Geological Considerations and Types of Wells designed in Oilfields
Site selection for drilling a well involves considerable geological study and commences long before a logging program is even considered .
Exploration wells ,confirmation wells ,development wells , injection wells , water supply wells , disposal wells , and exploratory holes for underground mining all require unique geological considerations .Decision-tree analysis and associated risk factors play a significant role in selecting frontier ventures ,offshore concessions,leases, partnership ventures , governmental exploration requirements , etc .Land locations can also be diverse , depending on locale . Both environnement and political climate are factors , as is ownership of surface and sub-surface rights .An oil company manager is responsible for the ultimate technical and economical decision .
Exploration Wells :
An exploration well (Wildcat well ) is drilled into a newly described structure or stratigraphic trap , including deeper targets in well-developed fields .This type of the wells are high risk ventures . They drilled on speculation that oil or gas will be found in certain location.
Costs to drill either well would differ considerably .Seismic data 3D are typically acquired and interpreted prior to drilling a well that is of high economic risk .
Appraisal or confirmation wells :
Once a discovery is made ,the petroleum companies usually attempt to define the reservoir limits by drilling wells near the projected limits of the reservoir as determined from geological and geophysical studies .Data acquired from the discovery well and any dry exploratory to re-calibrate previous geological and geophysical work and improve the risks of appraisal wells .Property or concession rights are also a prerequisite to determining offset wells locations .
Development Wells:
Infill-development drilling is meant to provide drainage of the entire reservoir , extracting the hydrocarbons and bring them to the surface where they are gathered in a collection system and transported elsewhere for refining and marketing .Government agencies often require a minimum spacing between wells in particular reservoirs (Drainage Radius) .
Additional surface seismic data are requested to help resolve the lateral geological peculiarities that led to unexpected . However , in many cases , dipmeter , borehole imaging , and or borehole seismic data resolve the problem effectively .
Injection Wells :
Are often drilled during the development and life of a producing oil reservoir in fields that have a gas drive mechanism forcing the oil into producing well bores , pressure depletion can become a severe problem early in the life of the field . Reservoir expert make effective use of the produced gas by reinjecting for pressure maintenance.
In reservoirs with a strong water drive , a waterflood operation is often implemented as oil production declines .The purpose is to force much of the remaining oil into the producing .
Relief Wells:
It is within a certain radius of the problem well.
Waste -Disposal Wells :
Another type of injection well is one used for disposing of unwanted materials :
- In many oil fields , it may simply be disposal of produced salt water.
- Chemical companies often dispose of their waste in this manner .Nuclear waste can be legally disposed of in this way in some countries .
Wells used for Underground Storage of Gas:
Depleted hydrocarbon-producing reservoirs are utilized for storage in some areas .It is much safer to store the gas under ground , and usually more economical in the long term .
Dry gas is injected into porous , permeable reservoirs rock . The injection cycle is typically from about April until late October .
Water Supply Wells:
Many wells are used to supply water . A shallow water supply well is often drilled to provide:
- Water for use in drilling a deeper wildcat well .
- Potable water needed for personnel .
Wells used for Mining Exploration :
Underground mining for coal , uranium , fluorite, and other rare minerals often uses boreholes to confirm the limits and direction of their underground .
Wells drilled for Geothermal Energy :
Well log data are used to evaluate many aspects of the geothermal reservoir .Unusually high temperatures are encountered in geothermal wells , and hostile logging equipment is almost always required that can perform above 400 ° F (204 °C), and many situations require tools rated for 500° F(260°C) or more .