• Appraisal Well
• Coal
• Drilling (Drilling Rig)
• Hydrocarbons
• Mobile Off-Shore Drilling Unit
• Natural gas
• Oil
• Oil (or gas) provine
• Reflection seismology
• Reserves (oil and gas reserves)
• Reservoir
• Submersible Drilling Rig

Appraisal Well

An appraisal well generally refers to a well drilled to evaluate or confirm size and quality and quantity of a reservoir and to evaluate its bankability. The appraisal well allows to evaluate if the oil or gas in the reservoir is effectively commercially viable. The appraisal is generally done after a discovery or seismic survey which highlights the presence of a reservoir. The appraisal well is done before production starts and after a reservoir is discovered. In order to completely evaluate the value and quantity of hydrocarbons in the reservoir, it may be the case that two or three appraisal wells are needed. The appraisal is the phase that immediately follows the exploration phase which has discovered a substantial and promising reservoir. The whole aim of the appraisal well is to better understand how to develop the reservoir and how to deal with its geological complexities. Delineation wells are drilled to establish the dimension of the reservoir [1].

COAL

Coal is a carbon-rick rock usually sedimentary which forms from remains of plants or other organic organisms buried for centuries underground. These remains experiment physical and chemical changes when buried and heated and exposed to pressure from . This process is called “coalification”. Since coal has an organic nature it is not classified as a mineral [2]. However, it assumes the form of a black or brown-colored rock which is composed by 50 to 70% of carbonaceous minerals which were created mainly from plants’ remains compacted and heated or pressured over time [3]. There are three types of coal: anthracite, bituminous coal and lignite. These have different organic characteristics and different calorific values as they are influenced by the way they have been formed [4].

Drilling (Drilling Rig)

Drilling is usually carried out nowadays through rigs although there are several types of drilling. Drilling techniques are different depending on off-shore or on-shore drilling. On-shore drilling rigs comprise every equipment or machine necessary to run the rig, including mud tanks, pumps, the derrick, the rotary table or topdrive, the drillstring, the power generation equipment and auxiliary equipment [5]. Off-shore drilling is usually carried out by rotary rigs. This comprises the derrick, source of power, lifting tackles and blocks, rotary table and Kelly to rotate the drill string , mud pump and circulation system, blow out preventer (especially important in off-shore drills) and handling drilling system pipe and casing. The off-shore drilling also sees a large variety of equipment used according to the type of reservoir that has to be developed [6].

HYDROCARBONS

The term generally refers to compounds of hydrogen and carbon which can be found in liquid, solid or gaseous forms. Hydrocarbons are therefore a mixture of hydrogen and carbon atoms [7]. Different forms of hydrocarbons indicate a different level of complexity of the way atoms are combined together. One of the simplest hydrocarbons is methane gas [CH4]. The molecules can have the shape of chains, branching chains, rings or other structures. “Petroleum” is the name used to indicate unprocessed oil (the product of the extraction process) and it is a complex mixture of hydrocarbons. These are therefore the basic components of all kind of fuels derived from oil or gas. The most common hydrocarbons are natural gas, oil and coal [8].

Hydrocarbons can be classified in different ways [9]:

• According to their chemical composition:

1. Pure hydrocarbons: also called aromatic hydrocarbons are entirely composed of carbon or hydrogen (arenes, alkanes, alkenes and alkyne-based);
2. Impure hydrocarbons: hydrocarbons which have “impurities” such as traces of sulphur or nitrogen compounded with carbon or hydrogen.

• According to the IUPAC nomenclature of organic chemistry:

1. Saturated hydrocarbons: (alkanes) are the most simple of the hydrocarbon species and are composed entirely of single bonds and are saturated with hydrogen; they are the basis of petroleum fuels and are either found as linear or branched species of unlimited number. The general formula for saturated hydrocarbons is CnH2n + 2 (assuming non-cyclic structures).
2. Unsaturated hydrocarbons: These hydrocarbons have one or more double or triple bonds between carbon atoms. Those with one double bond are called alkenes, with the formula CnH2n (assuming non-cyclic structures). Those containing triple bonds are called alkynes.
3. Cyclo-alkanes: hydrocarbons containing one or more carbon rings to which hydrogen atoms are attached. The general formula for a saturated hydrocarbon containing one ring is CnH2n.
4. Aromatic hydrocarbons: also known as arenes which have at least one aromatic ring.

