Oil and Gas
Exploration Drilling
A cylindrical drill commonly studded with diamonds is used to drill boreholes. The drill bit is cooled and lubricated by drilling mud, the density of this mud is controlled by adding Barite which is dense and allows the mud to reach the bottom of the hole. Rock chips and/or continuos drill core can be recovered and material is examined and logged. Continuous drill core is very expensive and only used in critical locations.
Sapropel
After plankton accumulate on the sea bed anaerobic bacteria cause them to partially decay forming an organic rich mud. Diagenesis of it forms an organic rich sedimentary rock called a source rock, for example black oil shales and mudstones. Their dark colour represents their high carbon content and their fine grain size represents the low energy conditions of deposition.
If Source Rocks Are Impermeable How Does The Oil Escape?
As maturation takes place Kerogen turns into Petroleum. Kerogen is a waxy solid composed of many different hydrocarbon compounds, where as petroleum is a viscous liquid. The transformation of a solid into a liquid corresponds with an increase in volume. This increase in volume induces an increase in pore fluid pressure which causes the rock to crack.
Maturation of Petroleum
As the source rock is buried, pressure and temperature increase. Over time the organic matter breaks down to form a mixture of organic compounds of C, H, O, N and S called Kerogen and finally Petroleum.
Salt Caverns
Evaporites are impermeable and therefore ideal for underground storage. They are constructed by injecting water to dissolve the salt and then pumping it out as brine, which can be used in the chemical industry. They are ideal as natural gas can be replenished and removed rapidly as they are hollow and do not contain grains to restrict flow.
Gamma-Ray Spectroscopy
Gamma rays emmited by rocks as a result of radioactive decay are counted. Potential source rocks give a high gamma ray count as they are mineralogically and chemically diverse where as reservoir rocks give a low reading as they are mineralogically and chemically simple.
Down-Hole Logging
It is possible to mount geophysical instruments on a wire or "Sonde" (probe) that is passed down the drill hole to record data as it is slowly pulled back up from the bottom.
British Oil and Gas Reserves
Most oil and gas reserves in the UK are located offshore, particularly in the North Sea, but gas fields also occur in the Irish Sea. There are a few onshore facilities, the largest of which is Wytch Farm in Dorset. Nearly all the British reserves are in Mesozoic (Triassic, Jurassic, Cretaceous) and Cenozoic (Tertiary, Quaternary) age sedimentary basins.
Gas Cap Drive
Natural gas (carbon dioxide or nitrogen) are injected into the reservoir rock above the oil to maintain the pressure.
Sustainability of Oil and Natural Gas
Oil and natural gas are non-renewable energy resources. They are fossil fuels that formed millions or years ago. Once they are burnt they are lost as gasses into the atmosphere and cannot be recycled on a human timescale. As a result the use of fossil fuels is ultimately unsustainable.
Migration
Once formed the petroleum undergoes it from the source rock to a reservoir rock. The main factors controlling it are: •Permeability of rocks - There must be permeable rocks for the petroleum to travel through. •Pressure - Oil & gas naturally move down the pressure gradient from high to low pressure. •Density differences - Petroleum is less dense than water so will percolate upwards until it reaches an impermeable layer. •Viscosity of the oil - Higher temperatures lower the viscosity of the oil. The lower the viscosity the easier it is to flow.
Source Rock Formation
Petroleum is formed from the remains of microscopic marine organisms called plankton. Plankton live in the photic zone of the oceans and after dying accumulate on the seabed. The environment of deposition must be low energy so they can settle out of the water and anoxic so the plankton won't decay or be scavenged.
Two-Way Time (TWT)
Seismic profiles are commonly constructed using two-way time, which is the time taken for a seismic waves to travel to a reflective layer and return to the surface
Unintentional Oil Fires and Burning of Fossil Fuels
Short term impacts: •Release of Carbon Monoxide, which binds with haemoglobin and reduces oxygen carrying capacity •Release of soot, which causes respiratory problems •Release of Volatile Organic Compounds, e.g. Benzene which are carcinogenic. Long term impacts: •Release of large amounts of Carbon Dioxide, which may contribute to the enhanced greenhouse effect.
