The magnetic method depends on measuring the variations in intensity of the magnetic field which reflects the magnetic character of the various rocks present, while the gravimetric method involves the measurement of small variations in the gravitational field at the surface of the earth. Measurements are made, on land and at sea, using an aircraft or a survey ship respectively.
Seismic surveys are the most common assessment methods used to pinpoint potential hydrocarbon reserves in geological formations.
Seismic technology uses the reflection of sound waves to identify subsurface geological structures. The surveys are conducted through the generation of seismic waves by a variety of sources ranging from explosives that are detonated in shot-holes drilled below the surface, to vibroseis machinery (a vibrating pad lowered to the ground from a vibroseis truck). Reflected seismic waves are measured with a series of sensors known as geophones laid out in series on the surface.
In modern marine seismic surveys, as many as 16 “streamers” (cables containing the hydrophones used to detect the sound reflected from the subsurface) are towed behind the seismic vessel, at a depth of 5 to 10 m. Each cable can be as long as 8 to 10 km. In addition to the hydrophone array, the vessel tows seismic source arrays comprising a number of air guns which discharge sound bursts of between 200–250 decibels (dB) downward. The sound bursts, repeated on an average of every 6 to 10 seconds, are reflected off deep geological formations and recorded by the hydrophone array.
Exploratory drilling activities onshore/offshore follow the analysis of seismic data to verify and quantify the amount and extent of oil and gas resources from potentially productive geological formations. Most of the exploratory boreholes are drilled to confirm the presence of hydrocarbons and the thickness and internal pressure of a reservoir. However, some are also drilled to gain knowledge of the geological formation.
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 formation.
For onshore operations, a well pad is constructed at the chosen location to accommodate a drilling rig, associated equipment and support services. A pad for a single exploration well occupies between 4000–15000 square meter (m2). The type of pad construction
depends on terrain, soil conditions and seasonal constraints. The drilling rig and support services are transported to site, typically in modules and assembled. Typical drilling rig modules, include a derrick, drilling mud handling equipment, power generators, cementing equipment and tanks for fuel and water.
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.
The following are the various types of mobile offshore drilling rigs:
- Jack-up rigs: Suitable for shallow water up to 100 m deep and transported to location by their own propulsion, or towed by tugs. Once there, electric or hydraulic jacks lower three or four legs to the seafloor to support the drilling platform above water.
- Semi-submersible rigs: Suitable for deep waters and transported to location by their own propulsion, or towed by tugs. The hull is partially submerged and the rig is held in place by a series of anchors.
- Submersible rigs: Limited to shallow waters and towed onto location. It consists of two hulls: an upper hull, or platform, and lower hull that is filled with water and submerged to the seafloor.
- Drilling barges as floating platform: Suitable for shallow waters, estuarine areas, lakes, marshes, swamps and rivers. Not suitable for open or deep water. Towed onto location.
- Drillships: Designed for drilling in deep water locations. Drilling takes place from a drilling platform and derrick positioned in the middle of the deck, from which drill stems are lowered through a hole in the hull (moonhole).
Once on location, a series of well sections of decreasing diameter are drilled from the rig. A drill bit, attached to the drill string suspended from the rig’s derrick, is rotated in the well. Drill collars are attached to add weight and drilling fluids are circulated through the drill string and pumped through the drill bit. Drilling operations are generally conducted round-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 off.
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 of the wellhead and the top joint of the casings are cut below the ground level and capped with a cement plug.
The fluid has a number of functions. It imparts hydraulic force that assists the drill bit for a cutting action and it cools the bit, removes cutting rock from the wellbore and protects the well against formation pressures. When each well section has been drilled, steel casing is run into the hole and cemented into place to prevent well collapse. When the reservoir is reached the well may be completed and tested by running a production liner and equipment to flow the hydrocarbons to the surface to establish reservoir properties in a test separator. Various drilling fluids are available, but they can generally be categorized into one or two fluid systems:
- Water-Based Drilling Fluids (WBDF): Fluids where the continuous phase and suspending medium for solids is seawater or a water miscible fluid. There are many WBDF variations, including gel, salt-polymer , salt-glycol and salt-silicate fluids;
- Non-Aqueous Drilling Fluids (NADF): The continuous phase and suspending medium for solids is a water immiscible fluid that is oil-based, enhanced mineral oilbased, or synthetic-based.
