access, site location and module size and weight. Once on site, the rig and a self-contained support camp are then assembled. Typical drilling rig modules include a derrick, drilling mud handling equipment, power generators, cementing equipment and tanks for fuel and water (see Figure 1). The support camp is selfcontained and generally provides workforce accommodation, canteen facilities, communications, vehicle maintenance and parking areas, a helipad for remote sites, fuel handling and storage areas, and provision for the collection, treatment and disposal of wastes. The camp should occupy a small area (typically 1000 m2), and be located away from the immediate area of the drilling rig upstream from the prevailing wind direction.

Once drilling commences, drilling fluid or mud is continuously circulated down the drill pipe and back to the surface equipment. Its purpose is to balance underground hydrostatic pressure, cool the bit and flush out rock cuttings. The risk of an uncontrolled flow from the reservoir to the surface is greatly reduced by using blowout preventers a series of hydraulically actuated steel rams that can close quickly around the drill string or casing to seal off a well. Steel casing is run into completed sections of the borehole and cemented into place. The casing provides structural support to maintain the integrity of the borehole and isolates underground formations. Drilling operations are generally conducted around-the-clock. The time taken to drill a bore hole depends on the depth of the hydrocarbon bearing formation and the geological conditions, but it is commonly of the order of one or two months. Where a hydrocarbon formation is found, initial well tests- possibly lasting another month- are conducted to establish flow rates and formation pressure. 

These tests may generate oil, gas and formation water—each of which needs to be disposed of. After drilling and initial testing, the rig is usually dismantled and moved to the next site. If the exploratory drilling has discovered commercial quantities of hydrocarbons, a wellhead valve assembly may be installed. If the well does not contain commercial quantities of hydrocarbon, the site is decommissioned to a safe and stable condition and restored to its original state or an agreed after use. Open rock formations are sealed with cement plugs to prevent upward migration of wellbore fluids. The casing wellhead and the top joint of the casings are cut below the ground level and capped with a cement plug.