Geoscientists use a variety of techniques to discover traps. From about 1920 onwards, geophysical methods such as seismology, geomagnetics, gravimetry and geoelectrics claimed their place alongside geological methods. Reflection seismic holds a key role in oil and gas exploration, and accounts for over 90% of the money invested in geophysical measurements.
The science of exploring for oil and gas is called petroleum geology. It identifies oil and gas prospective areas on the basis of geological characteristics.
Some 43,000 square kilometers (km²) or just half of the Austrian territory (approx. 84,000 km²) are oil and gas prospective. These areas are situated in the Vienna Basin, the Alpine Foreland Basin and the Calcareous Alps. Central Europe's largest oil and gas field, Matzen, was discovered in the mid-20th century (1949) using geological surface mapping and shallow exploration wells. Mapping is used to generate a color-coded geological map complete with symbols to display all the necessary information on the type and stratification of different geological formations.
Geophysical methods are used to determine geological conditions beneath the earth's surface as basis for geological interpretation of potential hydrocarbon traps: Seismic surveys enable geologists to identify the structure of geological formations below the surface. Other geophysical measurement techniques can be used to determine the physical properties of rocks such as magnetism, density, seismic velocity and gravitational acceleration (gravimetry).
Experts use the available data to construct a subsurface geological model as accurate as possible. These geological structures provide indications on the presence of hydrocarbon bearing traps.
If geologists feel confident of the evidence of an oil or gas trap, preparations for drilling a well can be initiated.
The most widely used geophysical exploration method is reflection seismic. Explosive charges or special vibroseismic (seismic) vehicles initiate vibrations (or compressional seismic waves). The waves propagate through the subterranean rock at speeds related to the age of the rocks. The seismic waves are reflected at acoustic boundaries between rock layers, similarly to an echo where sound waves bounce back off a cliff face.
The reflected seismic waves are recorded by a piece of equipment called a geophone placed at optimum positions at the earth's surface.
The electrical signals from the geophone are digitized by the recording system. This involves sampling the signal at regular time intervals and converting the measured voltage into binary code. These seismic measurements are recorded on storage data (magnetic tape, cartridges or CDs) in digital form (similar to compact disc music) , and can later be processed by special computer systems.
seismic data processing
The next step involves using this raw data to generate as accurate and detailed a picture of the subsurface as possible. This is performed by seismic data processing methods. Complex digital filtering techniques, and a wide range of adjustment and imaging methods can be used to suppress noise and enhance the reflected signals. The results give specialists an insight into the acoustic properties of the earth. These acoustic properties relate closely to geology and give specialists a good understanding of the geological structure and layering up to a depth of several thousand meters.
There are two types of seismic reflection measurements: two dimensional (2-D) and three dimensional (3-D) seismic.
2-D seismic: The measurements are taken along a straight or crooked line. The result is a 2-D image of the upper layers of the earth's crust. It is only possible to generate a reasonably accurate representation of an oil or gas bearing structure by combining the data acquired from a whole network of other 2-D seismic lines. This method is mainly used for reconnaissance surveys of large areas.
3-D seismic: Three-dimensional seismic has been in use since the late 1960s. This exploration technique uses wide arrays of geophone and exciter positions (receiver and vibrator points). The result is a three-dimensional image of the subsurface.
3-D seismic is chiefly used to make detailed surveys of structurally complex areas and to assess the size of reserves identified by drilling more precisely.
The final results of seismic measurements are acoustic models of the earths subsurface. These models are refined using other non-seismic information like surface geology, gravity and magnetic data, ground penetrating radar, aerial photography and last but not least well data. Seismic travel-times are transformed into depth values using knowledge of the propagation velocity of seismic waves. Thus a three-dimensional picture of the rocks in the earths subsurface is created.
This last step in the utilization of seismic data is referred to as interpretation. It is achieved with the use of high powered computer systems utilizing specialist software.
The ultimate objective of the seismic exploration method is the location of an oil or gas bearing reservoir, which can be drilled and produced to the surface.
Whether oil or gas is actually present can only be proven by drilling.
Drilling a well requires a drill bit that eats its way through the earth's crust and long pipes (drill string), linking the drill bit to the surface. The drill pipe is gradually fed into the well by the drilling rig.