Hydrocarbons can be gases (e.g. methane and propane), liquids (e.g. hexane and benzene), waxes or low melting solids (e.g. paraffin wax and naphthalene) or polymers (e.g. polyethylene, polypropylene and polystyrene). Hydrocarbons are the compound which constitute crude and refined oil, coal, gas and other fuels.

Major classes of hydrocarbons in crude oils include [10] :

• Paraffins
  • general formula: CnH2n+2 (n is a whole number, usually from 1 to 20)
  • straight- or branched-chain molecules
  • can be gasses or liquids at room temperature depending upon the molecule
  • examples: methane, ethane, propane, butane, isobutane, pentane, hexane

• Aromatics
  • general formula: C6H5 - Y (Y is a longer, straight molecule that connects to the benzene ring)
  • ringed structures with one or more rings
  • rings contain six carbon atoms, with alternating double and single bonds between the carbons
  • typically liquids
  • examples: benzene, napthalene

• Napthenes or Cycloalkanes
  • general formula: CnH2n (n is a whole number usually from 1 to 20)
  • ringed structures with one or more rings
  • rings contain only single bonds between the carbon atoms
  • typically liquids at room temperature
  • examples: cyclohexane, methyl cyclopentane

• Other hydrocarbons
  • Alkenes
    • general formula: CnH2n (n is a whole number, usually from 1 to 20)
    • linear or branched chain molecules containing one carbon-carbon double-bond
    • can be liquid or gas
    • examples: ethylene, butene, isobutene
  • Dienes and Alkynes
    • general formula: CnH2n-2 (n is a whole number, usually from 1 to 20)
    • linear or branched chain molecules containing two carbon-carbon double-bonds
    • can be liquid or gas
    • examples: acetylene, butadienes

Mobile Off-Shore Drilling Unit

There are several types of off-shore drilling unit generally floating and mobile. The term refers to all of these floating or floating drilling machines [11], including submersible and semi-submersible.

NATURAL GAS

Natural gas is a mix of hydrocarbons gases the majority of which is methane gas. It is common to refer to natural gas as to the one extracted from underground storages to distinguish it from other type of gases that have a different composition and use in the oil and gas industry. Natural gas is commonly found in association with oil. Methane [CH4] is the chief constituent of most natural gas (constituting as much as 85% of some natural gases), with lesser amounts of ethane [C2H6], propane [C3H8], butane [C4H10] and pentane [C5H12]. Impurities can also be present in large proportions, including carbon dioxide, helium, nitrogen and hydrogen sulfide [12].

OIL

Oil is typically a mixture of different hydrocarbons. It is often referred to as “crude oil” or “petroleum”. This is oil which has been extracted but not refined and therefore contains different hydrocarbons. Crude oil will go through the refining process in order to be commercialized as gasoline or other types of fuels. Petroleum can be found in the upper strata of the Earth’s crust in porous rock formations and also in sand (called then “crude bitumen”). The approximate length range is C5H12 to C18H38. Any shorter hydrocarbons are considered natural gas or natural gas liquids Long-chain hydrocarbons are more viscous, and the longest chains are paraffin wax. As found underground crude oil may contain other nonmetallic elements such as sulfur, oxygen and nitrogen. Crude oil is usually black or dark brown (although it may be yellowish or even greenish) but varies greatly in appearance, depending on its composition [13]. Crude oil can have therefore different qualities according to its composition. High sulfur oils are generally accounted as low-quality crude. The refining process for this type of crude is inevitably longer and more expensive than the one used for other type of petroleum with less sulfur. A longer refining process implies most costs and therefore a less profitable product deriving from refining.

