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Crude oil are liquid hydrocarbons that contain varying amounts of hydrocarbon components (ranging from C1 to C60), dissolved gases, bitumen, and other impurities. It is immiscible with water and has a density less than that of sea water. Crude oil is however, soluble in hydrocarbons like naphtha, carbon disulfide, ether, and benzene.
The physical properties of petroleum are controlled by the number of carbon & hydrogen molecules present along with the impurities. The most readily observed parameters are:
Specific Gravity is defined as the ratio between weight of a given volume of material and weight of an equal volume of water at 4°C.Paraffin-based oils are commonly light and asphalt-based (naphthenic components) oils are always heavy. The petroleum industry uses an API (American Petroleum Institute) scale to grade crude oil. Again it's the comparison with the density of water in its empirical index. As the API values increase specific gravity decreases. An API value greater than 30° means oils are considered light, API values between 30° and 22° mean an oil is considered to be of medium weight, while a value of below 22° implies the hydrocarbons are heavy crudes
Viscosity is defined as the internal friction of a liquid which causes resistance in the fluid to change its form. It is the ratio of stress to shear rate per unit time. Viscosity is the direct function of density of crude. It is a function of the number of carbon atoms and the amount of gas dissolved in the oil. As the gas content increases, viscosity decreases. This parameter is important when transporting petroleum products through pipelines across various geographies as the cost of pumping fluid is dependent on this.
Color is used along with refractive indices for identification of oil types. Paraffinic oils are light in color, yellow to brown in transmitted light and green in reflected light. Asphalt-based oils are black and are known as black oil.
Geological Time - Scale
The largest defined unit of time is theÂ supereon, composed ofÂ eons. Eons are divided intoÂ eras, which are in turn divided intoÂ periods,Â epochsÂ andÂ ages. The relative order of the three youngest eras is first Paleozoic, then Mesozoic, then Cenozoic. All the geologic time, from the Earth's origin till today, is divided into ten eras that makes up three eons. The first two eons are also informally referred to as Precambrian time (Super eon). Precambrian. is an established informal name for the 4 billion years of Earth history before hard-bodied organisms arose at the beginning of the Cambrian Period, earliest division of the Phanerozoic Eon. Phanerozoic means "visible life," Fossils are the recognizable remains, such as bones, shells, or leaves, or other evidence, such as tracks, burrows, or impressions, of past life on Earth. Fossils are fundamental to the geologic time scale. The names of most of the eons and eras end in zoic because these time intervals are often recognized on the basis of animal life. Rocks formed during the Proterozoic Eon may have fossils of relative simple organisms (maybe single celled), such as bacteria, algae, and wormlike animals. Rocks formed during the Phanerozoic Eon may have fossils of complex animals and plants such as dinosaurs, mammals, and trees. Petroleum geologists are mainly interested in rocks from the Cenozoic, Mesozoic, or Paleozoic Eras. This is because almost all of the oil and gas found so far is contained within these rocks. These rocks represent only a small fraction of the total age of the earth. referring to the first appearance of hard-shelled fossils at the beginning of the Cambrian Period.
Reservoir Rock Properties & its significance
A reservoir rock must be porous, permeable, and contain enough hydrocarbons (gas & liquid) to make it economically feasible for the operating company to drill for and produce them.
Porosity: PorosityÂ is a measure of the void spaces in a rock, and is a fraction of the volume of voids over the total volume, between 0-1, or as aÂ percentageÂ between 0-100 percent Porosity of a rock is a measure of rock's ability to hold a fluid. Porosity can be original inter granular porosity, solution porosity or fracture porosity.
Most oil and gas is produced from sandstones. Both porosity and permeability are required for production. Porosity creates the spaces to hold the oil or gas. Porosity is normally expressed as a percentage of the total rock which is taken up by pore space. For example sandstone may have 8% porosity. More important is the effective porosity, where pores are interconnected.
Permeability: Permeability is a measure of the amount of flow of a fluid through a rock. Permeability in petroleum-producing rocks is usually expressed in units called millidarcys. Permeability can be controlled by either porosity or fractures, which are formed due to some disjoint or fault in the rocks. Fractures can provideÂ permeabilityÂ for fluid movement, such as water orÂ hydrocarbons. Fractures are very important since they extend further than pores by orders of magnitude. Most reservoirs have permeabilities in the range of 100 - 500 md. Lower permeabilities may be used for gas production. In general, porosity decreases with burial.