Issues Associated With Extraction of Oil
The extraction of petroleum leads to a reduction in pore fluid pressure. If the lithostatic pressure exceeds the pore fluid pressure then pore spaces will close as grains move closer together. This can create minor earthquakes and subsidence. However if pore fluid pressure exceeds lithostatic pressure fractures can be generated releasing the pore fluid pressure and leading to compaction and subsidence.
Cap Rock
The reservoir rock must be overlain by it. It prevents further migration petroleum. Without it the petroleum would continue to rise, eventually forming oil seeps or tar pits on the surface. Suitable ones include clay, mudstone, shale and crystalline sedimentary rocks such as evaporites.
Travel Time
The time taken for the reflected waves to arrive back at the receivers.
Depleted Oil and Gas Reservoirs
These are reservoirs from which all the recoverable oil and gas has been extracted, leaving underground rocks capable of storing gas. They are ideal as the geology is already well known and it is cost effective as the site is already drilled with equipment left over from extraction.
Unconventional Sources of Petroleum
They include oil shales, tar sands and coal-bed methane. Historically the exploration of these sources was unpopular due to higher production costs and greater environmental impacts. But as global reserves of oil have dwindled, more attention is being focused on them.
Primary Recovery
To extract reserves the reservoir rock is drilled and a production well established. Initially oil comes to the surface under natural pressure caused by hydrostatic pressure of he water in pore spaces beneath the oil, gasses in the oil coming out of solution and the expansion of gas above the oil. After the natural pressure is released oil is pumped to the surface using nodding donkeys. Typically only 20-30% of the oil in a reservoir can be recovered by it.
Underground Gas Storage
UK gas consumption is rising and to meet this demand natural gas is imported. These facilities are being built to store the gas until it is needed. Onshore facilities are more convenient but may have to be located in environmentally sensitive locations. Gas can be stored in depleted oil and gas reservoirs and salt caverns.
Water Flood Drive
Water is injected into the reservoir rock beneath the oil to maintain the pressure.
Oil Shale
a general term for any fine grained sedimentary rock containing significant amounts of kerogen, that did not undergo enough maturation to produce petroleum. They can be processed by steam injection to produce oil that can be pumped out. Alternatively they can be mined and the kerogen converted into synthetic crude oil or burnt as a low grade fuel.
What is Petroleum?
a naturally occurring, viscous hydrocarbon mixture of Crude Oil, Natural Gas and Asphalt. In addition to fuel, it forms the basis of the petrochemical industry. Products include plastics, paints synthetic fibres, rubber, pharmaceuticals and fertilisers.
Tar Sands
are semi-solid fractions of petroleum mixed with clay and sand. Originally the theys would have been conventional crude oil, however, over time bacteria have converted it to bitumen.
Traps
are where the geological setting allows oil and gas to be concentrated in one place, making them economic to extract. All of them require a porous and permeable reservoir rock to be overlain by an impermeable cap rock.
Gravity Surveys
can be land-based or airborne. An instrument called a gravimeter measures small variations in the Earth's gravitational field strength in milligals (mGal). Survey point safe tied to each location via GPS. Maps are plotted with lines joining points of equal gravitational field strength and anomalies can then be identified.
Seismic Reflection Surveys
can be land-based or ship-based. Artificial seismic waves are generated by dropping a heavy thumper mounted on a vibrotruck on land or by air guns at sea. The seismic waves travel into the Earth and are reflected at layer boundaries within the strata. The reflected waves travel back to the surface where they are recorded by an array of receivers called geophones on land or hydrophones at sea. Their locations accurately pinpointed using Global Positioning Systems (GPS).
How Can Petroleum Be Destroyed?
if the temperature exceeds 200°C due to: •Heat from an igneous intrusion or volcanic activity. •Regional metamorphism. •Burial where the geothermal gradient results in high temperatures.
The North Sea
is a shallow continental sea that was subject to crustal extension during the Mesozoic when the Atlantic Ocean began to open. Faulting broke the area into a series of linear horsts and grabens that controlled sedimentation. It is split into two sedimentary basins by an east-west trending ridge or 'high'. The Northern North Sea basin contains oil and gas fields, whereas the Southern North Sea basin only contains gas fields.