Nowadays, the rotary drilling technique is used to sink virtually all wells. A rotary table or a Top Drive System ensures that the drill pipe and the drill bit are constantly rotating. Drill collars (thick-walled heavy weight tubular steel pipes immediately above the drill bit) help the bit exert enough weight to keep cutting deeper into the rock and maintain the tension on the drill string.
The material that the drill bit cuts through also has to be transported up to the surface. This is performed by the drilling mud - a water and clay mixture that is forced through the drill pipe by high-pressure pumps and constantly exits from the bit. The rock cuttings are washed back to the surface with the drilling mud, and are examined immediately, as they allow conclusions to be drawn about the geological strata being drilled through. The drilling mud is purified and recirculated into the well.
To prevent the hole from collapsing a telescopic lining of steel casing and cement is given in sequences. A well may have a diameter of up to 90 centimetres (36 inches) at the surface, and narrows in a number of stages as it deepens, down to a diameter of just a few centimeters.
Drilling a well like this can take a long time. Depending on the hardness of the rock strata and the planned depth, it can sometimes last longer than a year. However most wells are drilled through soft rock formations where the average drilling rate is about 300 meters per day (record for OMV Austria is more than 1,000 meters per day). Today's sophisticated exploration techniques already permit success rates of 50% or more, meaning that every second well in an area is commercial.
The most important piece of drilling equipment is the drill bit.
There are two types of drill bit:
Roller bits usually consist of three cones. These have teeth or insets, and are made of hardened steel (tooth or rock bit) or tungsten carbide (tungsten carbide insert bit). The rock is drilled with a percussive action.
Fixed cutter bits were originally impregnated with natural diamonds. They were used to drill through extremely hard geological strata. The PDC bit (polycrystalline diamond compact) is a newer development. Its main cutting elements are coated with synthetic diamond layers. These bits were originally used only for very soft formations, but newer PDC developments are suitable for medium-hard strata.
Environmental restrictions, difficult terrain, and the nature of offshore technology made it necessary to find a way of developing reserves at a distance from the surface drilling location.
Directional drilling equipment, capable of deviating both the inclination and the direction of a well, was developed for this purpose. This was made possible by the use of down hole motors in combination with an integral bent. If the drill string and hence also the bent sub are not rotated, the bit - which is driven by the down hole motor - drills a crooked well.
Measuring instruments are positioned above the drill bit and constantly send data to the directional driller via pulses through the drilling fluid that tell him the angle and direction of the bit, enabling him to adjust the well path.
Because of the complexity of modern drilling techniques the length of the well is no longer the sole yardstick for a record. Two aspects must be taken into consideration:
1. The true vertical depth (TVD), i.e. the vertical distance from a point at the bottom of the hole to the surface:
- Continental Deep Drilling (KTB) well, Germany: 9,142 m TVD (scientific borehole to investigate the earth's crust
- Kola SG-3, Russia: 12,262 m TVD (scientific borehole to investigate the earth's crust
- Both wells were suspended because the extreme temperatures at these depths made further use of the drilling tools impossible.
- Bertha Rogers, Oklahoma, USA: 9,586 m TVD (the deepest well ever sunk with the intention of discovering oil and gas)
2. The measured depth (MD) of a well denotes its actual length.
Directional and horizontal wells can be extremely long despite being relatively shallow. The ratio of the horizontal deviation to the true vertical depth (TVD) is known as the aspect ratio, and is an indication of the difficulty of drilling a well of this type. Aspect ratios of over 5:1 are no longer uncommon. This means that a reservoir located about 2,000 m below the surface can be reached at a distance of over 10 kilometers from the surface location. In other words, the measured depth (MD) of a well can extend beyond 12,000 meters (m).
The limits to the length of directional wells are set by the capacity of the rig, which can only accommodate a given length of drill pipe in the derrick. Handling drill pipes is time consuming and hazardous.
Total, Tierra del Fuego: MD 11,184 m; TVD 1,666 m; horizontal deviation 10,585 m; aspect ratio 10,585/1,666 = 6.3:1
BP, Wytch Farm, UK: MD 11,280 m; TVD 1,900 m; horizontal deviation 10,670 m; aspect ratio 5.6:1
Chevron Texaco, North Sea: 4,606 m longest horizontal distance (Captain field)
Core samples are taken if the stone cuttings conveyed back to the surface fail to deliver sufficient information about the geological strata.