Porosity and permeability may also be decreased by secondary precipitation of minerals as cement or grain overgrowths. On the other hand, solution of material by migrating ground waters can increase porosity and permeability, such as the preferential solution of fossil fragments and widening of fractures. About 60% of the world's petroleum reserves are held in sandstones, and 40% in limestones.
Different types of traps
TheÂ trapÂ is the impervious feature that ensures the juxtaposition of reservoir and seal such that hydrocarbons remain trapped in the subsurface, rather than escaping (due to their naturalÂ buoyancy) and being lost. Material of trap rock is generally having lesser permeability than the source rock through which fluid flows. The traps have been classified by petroleumÂ geologistsÂ into two types: structural and stratigraphic. A reservoir can be formed by one kind of trap or a combination of both.
Structural TrapsÂ are formed by a deformation in the rock layer that contains the hydrocarbons.Â Domes, anticlines and foldsÂ are common structures. Fault-related features also may be classified as structural traps if closure is â€Žpresent. Structural traps are the easiest to locate by surface and subsurface geological and geophysical studies. They are the most numerous among traps and have received a greater amount of attention in the search for oil than all other types of traps. These are again of two types. First one is the anticline trap in which rocks undergo bending for quite a long period of geologic time. These are found mainly in mountain ranges. The other one is the fault trap which is the result of fractures where one rock shifts relative to other.
Stratigraphic Traps are formed when other beds seal a reservoir bed or when theÂ permeabilityÂ changes within the reservoir bed itself. These are formed when lithological variation occurs in rock due to the deposition of reefs, channels or sand bars.This is referred as pinch type of trap. Stratigraphic traps can form against either younger or older time surfaces. It can also form due to the erosion or truncation of rock which is a truncated type of trap.
Combination Traps are formed due to the combination of tectonic movements & lithological variations in rock. In other words it is the combination of structural & stratigraphic traps. An example of this type of trap is the salt dome.
Classification of Reserves
ReservesÂ are those quantities of petroleum claimed to be commercially recoverable by application of development projects to known accumulations under defined conditions. All reserve estimates involve uncertainty, depending on the amount of reliable geologic and engineering data available and the interpretation of those data. The relative degree of uncertainty can be expressed by dividing reserves into two principal classifications:
These are those reserves claimed to have aÂ reasonable certaintyÂ (normally at least 90% confidence) of being recoverable under existing economic, operational method and political conditions, with existing technology. Industry specialists refer to this asÂ P90Â (i.e. having a 90% certainty of being produced).Â Proved reserves are further subdivided intoÂ Proved Developed andÂ Proved Undeveloped.Â Proved developed reserves are reserves that can be produced with existing wells and perforations, or from additional reservoirs where minimal additional investment (operating expense) is required. Proved Undeveloped reserves require additional capital investment (e.g. drilling new wells) to bring the oil to the surface. In general reserves are only proved when if the commercial producability is supported by actual production or formation test.
Based on geologic & engineering data these reserves are similar to proved reserves but due to technical, contractual, economic or other regulations prevents it from being proved. Unproved reserves may be used internally by oil companies and government agencies for future planning purposes. These again are classified into two reserves. First is probable reserves which are attributed to known accumulation & claims 50% possibility in recovery. The other one is possible reserve which is attributed to known accumulations which have a less likely chance of being recovered than probable reserves. This term is often used for reserves which are claimed to have at least a 10% certainty of being produced. Reasons for classifying reserves as possible include varying interpretations of geology, reserves not producible at commercial rates, uncertainty due to reserve infill (seepage from adjacent areas) and projected reserves based on future recovery methods.Â
Various methods of geological surveys which are used after knowing available literature & remote sensing are:
Oil Seeps: The knowledge that surface seepage has a direct link to subsurface oil and gas accumulations is not new and has been the stimulus for many of the world's early major oil and gas discoveries. It is just the expression of petroleum on the surface driven by the buoyant forces from the sub surface origins. It is a key bit of evidence which goes a long way in proving the presence of source in the basins.
Litho-stratigraphic succession establishment: The establishment of rock type change or lithologic change, both vertically in layering or bedding of varying rock types goes a long way in establishment of presence of confirmed traps. In an un-deformed stratigraphic sequence the oldest data occurs at the base of the sequence. Using the data of base strata the data for thickness and succession of rock column is computed which are used in the preparations of cross section.
