Content – Oil & Gas
Oil and gas exploration and production is divided into two main categories; onshore and offshore fields.
The main differences are the environment the production is carried out under, some of the equipment used and the logistics necessary.
Drilling and production of offshore oil and gas fields in most cases is relatively more complex than for drilling and producing onshore fields. However the principles of finding and draining the reservoirs are the same.
Oil and gas exploration and production may be divided into a few main processes:
- Seismic surveys and collection of geological data
- Survey of available geological data
- Collection of additional geological data.
- Exploration and appraisal drilling
- Field development
- Field development planning
- PDO (Plan for Development and Operations) establishment and submission
- Design, procurement, fabrication, installation and testing of production equipment, facilities and organisation.
- Production drilling and well completion
- Storage and Transportation
Petroleum geologists and geophysicists search for sub surface (beneath the Earth`s surface) hydrocarbon deposits initially by assessing data from seismic surveys.
Exploration is normally an expensive and high-risk activity since both seismic surveys and exploration drilling are complex activities that require costly resource. Furhtermore many surveys and drilling of multiple wells might be required until oil and gas is discovered if at all.
Seismic surveys – Sound waves are used to determine the sub surface rock layers.
Electromagnetic surveys – Electromagnetic waves are used to to explore the sub surface rock layers
Wildcat drilling – Wells drilled on a very loose information/knowledge (satelite views etc)
Exploration drilling – A hole/well drilled to find a new reservoir in a field where oil and/or gas is expected. Rock samples are collected together with other data to determine if the criteria for finding oil and gas are present.
Reservoir modelleing and simulation – Computer models made to enable estimation of available volumes of oil and gas and to investigate possible reservoir fluid flow, location of wells and possible production volumes
Appraisal drilling – Wells drilled after the discovery of oil or gas to establish the limits of the reservoir, the productivity of wells in it and the properties of the oil or gas.
Criteria to be investigated/explored
- Determine if there is a geological feature that might act as an accumulator to hold the petroleum (Trap)
- Determine if the accumulator is sealed such that the petroleum cannot escape.
- Determine if there is migration path that will allow the petroleum to flow towards the accumulator (Trap)
- Determine if the area investigated contain source material of petroleum.Only when a structure is rated as prospective in terms of trap, seal, migration and source, it is drilled. A chance of success of 20% to 35 % is often considered risk worthy.After a well is drilled, it is logged, taking parametric measurements using electric, nuclear and sonic methods.
- Porosity, permeability, gas/ water saturation, pressure, salinity are normally calculated. Indicative zones are then perforated to test the flow of gas/oil.
Seismic surveys (seismology)
Sound waves are sent towards the surface of the Earth. When offshore through the water towards the seabed, and then through the Earth.
Since the acoustic impedance (speed of sound) vary with the density of the material it flows through, some of the sound waves will be reflected at the interface between materials (rock types). The sound wave reflection is captured by sensors (geophones or hydrophones), analysed and used to map and determine the various materials (source rocks) in the underground. Based on data from these surveys and other geological knownledge of an area, data models and geological maps are generated.
2D Seismic: Data is collected over a simple pattern (single line), providing a good general understanding of the area surveyed.
3D Seismic: Data is collected over a dense array of geophones or hydrophones, providing a detailed set of data for computer processing.
There are various options for seismic source to be used for land surveys. The most common methods are dynamite or Vibroseis.
Dynamite or Primacore is the simpler source, but for obvious reasons it is limited to open areas. Dynamite is the only practical energy source in swampy areas.
Dynamite is buried and then set off and the explosion generates the underground sound waves and the reflections are picked up by geophones.
Vibroseis, requiring a relatively flat surface, uses large trucks vibrating a steel plate to the ground as the source of the seismic waves. Several trucks are normally used to create enough energy. Like with the dynamite method, the reflecting sound waves are picked up by geophones.
The geophones are contained in cables that are layed out on the ground.
Offshore seismic surveys are conducted by using vessels towing one or more streamers (cables) containing a series of hydrophones. Performing 2D surveys using only 1 streamer and 3D surveys employing 6 – 8 streamers (could be as many as 12 or more).
The streamers are towed just below the water surface of the water.
The seismic source (air guns) is deployed just below the water surface between the vessel and the first hydrophone and would have a length up to 8 km.
Offshore surveying also include more permanent type of systems where a surface vessel provides the seismic source and where the cables of geophones or hydrophoenes is layed on the seabed.
Reapeated surveyeing utilizing “permanent” equipment is often called 4D suveing due to the repetition and the ability to record changes in the subsurface structure.
Electromagnetic surveys (EMS)
Controlled Source Electro Magnetic (CSEM)
Electromagnitic surveying in connction with oil and gas exploration would normally be complimentary to seismic surveying. EMS may in some cases be used to differentiate between fluids contained in a geological reservoir something that is not possible with seismic data.
While seismic surveying detect contrast in acoustic impedance, electromagnetic surveying do detect contrast in electrical conductivity.
The source creates a localized EM field and induced current lines are deflected by the different geological structures they encounter. Sand or gravel affect current flow lines differently than solid rock, water affects current flow lines differently than oil and gas.
The physics of seismic and EM is quite different since the seismic signal propegates like a wave and comes back with a clear echo, while the EM signal spreads out diffusively, is smoothened and takes a time that varies quadratically with distance before bringing the information back.
The induced currents will modify both the electric and magnetic fields, which are recorded by the seafloor receivers.
EMS do not give a direct image as is the case with seismic surveys, it do rater rather provide a huge amount of data which neeeds to be analysed with specially developed computer programs to devlop conductivity profiles.
Airborne magnetic surveying has been used extensively in oil exploration, for mapping bedrock structure and depth. Capable of detecting subtle faulting and folding deep within the Earth, aeromagnetic surveys are fast, low-cost methods of covering large areas. Salt dome mapping is another application where the susceptibility contrast between the sediments and the salt is measurable and salt domes and ridges can be mapped. Airborne gravity surveys are a more recent addition to the exploration toolbox, and can map structures represented by density variations. The regional coverage of aeromagnetic and airborne gravity surveys can be used to characterize entire basins, linking together more sparsely distributed seismic data.
Electromagnetic survey equipment
There are a great variety of technology and solutions used, however simplified there are two main solutions; Horizontal systems and vertical systems
The most common method is by use of a horizontal current transmitter, towed a few meters above the sea bed and horizontal receivers positioned at the seabed (“ Sea bed logging”).
This method uses a continuous periodic signal.
This method use vertical electric lines both as transmitters as well as receivers (Transient ElectroMagnetic Prospecting using Vertical Electric Lines).