Core samples, which measure up to nine meters in length and have a diameter of five to ten centimeters, provide an accurate picture of the rocks traversed. The sample is taken using a special drill bit, known as a core bit, and retrieved in a special protective lifter tube.
Light drills can also be lowered into the well on cables to cut small samples from the wall.
A core sample yields a wealth of information. It reveals the type of rock, the inclination of the geological strata and faults, as well as oil and gas deposits.
A historic example of a core sample is "the deepest bit of Vienna", from the Aderklaa Ultratief 1 well, taken at a depth of 6,630 m and now on display at the Vienna Museum of Technology. The educational trail in Prottes gives visitors a chance to take a look to other core samples.
Production tests (tests) provide information about the reservoir fluids (oil, gas or water), reservoir pressure and the permeability of the reservoir rock, flow potential (achievable production rates) and much more. Such tests are normally performed if there is not enough information available on about the type of the reservoir fluid and its chemical properties (oil, gas and water) or potential production rates.
Well tests are short-term production trials carried out using special test equipment which mainly consists of packers (sealing devices made of reinforced rubber) and control and shut-in tools. This equipment is normally run in hole on drill pipe. By applying weight, the rubber elements will expand and isolate the formation to be tested from the rest of the wellbore. A valve in the testing unit is opened and the reservoir is exposed to a differential pressure (so called drawdown) which causes the formation contents (oil, gas and water) to start flowing.
With the help of control and shut-in valves, which are integral part of the test string, the pressure - which is often very high if the well is deep - can kept under control at all times.
The crude produced in a well test is stored in special tanks; during the test, at all times important parameters (flow temperature, pressure, oil density, water content, flow rates and cumulative volume) are recorded. Normally the produced fluid is trucked to a waste disposal site and properly disposed, but if an existing production pipeline is in place near the well the test output is fed into this.
Production tests can last for anything from a few hours to several weeks, depending on the complexity of the information required and the amount of work involved in obtaining it. The information obtained from short tests is generally limited to indications of the nature of the hydrocarbons produced, potential production rates and reservoir pressure, whereas longer tests also permit conclusions about the size of the reservoir.
With the help of well logging, information about the geological strata a well has encountered can be obtained. The main purpose is to obtain geological and lithological information, and to identify the formation fluid contained in the pore space (oil, gas and water).
Electrical measurements determine the specific electrical resistance of the strata drilled through. Formations containing oil and/or gas have a higher electrical resistance than formation water (brine).
A special instrument is used to give a lifelike image of the borehole wall, complete with valuable geological and lithological information (fissures and formation dip).
Radioactive measurements utilize the fact that the proportions of radioactive elements (uranium, thorium and potassium 40) vary according to the kind of rock encountered. This makes it possible to distinguish between clay and sand or limestone, as clays are normally more radioactive. It is also possible to bombard formations with neutrons or gamma rays from a radiation source in the measurement probe. This technique is used to identify porous layers and determine their porosity.
With acoustic measurements, short ultrasound pulses are emitted in a similar manner to seismic. The propagation speed of the sound waves is measured, and the data acquired gives an indication of the porosity of the rock layers.
Not all oil and gas accumulations are located onshore. Most of Europe's hydrocarbon resources are in the North Sea, where Denmark, Germany, the Netherlands, Norway and the United Kingdom operate production platforms.
There are various types of production platforms: The design used depends on the depth of the water. As a rule, the deeper the water, the larger and more costly the structure must be. The first offshore production platforms were built in the 1940s, in shallow coastal waters. Technical advances over the past 60 years have made it possible to produce oil and gas from depths of over 3,000 m.
The management of all OMV offshore wells is covered by the OMV well integrity management system (WIMS). The standards defining OMV WIMS1 are based on the industry-wide recognized ISO 16530-1 standard, NORSOK D-010 standard and API standards, as well as on multiple recommended practices and technical requirements related to well integrity management. WIMS regulates the full cycle of well management, starting from the base design, and through to the abandonment, including requirements established in case of well handover from one operator to another. WIMS includes regular monitoring, preventive-corrective maintenance, an established schedule of testing for pressure and for regular functioning of all well barriers and barrier elements. This practice enables an early detection of a possible well integrity issue and timely reaction to fix them in order to prevent any escalation, thus providing operations that are safe for people, environment and asset integrity.
