Basic Petroleum Engineering interview questions

150 questions with answers (PET-001 to PET-150). Try each one aloud before reading the answer.

Petroleum Geology

PET-001 · What is petroleum?

A naturally occurring mixture of hydrocarbons, with minor nitrogen, sulfur, oxygen compounds and trace metals, found as gas, liquid or solid. It forms from organic matter buried in sediments and transformed by heat and pressure over geological time.

PET-002 · What is a source rock?

A fine-grained, organic-rich sedimentary rock, usually shale or carbonate mudstone, that has generated or can generate hydrocarbons when buried and heated enough. Good source rocks commonly have total organic carbon above about 1 wt%.

PET-003 · What is a reservoir rock?

A porous and permeable rock that stores hydrocarbons and lets them flow to a well. Most reservoirs are sandstones or carbonates (limestone and dolomite); fractured basement and volcanic rocks are less common examples.

PET-004 · What is a cap rock or seal?

A low-permeability rock above the reservoir, typically shale, salt or anhydrite, whose high capillary entry pressure stops hydrocarbons from migrating further upward.

PET-005 · What is a reservoir trap?

A geological configuration that stops hydrocarbon migration so they accumulate. Structural traps include anticlines and fault traps; stratigraphic traps include pinch-outs, unconformities and reefs; combination traps have both elements.

PET-006 · What is kerogen and what are its types?

Kerogen is insoluble organic matter in sedimentary rock that yields oil and gas when heated. Type I (lacustrine algae) and Type II (marine) are oil-prone, Type III (terrestrial plants) is gas-prone, and Type IV is inert.

PET-007 · What is the oil window?

The temperature range, roughly 60–120 °C, in which kerogen mainly generates oil. At higher temperatures oil cracks to gas (the gas window). The matching depth depends on the local geothermal gradient.

PET-008 · What are primary and secondary migration?

Primary migration is expulsion of hydrocarbons from the source rock into permeable carrier beds. Secondary migration is their movement through carrier beds into a trap, driven mainly by buoyancy.

PET-009 · What is a petroleum system?

The set of geological elements (source, reservoir, seal, overburden rock) and processes (trap formation, generation, migration, accumulation) needed for a hydrocarbon accumulation. Timing is critical: the trap must exist before migration.

PET-010 · What is an anticline trap?

An upward-arched fold in which hydrocarbons rise to the crest and are held beneath an impermeable seal. It is the most common type of structural trap.

PET-011 · How do clastic and carbonate reservoirs differ?

Clastic reservoirs (sandstones) are made of transported grains with mainly intergranular porosity. Carbonate reservoirs (limestone, dolomite) form largely from biological and chemical processes and often have vuggy, moldic or fracture porosity, making them more heterogeneous.

PET-012 · What is a seismic survey?

A survey that sends sound waves into the earth and records the reflections from layer boundaries with geophones (land) or hydrophones (marine). Processed data image subsurface structure and help locate traps.

PET-013 · What is the difference between a play, a lead and a prospect?

A play is a group of fields or prospects that share common geology. A lead is a poorly defined potential trap. A prospect is a lead that has been mapped and evaluated well enough to be drilled.

Petrophysics & Well Logging

PET-014 · What is the difference between porosity and permeability?

Porosity is the fraction of rock bulk volume that is pore space, so it measures storage capacity. Permeability measures the rock's ability to transmit fluid, usually in millidarcies, and depends on how well the pores are connected.

PET-015 · What is the difference between total and effective porosity?

Total porosity is all pore volume divided by bulk volume. Effective porosity counts only the interconnected pores that can contribute to flow, excluding isolated pores and clay-bound water.

PET-016 · What is fluid saturation?

The fraction of pore volume occupied by a given fluid. Oil, water and gas saturations add up to one: So + Sw + Sg = 1.

PET-017 · What is a well log?

A continuous record of rock and fluid properties against depth, measured by tools run on wireline after drilling or by logging-while-drilling (LWD) tools in the drill string.

PET-018 · What does a gamma ray log measure?

Natural radioactivity from potassium, thorium and uranium. Shales usually read high and clean sands and carbonates read low, so the log is used for lithology, shale volume and well-to-well correlation.

PET-019 · What does a resistivity log indicate?