Geologic Maps: AÂ geologic mapÂ orÂ geological mapÂ is a special-purposeÂ mapÂ made to showÂ geologicalÂ features.Â RockÂ units orÂ geologic strata are shown by color or symbols to indicate where they are exposed at the surface.Â Bedding planesÂ and structural features such as faults, folds,Â foliations, andÂ lineatesÂ are shown withÂ strike and dipÂ orÂ trend and plungeÂ symbols which give these features' three-dimensional orientations.
Basin Modelling: It is used to analyze the formation and evolution of sedimentary basins which may or may not aid in the evaluation of hydrocarbon reserves. It's just aid in the information of burial & thermal history of the basin.
Installing well casing is an important part of the drilling and completion process. Well casing consists of a series of metal tubes installed in the freshly drilled hole. Casing serves to strengthen the sides of the well hole, ensure that no oil or natural gas seeps out of the well hole as it is brought to the surface, and to keep other fluids or gases from seeping into the formation through the well. A good deal of planning is necessary to ensure that the proper casing for each well is installed. Types of casing used depend on the subsurface characteristics of the well, including the diameter of the well (which is dependent on the size of the drill bit used) and the pressures and temperatures experienced throughout the well. In most wells, the diameter of the well hole decreases the deeper it is drilled, leading to a type of conical shape that must be taken into account when installing casing.Â
Reservoir Drive Mechanism
Producing oil and gas needs energy. Usually some of this required energy is supplied by nature. The hydrocarbon fluids are under pressure because of their depth. The gas and water in petroleum reservoirs under pressure are the two main sources that help move the oil to the well bore and sometimes up to the surface. Depending on the original characteristics of hydrocarbon reservoirs, the type of driving energy is different.
i) Solution gas drive reservoirs: When a newly discovered reservoir is below the bubble point pressure, there will be free gas as bubbles within the oil phase in reservoir. The reservoir pressure decreases as production goes on and this causes emerging and expansion of gas bubbles creating extra energy in the reservoir. These kinds of reservoirs are called as solution gas drive reservoirs. Crude oil under high pressure may contain large amounts of dissolved gas. When the reservoir pressure is reduced as fluids are withdrawn, gas comes out of the solution and displaces oil from the reservoir to the producing wells. The efficiency of solution gas drive depends on the amount of gas in solution, the rock and fluid properties and the geological structure of the reservoir. Recoveries are low, on the order of 10-15 % of the original oil in place (OOIP). Recovery is low, because the gas phase is more mobile than the oil phase in the reservoir.
ii) Gas Cap drive reservoir: Sometimes, the pressure in the reservoir is below the bubble point initially, so there is more gas in the reservoir than the oil can retain in solution. This extra gas, because of density difference, accumulates at the top pf the reservoir and forms a cap. These kinds of reservoirs are called a gas cap drive reservoir. In gas cap drive reservoirs, wells are drilled into the crude oil producing layer of the formation. As oil production causes a reduction in pressure, the gas in gas cap expands and pushes oil into the well bores. Expansion the gas cap is limited by the desired pressure level in the reservoir and by gas production after gas comes into production wells.
iii) Water gas drive reservoir: Most oil or gas reservoirs have water aquifers. When this water aquifer is an active one, continuously fed by incoming water, then this bottom water will expand as pressure of the oil/gas zone is reduced because of production causing an extra driving energy. This kind of reservoir is called water drive reservoirs. The expanding water also moves and displaces oil or gas in an upward direction from lower parts of the reservoir, so the pore spaces vacated by oil or gas produced are filled by water. The oil and gas are progressively pushed towards the well bore.
Enhanced oil recovery or EOR refers to all methods other than that of primary & secondary processes which aims at increasing the capacity of the well. It enhances the flowability of the oil in the reservoir by one mean or the other.
EOR introduces fluids that reduce viscosity and improve flow. These fluids could consist of gases that are miscible with oil (typically carbon dioxide), steam, air or oxygen, polymer solutions, gels, surfactant-polymer formulations, alkaline-surfactant-polymer formulations, or microorganism formulations.
Some methods for enhanced oil recovery are:
Thermal EOR process: It involves pumping in steam or using the capped gas for combustion to increase temperature inside the reservoir so that viscosity of the fluid decreases & the amount of fluid flowing out increases.