- Diesel-based fluids are also available, but the use of systems that contain diesel as the principal component of the liquid phase is not considered a good practice at present for offshore drilling programs and should be avoided.
Typically, the solid medium used in most drilling fluids is barite (barium sulfate) for weight, with bentonite clays as a thickener. Drilling fluids also contain a number of chemicals that are added depending on the downhole formation conditions.
Drilling fluids are either circulated downhole with direct loss to the seabed along with displaced cuttings, particularly while drilling well sections nearest to the surface of the seabed, or are recirculated to the offshore facility where they are routed to a control system that filters solids. In this control system, the drilling fluids are separated from the cuttings, so that they may be recirculated downhole leaving the cuttings behind for disposal. These cuttings contain a proportion of residual drilling fluid. The volume of cuttings produced will depend on the depth of the well and the diameter of the hole drilled.
The drilling fluid is either replaced when its rheological properties or density of the fluid can no longer be maintained or at the end of the drilling program. These spent fluids are then contained for reuse or disposal.
As the hole is drilled, casing is placed in the well to stabilize the hole and prevent caving. The casing also isolates water-bearing and hydrocarbon-bearing zones. In locations where surface soils may cave in during drilling, a “conductor” casing may be placed at the surface, extending only twenty to one hundred feet from the surface. This string is often placed before the drilling starts with a pile driver (Berger and Anderson, 1992). The next string, or “surface” casing, begins at the surface and may penetrate two thousand to three thousand feet deep. Its primary purpose is to protect the surrounding freshwater aquifer(s) from the incursion of oil or brine from greater depths.
The “intermediate” string begins at the surface and ends within a couple thousand feet of the bottom of the wellbore. This section prevents the hole from caving in and facilitates the movement of equipment used in the hole, e.g., drill strings and logging tools.
The final “production” string extends up to the full length of the wellbore and encases the downhole production equipment. Shallow wells may have only two casing strings, and deeper wells may have multiple intermediate casings. After each casing string has been installed, cement is forced out through the bottom of the casing up the annulus to hold it in place and surface casing is cemented to the surface.
Casing is cemented to prevent migration of fluids behind the casing and to prevent communication of higher pressure productive formations with lower pressure nonproductive formations. Additional features and equipment will be installed during the completion process for production. Perforations will allow reservoir fluid to enter the wellbore; tubing strings will carry the fluid to the surface; and packers (removable plugs) may be installed to isolate producing zones.
Casing is important for both drilling and production phases of operation, and must therefore be designed properly. It prevents natural gas, oil, and associated brine from leaking out into the surrounding freshwater aquifer(s), limits sediment from entering the wellbore, and facilitates the movement of equipment up and down the hole. Several considerations are involved in planning the casing.
First, the bottom of the wellbore must be large enough to accommodate any pumping equipment that will be needed either upon commencement of pumping, or in the later years of production. Also, unusually pressurized zones will require thicker casing in that immediate area. Any casing strings that must fit within this string must then be smaller, but must still accommodate the downhole equipment. Finally, the driller is encouraged to keep the size of the hole to a minimum; as size increases, so does cost and waste.
When exploratory drilling is successful, more wells are drilled to determine the size and the extent of the field. Wells drilled to quantify the hydrocarbon reserves found are called ‘outstep’ or ‘appraisal’ wells. The appraisal stage aims to evaluate the size and nature of the reservoir, to determine the number of confirming or appraisal wells required, and whether any further seismic work is necessary.
The technical procedures in appraisal drilling are same as those employed for exploration wells, and description provided above applies equally to appraisal operations. A number of wells may be drilled from a single site, which increases the time during which the site is occupied. Deviated or directional drilling at an angle from a site adjacent to the original discovery bore hole may be used to appraise other parts of the reservoir, in order to reduce the land used or ‘footprint’.