The electrical resistivity of the formation. Oil and gas are non-conductive, so a hydrocarbon-bearing zone reads higher resistivity than the same rock saturated with conductive brine.

PET-020 · What is the spontaneous potential (SP) log?

A record of natural voltage caused by electrochemical differences between mud filtrate and formation water. It identifies permeable beds and helps estimate formation water resistivity, but it does not work in oil-based mud.

PET-021 · How does a density log measure porosity?

A gamma source (usually Cs-137) irradiates the formation and detectors count scattered gamma rays, giving bulk density ρb. Porosity is φ = (ρma − ρb)/(ρma − ρf), with ρma ≈ 2.65 g/cm³ for sandstone.

PET-022 · What does a neutron log measure?

The hydrogen index of the formation. Hydrogen in water and oil slows neutrons, so the reading tracks liquid-filled porosity. Gas has a low hydrogen density and makes neutron porosity read too low.

PET-023 · What does a sonic log measure?

The interval transit time Δt of a compressional wave, in μs/ft. Porosity is estimated with the Wyllie time-average equation φ = (Δt − Δtma)/(Δtf − Δtma). Sonic data also calibrate seismic.

PET-024 · What is Archie's equation?

Swⁿ = a·Rw/(φᵐ·Rt), relating water saturation to formation water resistivity Rw, true resistivity Rt and porosity φ. Typical clean-sand constants are a = 1 and m = n = 2.

PET-025 · What is net pay?

The thickness of reservoir that contains producible hydrocarbons, after applying cutoffs on porosity, shale volume, water saturation and sometimes permeability to the gross interval.

PET-026 · What is a core and why is it taken?

A cylindrical rock sample cut from the formation. It gives direct measurements of porosity, permeability, saturations, lithology and rock mechanics, and it is used to calibrate log interpretation.

Reservoir Engineering

PET-027 · What is API gravity?

A measure of how heavy or light a liquid petroleum is relative to water: °API = (141.5/SG) − 131.5, with SG at 60 °F. Water is 10 °API; higher API means lighter crude.

PET-028 · What is bubble point pressure?

The pressure at reservoir temperature at which the first bubble of gas comes out of solution as pressure falls. Above it the oil is undersaturated; below it free gas forms.

PET-029 · What is the oil formation volume factor (Bo)?

The ratio of oil volume at reservoir conditions, including dissolved gas, to its volume at stock-tank conditions, in rb/STB. It is greater than 1 because oil shrinks as gas leaves and it cools.

PET-030 · What is the solution gas-oil ratio (Rs)?

The volume of gas, in standard cubic feet, dissolved in one stock-tank barrel of oil at given reservoir pressure and temperature. It is constant above bubble point and falls below it.

PET-031 · What is the difference between saturated and undersaturated reservoirs?

An undersaturated reservoir is above bubble point, so all gas is dissolved and there is no free gas. A saturated reservoir is at or below bubble point and may have a gas cap or free gas.

PET-032 · What is the volumetric equation for oil in place?

N = 7758·A·h·φ·(1 − Sw)/Boi, giving stock-tank barrels, with A in acres, h in ft and Boi in rb/STB. 7758 is the number of barrels in one acre-foot.

PET-033 · What is recovery factor?

The fraction of the initial hydrocarbons in place that is produced or expected to be produced. It depends on drive mechanism, rock and fluid properties, development plan and economics.

PET-034 · What is the difference between reserves and resources?

Resources are all estimated quantities of hydrocarbons, discovered or not. Reserves are the subset that is discovered, commercially recoverable from a given date under defined conditions, and they are classified as proved (1P), probable (2P) and possible (3P).

PET-035 · What is relative permeability?

The ratio of effective permeability to one phase to the absolute permeability, kr = ke/k, ranging from 0 to 1. It depends on saturation and describes how fluids interfere when flowing together.

PET-036 · What is wettability?

The tendency of a rock surface to be preferentially in contact with one fluid when others are present. Reservoirs can be water-wet, oil-wet or mixed-wet; it is measured by contact angle, Amott or USBM tests.

PET-037 · What is capillary pressure?

The pressure difference across the interface of two immiscible fluids in a pore: Pc = Pnw − Pw = 2σ·cosθ/r. It controls the transition zone above the free water level and initial saturation distribution.

PET-038 · What are the five main reservoir fluid types?