Miscible EOR process: It involves pumping in supercritical carbon dioxide, nitrogen or other gas which mixes with the fluid at high pressure & increase the flowability of the fluid such that the capacity of the reservoir goes up. Also due to the pressure of these gases the oil is displaced out.
Chemical EOR process: The chemical flooding processes include polymer flooding, surfactant polymer flooding & alkaline surfactant polymer flooding. The mechanisms of chemical methods vary depending on the chemical material added to the reservoir; chemical methods may achieve one or more effects: interfacial tension reduction, wettability, alteration, emulsification, or mobility control
NELP and significance in Indian oil and gas scenario
In order globalize the activities of domestic oil and gas sector; various initiatives have been taken to basically mobilize the large amount of capital risk required for this sector. The new exploration and licensing policy (NELP) formulated by Indian government is a part of those initiatives which has brought in new investment in the oil and gas exploration activities besides the contribution of national oil company. The salient features associated with it are:
Award of licenses through international competitive bidding.
Fast track approval mechanism through single window empowered committee of secretaries.
Up to 100% foreign participation.
No acreages on nomination basis
NOCs to compete in the bidding rounds
An internationally competitive fiscal regime
Model production sharing contract (MPSC) & petroleum tax guide in place
Exploration investment of Rs 200 billion in three phases of 90 NELP blocks - which is expected to go up substantially with discoveries. 90 contracts signed under NELP in the last four years, as against 22 contracts in the 10 years preceding NELP.
With the coming of NELP in the Indian oil and gas scenario, market has opened up to a lot many players which have requisite experience & technology in the exploration and drilling of oil and gas. The discovery of blocks in Krishna Godavari basin by Reliance in joint venture with Niko or the fields in Barmer (in Rajasthan) discovered by the Cairns Energy are few of the results of Indian ministry going for a liberal change as far as its exploration policy is concerned. Also partnership with the big public players have also ensured that some control remains in the natural resources produced by these players.
a) What is an offshore platform and how is it different than onshore drilling rig?
AnÂ offshore platform, often referred to as anÂ oil platformÂ or anÂ oil rig, is a lÐ°rge structure used to house workers and machinery needed to drill wells in the ocean bed, extractÂ oilÂ and/orÂ natural gas, process the produced fluids, and ship or pipe them to shore. Depending on the circumstances, the platform may beÂ fixedÂ to the ocean floor, may consist of anÂ artificial island, or mayÂ float. Most offshore platforms are located on theÂ continental shelf, though with advances in technology and increasing crude oil prices, drilling and production in deeper waters has become both feasible and economically viable. A typical platform may have around thirty wellheads located on the platform andÂ directional drilling allows reservoirs to be accessed at both different depths and at remote positions up to 5 miles (8 kilometers) from the platform. Some of the common offshore structures are shallow water complex, gravity base, compliant towers and floating production or FPSO.
Offshore oil and gas production is more challenging than land-based installations due to the remote and harsher environment. Most of the innovation in the offshore petroleum sector concerns overcoming these challenges, including the need to provide very large production facilities.
b) What are the differences between (i) a jack-up, (ii) a submersible and (iii) a semisubmersible drilling rig?
A Jack Up: AÂ jack-up rigÂ is a type of mobile platform that is able to stand still on theÂ sea floor, resting on a number of supporting legs. The most popular designs use 3 independent legs; although some jack up have 4 legs or more. It is a floating barge fitted with long support legs that can be raised or lowered. The jack up is towed (or self propelled) onto location with its legs up and the barge section floating on the water. Upon arrival at the drilling location, the legs are jacked down onto the seafloor. Then preloading takes place, where the weight of the barge and additional ballast water are used to drive the legs securely into the sea bottom so they will not penetrate further while operations are carried out. After preloading, the jacking system is used to raise the entire barge and drilling structure above the water to a predetermined height or air gap, so that wave, tidal and current loading acts only on the relatively slender legs and not on the barge hull.
Submersible Drilling Rig: A submersible drilling rig is aÂ marine vesselÂ design that can be floated to location and lowered onto the sea floor forÂ offshore drillingÂ activities. The submersibleÂ drilling platformÂ is supported on largeÂ pontoon-like structures. These pontoons provide buoyancy allowing the unit to be towed from location to location.