1 “Well Integrity Management System For The Well Life Cycle” and “Well Integrity Management System For Well Production/Injection Phase”
This is the oldest type of offshore drilling rig, dating back to the 1950s. The rig is towed into position by tugs, and its support legs (at least three) are lowered on to the seabed. As the legs are jacked down, the platform is raised above the water. The working platform is about 20 m above the surface of the water to keep it out of reach of high waves. Jack-up platforms are used in depths of up to 150 m.
Semisubmersibles are used in deeper waters and are typically towed to site by tugs. They do not have support legs, and instead have giant pontoons with ballast tanks which ensure the center of gravity of the floating platform is as far as possible below the surface in order to maximize stability. Semis can either be anchored to the seabed (using a minimum of eight special anchors) or kept in position by a computerized dynamic positioning system, which controls the thrusters (propellers) to ensure that the rig remains in position over the well even in high seas or during heavy storms. The working deck is high above water level to prevent it from flooding even in the highest seas. Semis can be used in water depths of up to 3,000 m.
Drill ships can be used at depths of over 3,000 m. They have the added advantage that they can simply transit from drill site to drill site under their own power. This makes them ideal for exploration, since often only a single well is drilled at a remote location. As with semisubmersibles, the vessel is held in position by anchors or by dynamic positioning.
All of the above drilling vessels can also be used as production platforms if moored permanently. They can be used to drill additional production wells or injection wells and bring them on stream. Most often however, a dedicated wellhead or production platform is fabricated and installed.
To develop a large oil field, drill rigs use directional drilling techniques. This technology dispenses with the need to build a separate production platform above each well. The animation shows an offshore field with three directional wells drilled from a single platform so as to tap the deposit as efficiently as possible.
The oil and gas industries are highly capital intensive and it takes a relatively long time for the capital employed to deliver returns to investors.The total duration of a project, from the start of exploration for hydrocarbons, through to field development, production and the decommissioning of a production system is known as its life cycle. This can last for anything from several years to decades.The process begins with exploration in a frontier area. This phase usually takes between one and seven years. During this period initial geological and geophysical studies are carried out, and the first exploration wells are drilled. If they are successful, additional appraisal wells are drilled to determine the size of the discovery.Assuming that the appraisal wells confirm that field development is commercially viable, the next task is to develop the field by drilling the number of production and injection wells required to maximize oil and/or gas output or the recovery rate. During the initial development drilling the processing facilities and any required export pipelines are constructed. Production and export of oil and gas usually starts as soon as processing facilities are available, and frequently before completion of drilling all the planned production wells. The production phase can continue for several decades, depending on how prolific the field is. A field is nearing the end of its economic life when some wells are only producing small quantities of hydrocarbons together with very large amounts of water.Then the task is to decommission the installations properly, and restore the surface to its original state.
In oil and gas industry parlance, the term "abandonment" refers to the plugging of exploration or production wells: Every well that fails to encounter commercial quantities of oil or gas, and every production well that has reached the end of its useful economic life has to be safely plugged.Well abandonment is coordinated with the relevant authorities and landowners. It has three main aims:1. To prevent oil and gas from escaping to the surface;2. To permanently prevent water horizons from mingling, and in particular, to protect them from oil and gas bearing horizons; and3. To restore the surface to its original state.Decommissioning a well involves plugging the artificially created access to the oil or gas formation (perforations) with a special cement, which is injected into the tubing under pressure. The well is then mechanically sealed (steel structure with special plastic seals). Next the well is cleaned to ensure that there are no more traces of hydrocarbons in the pipes. Depending on the geological characteristics, the well is plugged with additional cement plugs measuring at least 100 m in length.Finally all the metallic parts are cut down to a depth of 2.5 m below ground level and removed. A plate is welded on top of the well and then covered with a large slab of concrete. This in turn is covered with a layer of topsoil to recreate the natural terrain, and the site is returned to its owner. In Austria, well plugging is subject to statutory regulations. The most important are the Mineral Resources Act and the Well Order.
Research in exploration and production is geared towards the development of environmentally friendly and economic exploration and production methods.