Black oil, volatile oil, gas condensate (retrograde gas), wet gas and dry gas. They are classified by phase behaviour, initial GOR and stock-tank liquid colour and gravity.

PET-039 · What does a pressure-temperature phase diagram show?

The bubble-point and dew-point curves that bound the two-phase envelope and meet at the critical point. It also marks the cricondenbar (highest pressure) and cricondentherm (highest temperature) at which two phases can coexist.

Drilling Engineering

PET-040 · What are the main components of a drilling rig?

The hoisting system (derrick or mast, drawworks, crown and travelling blocks), rotating system (rotary table or top drive), circulating system (mud pumps, standpipe, shale shakers), power system, well control system (BOP stack) and the drill string.

PET-041 · What makes up the drill string?

Drill pipe, heavy-weight drill pipe and the bottomhole assembly (BHA). The BHA contains drill collars, stabilisers, measurement tools, possibly a motor or rotary steerable tool, and the bit.

PET-042 · What is the purpose of drill collars?

Drill collars are heavy, thick-walled tubulars at the bottom of the string. They supply weight on bit and keep the lighter drill pipe above them in tension, which prevents buckling and fatigue.

PET-043 · What is the difference between roller cone and PDC bits?

Roller cone (tricone) bits have rotating cones with teeth or inserts that crush and gouge rock. PDC bits have fixed polycrystalline diamond cutters that shear rock, and usually drill faster and last longer in soft to medium formations.

PET-044 · What is rate of penetration (ROP)?

The speed at which the bit deepens the hole, in ft/hr or m/hr. It depends on weight on bit, rotary speed, bit type, hydraulics, mud properties and rock strength.

PET-045 · What types of offshore drilling rigs exist?

Jackups for shallow water, semisubmersibles and drillships for deep water, platform rigs on fixed or floating production structures, and barges or tenders for very shallow or sheltered water.

PET-046 · Why is casing run in a well?

Casing is steel pipe cemented in the hole. It prevents the hole from collapsing, protects freshwater zones, isolates zones with different pressures, and provides a pressure-tight anchor for the BOP and wellhead.

PET-047 · What are the main casing strings?

From the outside in: conductor, surface casing, intermediate casing and production casing. A liner is a casing string hung from the bottom of the previous string rather than run back to surface.

PET-048 · What is directional drilling and why is it used?

Deliberately steering a well along a planned non-vertical path. It is used to drill many wells from one pad or platform, reach targets under inaccessible locations, drill horizontal wells, sidetrack and drill relief wells.

PET-049 · What is the difference between measured depth and true vertical depth?

Measured depth (MD) is the length along the wellbore. True vertical depth (TVD) is the vertical distance from the reference point. Hydrostatic pressure depends on TVD; friction and pipe length depend on MD.

PET-050 · What is the kick-off point?

The depth at which the well begins to deviate from vertical in a directional well.

PET-051 · What is a mud motor?

A positive displacement motor in the BHA that uses mud flow through a rotor-stator (Moineau) section to turn the bit. With a bent housing it can steer the well while the drill string slides without rotating.

PET-052 · What is tripping?

Pulling the drill string out of the hole (POOH) or running it back in (RIH), for example to change the bit or BHA, stand by stand.

Drilling Fluids & Cementing

PET-053 · What is drilling mud?

The fluid circulated down the drill string, out through the bit and up the annulus. It can be water-based, oil- or synthetic-based, or a gaseous system such as air, mist or foam.

PET-054 · What does bentonite do in drilling mud?

Bentonite, a sodium montmorillonite clay, hydrates and swells in fresh water. It gives the mud viscosity and gel strength and helps form a thin, low-permeability filter cake.

PET-055 · What is barite used for?

Barite (barium sulfate, SG about 4.2) is the standard weighting material added to raise mud density so that hydrostatic pressure controls formation pressure.

PET-056 · How is hydrostatic pressure of a mud column calculated?

P (psi) = 0.052 × mud weight (ppg) × TVD (ft). In SI units P = ρ·g·h.

PET-057 · What is a filter cake?

A layer of mud solids deposited on the borehole wall as liquid filtrate is pushed into a permeable formation. A thin, tough, low-permeability cake limits fluid loss and formation damage.

PET-058 · What is mud filtrate?

The liquid phase of the mud that passes through the filter cake into the formation. Its volume and chemistry affect invasion, formation damage and log readings.