Once on the desired location, the pontoon structure is slowly flooded until it rests on the sea floor. The operating deck is elevated 100 feet above the pontoons on large steel columns to provide clearance above the waves. After the well is drilled, the water is pumped out of the buoyancy tanks and the vessel is re-floated and towed to the next location.
Submersibles, as they are known informally, operate in relatively shallow water, since they must rest on the sea floor.Â
Semi-submersible Drilling rigs: AÂ semi-submersibleÂ drilling rig is a specializedÂ vesselÂ with good stability and sea keeping characteristics. Semi-submersible rigs are typically configured with large buoyantÂ pontoonÂ structures below the water surface and slender columns passing through the water surface supporting a platform deck at a significant height above the sea surface. A semi-submersible vessel may be able to transform from a deep to a shallowÂ draftÂ by de ballasting (removing ballast water from the hull), and thereby become a surface vessel. With its hull structure submerged at a deep draft, the semi-submersible is less affected by wave loadings than a normal ship. With a small water-plane area however, the semi-submersible is sensitive to load changes, and therefore must be carefullyÂ trimmedÂ to maintain stability. Unlike submersible, a semi-submersible vessel is never entirely underwater.
a) What are the functions of drilling fluid used in drilling a well?
The main functions of aÂ drilling mudÂ can be summarized as follows:
Remove cuttings from the well: Drilling fluid (mud in general) carries the rock excavated by the drill bit to the surface. It depends on the various properties of the fluid some of which are viscosity & density. The velocity of the fluid is very much important in order to carry the rock bits up to the surface.
Suspend and release cuttings: Drilling fluid helps in suspending drill cuttings, weight materials and additives under wide range of conditions so that it does not cause bridges and stuck pipes.
Controls formation pressure: If formation pressure increases, mud density should also be increased to balance pressure and keep the wellbore stable. Unbalanced formation pressures will cause an unexpected influx of pressure in the wellbore possibly leading to aÂ blowoutÂ from pressured formation fluids.
Maintain wellbore stability: Chemical composition and mud properties must combine to provide a stable wellbore. Weight of the mud must be within the necessary range to balance the mechanical forces.
Minimizing formation damage: Skin damage or any reduction in natural formation porosity and permeability (washout) constitutes formation damage. Specially designed drill-in fluids or work over and completion fluids minimize formation damage.
Cool, lubricate and support the bit and drilling assembly: Heat is generated from mechanical and hydraulic forces at the bit and when the drill string rotates and rubs against casing and wellbore. Cool and transfer heat away from source and lower temperature than bottom hole. Also works as lubricant for the drilling bit.
Controls corrosion: By removing the drill & rock bits it helps in preventing erosion. Specific chemicals are mixed in the drilling fluids to prevent piping from corrosion also.
b) Which are the basic groups of well logs to be taken in an exploratory well? Explain their purpose.
Well loggingÂ is the practice of making a detailed record of theÂ geologicÂ formations penetrated by a borehole. The basic groups of well logs taken in exploratory wells are:
Mud Logging: Mud logging is the creation of a detailed record (well log) of a borehole by examining the bits of rock or sediment brought to the surface by the circulating drilling medium (most commonly mud). Mud logging is usually performed by a third-partyÂ mudÂ logging company. This provides well owners and producers with information about theÂ lithologyÂ and fluid content of the borehole while drilling.
Wireline Logging: In wireline measurements, the logging tool (or probe) is lowered into the open wellbore on a multiple conductor, contra-helically armored wireline. Once lowered to the bottom of the interval of interest, the measurements are taken on the way out of the wellbore. This is done in an attempt to maintain tension on the cable (which stretches) as constant as possible for depth correlation purposes.
Caliper Log: The caliper log measures the variation in bore hole diameter as it is withdrawn from the bottom of the hole. It is constructed with two or more articulated arms that push against the bore hole wall to takeÂ measurements. The arms show variable movements of the cursor by measuring electrical resistance, creating electrical variation. The variation in output is translated into changes of diameter after a simple calibration. The caliper log is printed as a continuous series of values of hole diameter with depth.
Electrical Logs: The logging procedure consists of lowering a 'logging tool' on the end of a wireline into anÂ oil wellÂ (or hole) to measure the rock and fluid properties of the formation. An interpretation of these measurements is then made to locate and quantify potential depth zones containing oil and gas. This data is recorded to a printed record called a "well log" and is normally transmitted digitally to office locations.
What are the conditions necessary for generation, migration and accumulation of petroleum in a basin?