Reservoir Rock
it must have a high porosity to be able to store significant amounts of petroleum, it must also have a high permeability to allow oil to migrate into it and be extracted. The properties of a it are the same as the properties of an aquifer. Suitable reservoir rocks include poorly cemented sandstones, limestones and fractured chalk.
Requirements For The Formation of Oil and Gas
originate in sedimentary basins. The main requirements to form economic accumulations are: •A source rock •Maturation •Migration •A reservoir rock •A cap rock •A trap
Seismic Profile
shows the subsurface layering and can be interoperated by geologists to identify possible traps.
Resistivity
the ability of a material to resist the flow of electricity. Water is the main conductor present in rocks and gives a low resistance. However the presence of hydrocarbons results in a high resistance.
Reserves of Oil and Natural Gas
the amount of the resource that can be extracted at a profit using existing technologies.
Directional and Extended Reach Drilling
the techniques are used so that a number of wells from one well head can tap the reservoir rock over a large area.
Blowout
when oil gushes up the well and flows out onto the surface. Production wells must be quickly capped off to prevent it.
Causes of Gravity Anomalies
•A positive- caused by an excess of mass or materials that are denser than their surroundings. Positive anomalies are usually associated with anticlinal traps. •A negative- caused by a deficit of mass or materials that are less dense than their surroundings. Negative anomalies are usually associated with salt dome traps.
Spills of Light Fractions
•Covers seabirds leading to a loss of buoyancy and drowning. •Enters the plumage of birds and impacts upon their thermoregulation. •Blocks out sunlight leading to a reduction in photosynthetic algae (Phytoplankton), which in turn has knock-on effects for the ecosystem.
How Can Petroleum Be Lost?
•Erosion or removal of the overlying cap rock. •Escaping upwards along an unsealed fault plane.
Oil and Natural Gas Reserves Increase If...
•Exploration discovers more. •Technology improves so more can be extracted from existing wells •Smaller oil fileds become economic when prices rise.
Why Do Older Rocks In the UK Not Contain Oil and Gas?
•Metamorphism in parts has raised temperatures denaturing the oil. •Intrusions of igneous rocks denatures the oil. •Older rocks are generally buried deeper raising the temperature and denaturing the oil.
Advantages of Offshore versus Land-Based Surveys
•Offshore surveys are particularly effective as a large number of hydrophones an be towed behind a ship. •Also a ships path is not restricted like vibrotrucks are on land.
The Northern North Sea Basin
•The main source rock for oil is Jurassic age Kimmeridge Clay •Reservoir rocks are mainly marine sandstones and fractured chalk. •Cap rocks are mainly clays. •Oil and gas are contained by a variety of traps.
Secondary Recovery
•Thermal methods - Steam is injected to increase the oils temperature and lower the viscosity of the oil •Detergents - Used to reduce surface tension of the oil to loosen it from the grains •Bacteriological - Digest and break down large hydrocarbon molecules to decrease viscosity. After this 20-30% of the oil still remains in the reservoir.
Oil and Natural Gas Reserves Decrease If...
•They are being extracted •Calculations of reserves are incorrect •Smaller oil fileds become uneconomic when prices fall.
Southern North Sea Basin
•This is because the source rock for this area is the carboniferous coal measures. •Coal forms from the breakdown of vegetation so the source rock did not at any time contain plankton needed for oil. •Escaped gas is mainly held in desert sandstone reservoirs. •The cap rocks are often clays and evaporites and the traps are mainly salt domes and their associated anticlines.
Spills of Heavy Fractions
•Toxic heavy fractions sink to the seabed, where they persist for a long time and smother organisms. •The oil reacts with dissolved oxygen in the water and thus reduces the available oxygen for marine life.
Factors Affecting Recovery
•Viscosity of oil - Higher viscosity oils don't flow as easily. •% of matrix or cement - A Higher % will reduce pore space/storage capacity and increase grains per unit volume therefore increasing surface tension. •Grain shape - Rounded grains have a higher porosity •Sorting - Poorly sorted rocks contain less oil as smaller grains infill pore spaces. •Grain size - Smaller grains have a larger surface area producing more friction and reducing permeability. •Joints - Joints increase porosity and permeability allowing oil to be stored and transmitted.