OIL (OR GAS) PROVINCE

An oil (or gas province) is a geographical area where it is possible to find a large number of oil or gas fields capable of sustaining extraction and production of oil or gas for several years. An oil province is usually delimited after a series of geological explorations and once there is sufficient certainty regarding the total amount of proven reserves and possible or probable reserves.

REFLECTION SEISMOLOGY

Reflection seismology (or seismic reflection) is one of the available methods to explore geologic formations using the principle of seismology to estimate the properties of the rocks and liquids underground from reflective seismic waves. In order to use this method it is necessary to produce seismic waves through a source of energy such as dynamite or a specialized air gun. The explosion will produce seismic wave similar to the ones produced by earthquakes. It is then possible to study the geologic formations underground evaluating the time it is needed for the wave to cross them and get back to the Easrth’s surface. Different materials have different reflective ratios and by the time it takes for a reflection to a receiver on the surface it is possible to evaluate what kind of material generated that reflection. Waves reflected to the surface are measured through seismographs and data is elaborated through computer software.

Reflection seismology is therefore extensively used in hydrocarbons research (oil, gas, coal, etc.) and other applications which are related to the study of the Earth’s surface or underground formations. Seismic technology is used by geologist who then interpret the data produced by calculating the time necessary for reflection by underground formations or liquids. Once data are complete it is possible to identify structural traps that could contain hydrocarbons. Although there has been a significant improvement in exploration technology in the last 20 years, the principles for acquiring systemic data have remained the same [14].

Reflection seismology or seismic reflection is strictly related to seismic reflection tomography. This is a technique to measure and display the three-dimensional distribution of velocity or reflectivity of a geologic formation or liquid underground. Using the waves produced in seismic reflection and various receivers (seismographs) on the Earth’s surface it is possible to draw a sort of “map” of the geologic formations underground. Space underground is divided into cells which have different velocities and reflectivity values because of the different composition of rocks underground. The final model is the one that have the reflectivity and velocities closer to the data acquired through seismic reflection [15].

RESERVES (OIL AND GAS RESERVES)

The term is generally used to refer to the recoverable portion of resources (namely oil and gas) available based on current knowledge, technology and economics [16]. Recoverable reserves are generally those which can be recovered profitably on the basis of costs of extracting them and of quantity of oil and gas available in the reservoir (pool).

Reserves can be divided in four categories [17] :

• Proven reserves: Reserves that have been sampled extensively, for example—in mining—by closely spaced diamond drill holes or by underground working giving an accurate picture of size and quality of reserves. Also called measured reserves. In oil, proven reserves are an estimate based on seismic and other exploratory data, linked to an assessment of economic and operating viability.

• Recoverable reserves: see above. In oil and gas, recoverable reserves are a portion of a field’s total estimate reserves that can actually be recovered from the field using currently available technology. Recoverable reserve estimates are dependent on factors such as reservoir pressure (see reservoir) and the density of strata. Reserves can be defined as recoverable depending on total costs of the recovery and local economic conditions.

• Probable reserves: Valuable mineralization not sampled enough to estimate accurately the terms of tonnage and grade. Also called indicated reserves. These reserves are only “probable” as there is no certainty regarding their recovery as the cost of this operation cannot be accurately established without specific information on the quality and quantity of gas or crude underground. These information are essential as a low-quality grade crude could be very expensive to refine and consequently unprofitable to extract.

• Possible reserves: Valuable mineralization not sampled enough to estimate accurately its tonnage and grade, or even verify its existence. Also called inferred reserves. In oil, possible reserves are an estimate of reserves from an undrilled site or one that has not bee seismically tested. These reserves are only virtually profitable. Their quantity and quality has not been assessed as the site has not been drilled for geological tests aimed at estimating the type of resources underground. Further studies and tests are therefore required to characterize these reserves as probable or recoverable.

The total amount of probable and possible reserves constitutes the level of hydrocarbon “resources” available.

