Upon successful completion of geophysical surveys, petroleum or Natural Gas Company drills a deep hole, in the earth notably known as Exploratory wells, in the hopes of locating a new source of fossil fuel. These types of wells are drilled to confirm the availability of oil and gas in place. An exploratory well represents a risk for the company drilling it, because it is not known, before investing in the well, how much oil or natural gas it might contain.
The expenditure associated with drilling wells is usually valued at millions of dollars and every two out of three wells, on average, contain no traces of hydrocarbons. It therefore require companies to drill many wells in one area before they are able to find an oil or gas discovery, and this may take decades or more years. Some explorers sometimes find nothing at all after the exploration exercise.
Generally exploratory wells are drilled in areas that have not been previously shown to contain oil or natural gas. They may also be drilled in areas near existing wells to find another reservoir of oil or natural gas. Exploration wells are mainly used for data collection from potential reservoirs by making use of formation evaluation methods which might be coring, logging and well testing.
Wells are completed for production if the value of the recoverable hydrocarbons is greater than the cost of drilling, producing, and delivery to market. If not, the exploratory well would be plugged, all drilling equipment and materials would be removed from the drill site, and the site would be restored as near as possible to its original condition. If enough hydrocarbons are present to possibly warrant commercial production, additional exploratory wells would be drilled to test the production conditions and further delineate the boundaries of the reservoir.
Exploratory drilling involves:
- Using rotary equipment and hardened drill bits, weighted and lubricated by drilling fluids, to penetrate the earth’s surface;
- Running electric logs and collecting core samples to characterize selected subsurface formations;
- Inserting casing and tubing into each well to protect the subsurface and control the flow of fluids (oil, gas, and water) from the reservoir;
- Perforating the well casing at the depth of the prospective formation to allow flow of fluids from the formation into the wellbore; and
- Installing a wellhead at the surface to regulate and monitor fluid flow and prevent potentially dangerous blowouts.
As the wells are being drilled, the engineers collect vital information and samples (known as well log) about the rocks and fluids (water, gas and oil) while drilling Well log is a continuous record of measurement made in bore hole respond to variation in some physical properties of rocks through which the bore hole is drilled.
Well logging is done for determining depth to lithological boundaries, lithology identification, minerals grade / quality, inter borehole correlation, structure mapping, dip determination, rock strength, in situ stress orientation, fracture frequency, porosity and fluid salinity. In general well logs are done during exploration, production and well inspection. The typical well logs at various phases are as follows:-
- Geophysical Logs include Resistivity, Porosity, Gamma Ray, Dip Meter, Borehole Imaging
- Production Logs include Pressure, Temperature, Spinner, Fluid Density
- Well Inspection Logs include Sonic, Caliper, Electro Magnetic, Ultrasonic, RA tracer, Video
The well logging units logging unit typically contains logging cable, winch to raise and lower the cable in the well, self contained generator (typically 120 V AC), set of surface control panels, set of down hole tools (sondes and cartridges) and digital recording system.
Upon completion of well logging, the Well Log interpretation is done typically for two purposes Quantitative and Geological & Geophysical.
Quantitative analysis : Porosity, water saturation, fluid type (oil/gas/water), lithology, permeability, hydrocarbons in place, reserves, mapping reservoir parameters,
Geological & Geophysical – well to well correlation, facies analysis, regional structural and sedimentary history, In quantitative log analysis to define the type of reservoir (lithology), its storage capacity, its hydrocarbon type and content (Saturation), its productivity (permeability)
The following video provides an overview of Well Logging tools and Techniques
https://www.youtube.com/watch?v=JmTxpMQ7LhA
Video courtesy: IHRDC, USA
For more detailed information on petrophysics and well logging visit Crain’s Petrophysical Handbook Website
Appraisal Wells
In the event of oil discovery in the exploration well, an appraisal process has to be carried out and all the alternatives must be considered, which includes the selection of development plan, implementation of the plan along with the construction of wells, infrastructure and other facilities.
The design, planning process and construction of appraisal wells are identical to exploration wells. However, appraisal wells are drilled only when a discovery is made, with the motive of assessing the size and viabilities of the reservoirs. They can also be drilled to obtain more information about the geology or geometry of a well present. Once the sought after information about an appraisal well is met, the drilling can be called successful.
During appraisal, delineation wells are drilled to define the extent of reserves. Appraisal may take several years to complete and is very costly. In certain countries, the oil and gas company may be required to enter into a production sharing contract, which is usually done later in this phase, if a company decides to develop the reserves, based on the exploration and appraisal results.
Once appraisal drilling is carried out, the data obtained is further analyzed to determine whether the reservoir is economically viable to produce, and if so, the geologists and reservoir engineers build models of the reservoir, to further determine reserve volumes which could be accessed through development drilling.