RESERVOIR

The reservoir represents an underground area (subsurface), generally a body of rock, which is capable of store and transmitting fluids such as crude oil. The rock will then have sufficient porosity and permeability to be able to transmit fluids and especially the hydrocarbons contained or accumulated during the centuries. The reservoir indicate a permeable rock containing petroleum or other hydrocarbons. This type of rock is usually sedimentary because it has more porosity than most igneous and metamorphic rocks and at the same time they form under temperature conditions at which hydrocarbons can be preserved. The reservoir is therefore a crucial component of the petroleum system [18].

A reservoir has many important characteristics which can affect the entire petroleum system such as porosity, permeability, viscosity and density (which influence the possibility of extracting crude oil from the reservoir). Given its importance, there are some peculiar activities which are connected in different ways to the reservoir and which are carried out in order to evaluate porosity, permeability and other characteristics of the reservoir and then start the extraction process. Here are some of the most important activities or characteristics related to the reservoir:

• Reservoir Characterization (or description) : A model of a reservoir that incorporates all the characteristics of the reservoir that are pertinent to its ability to store hydrocarbons and also to produce them. Reservoir characterization models are used to simulate the behavior of the fluids within the reservoir under different sets of circumstances and to find the optimal production techniques that will maximize the production. Otherwise stated the reservoir characterization identifies the act of building a reservoir model based on its characteristics with respect to fluid flow [19] (also known as reservoir modeling - The act of producing a model of a reservoir. The model could include any of the geological, fluid or other characteristics of the reservoir [20]) - or reservoir simulation - A computer run of a reservoir model over time to examine the flow of fluid within the reservoir and from the reservoir. Reservoir simulators are built on reservoir models that include the petrophysical characteristics required to understand the behavior of the fluids over time. Usually, the simulator is calibrated using historic pressure and production data in a process referred to as "history matching." Once the simulator has been successfully calibrated, it is used to predict future reservoir production under a series of potential scenarios, such as drilling new wells, injecting various fluids or stimulation [21].

• Reservoir Communication: The flow of fluids from one part of a reservoir to another or from one reservoir to another. The term is often used to describe crossflow from one reservoir compartment to another [22].

• Reservoir height (or well completions): A measurement of the vertical thickness of reservoir formation that is open to flow. The reservoir height is used in calculations and mathematical models to assess reservoir performance or potential productivity [23].

• Reservoir heterogeneities (or well testing): The variations in rock properties in a reservoir. The variations can result in directional variations in permeability. Geological processes, such as sedimentation, diagenesis and erosion, act to produce nonuniformities in rock formations. Because there are so many types of reservoir heterogeneities, a unique interpretation of test results from pressure data alone is often impossible. Expert test interpreters rely heavily on experience, core analysis, well logs and knowledge of the geology specific to the region [24].

• Reservoir pressure (also known as formation pressure, hydrostatic pressure, pore pressure): The pressure of fluids within the pores of a reservoir, usually hydrostatic pressure, or the pressure exerted by a column of water from the formation’s depth to sea level. Because reservoir pressure changes as fluids are produced from a reservoir, the pressure should be described as measured at a specific time, such as initial reservoir pressure [25].

• Reservoir-drive mechanisms (production enhancement): Natural forces in the reservoir that displace hydrocarbons out of the reservoir into the wellbore and up to surface. Reservoir-drive mechanisms include gasdrive (gas cap or solution gasdrive), waterdrive (bottomwater drive or edgewater drive), combination drive, and gravity drainage. Waterdrive is the most efficient drive mechanism, followed by gasdrive and gravity drainage. Reservoir-drive mechanisms are also called natural drives [26].

Submersible Drilling Rig

Submersible rigs are used in off-shore drilling. The term refers to mobile drilling rigs and floating units standing on large “pantoons” which are submerged under the water. The operating deck is positioned over the pantoons generally at a high of 30 metres above these pantoons thanks to steel supports. Once in position, the submersible rig is flooded until it touches the seabed. Drilling can then start. Because of their nature and the fact that they have to rest on the seabed, submersible rigs are generally used in shallow waters [27].