PET-059 · What are the pros and cons of water-based versus oil-based mud?

Oil-based and synthetic muds give better shale inhibition, lubricity and temperature stability, but cost more and have stricter disposal rules. Water-based muds are cheaper and environmentally easier but can let reactive shales swell.

PET-060 · What is a shale shaker?

A vibrating screen that removes drilled cuttings from returning mud. It is the first stage of solids control, followed by desanders, desilters and centrifuges.

PET-061 · Why are wells cemented?

To bond the casing to the formation, isolate zones so fluids cannot move behind pipe, support the casing and protect it from corrosive formation fluids.

PET-062 · What is the difference between primary and remedial cementing?

Primary cementing places cement behind casing right after it is run. Remedial cementing repairs problems later, either by squeeze cementing into channels or perforations or by setting cement plugs for abandonment, sidetracks or lost circulation.

PET-063 · What are API classes of well cement?

API cement classes A to H are defined by depth and temperature range and sulfate resistance. Classes G and H are the most widely used basic well cements, adjusted with additives for each job.

PET-064 · What is a centralizer?

A device fitted on casing that keeps it centred in the hole so cement can form a uniform sheath all around the pipe.

Well Control

PET-065 · What is a kick?

An unwanted influx of formation fluid (gas, oil or water) into the wellbore, which happens when formation pressure exceeds the pressure exerted by the wellbore fluid.

PET-066 · What is a blowout?

An uncontrolled flow of formation fluids, either to surface or from one formation into another (an underground blowout), after well control has been lost.

PET-067 · What are primary, secondary and tertiary well control?

Primary control is keeping bottomhole pressure above pore pressure with the mud column. Secondary control is closing the BOP and circulating out a kick. Tertiary control covers measures after secondary control fails, such as relief wells, capping or barite and cement plugs.

PET-068 · What is formation (pore) pressure?

The pressure of fluids in the pore space. A normal pore pressure gradient equals the gradient of formation water, about 0.433–0.465 psi/ft depending on salinity.

PET-069 · What are abnormal and subnormal pressures?

Abnormal (over-) pressure is above the normal hydrostatic gradient, often caused by undercompaction or fluid expansion. Subnormal pressure is below it, as in depleted reservoirs.

PET-070 · What is fracture pressure?

The pressure at which the formation breaks down and accepts fluid. It sets the upper limit for mud weight and equivalent circulating density.

PET-071 · What are the warning signs of a kick?

Pit gain, increase in return flow, flow with pumps off, a sudden drilling break, falling pump pressure with rising pump strokes, and incorrect hole fill during trips.

PET-072 · What is the difference between an annular preventer and a ram preventer?

An annular preventer uses a doughnut-shaped elastomer element that can seal around almost any pipe size or an open hole. Ram preventers close steel rams on a specific pipe size (pipe or variable-bore rams), on an open hole (blind rams) or cut pipe (shear rams).

PET-073 · What is a choke manifold?

An assembly of valves and adjustable chokes connected to the BOP. It holds back-pressure on the well so bottomhole pressure stays constant while a kick is circulated out.

PET-074 · What are swab and surge pressures?

Pulling pipe out reduces bottomhole pressure (swab) and can draw in a kick. Running pipe in raises bottomhole pressure (surge) and can fracture the formation. Both increase with tripping speed.

PET-075 · What are SIDPP and SICP?

Shut-in drill pipe pressure and shut-in casing pressure, read after the well is closed on a kick. SIDPP shows how far pore pressure exceeds the drill pipe mud hydrostatic.

PET-076 · What is a trip tank?

A small, calibrated tank used during trips to measure whether the hole takes the right amount of mud when pipe is pulled, or returns the right amount when pipe is run.

Well Completion & Stimulation

PET-077 · What is well completion?

The work done after drilling to make a well ready to produce or inject safely. It includes perforating, running tubing, packers and safety valves, installing the wellhead and tree, and any sand control or stimulation.

PET-078 · What is the difference between open-hole and cased-hole completions?

In an open-hole completion the reservoir section is left uncased, giving maximum contact but little zonal control. In a cased-hole completion casing or a liner is cemented across the reservoir and then perforated, allowing selective zone isolation.

PET-079 · What is perforating?

Creating flow paths through casing and cement into the formation, normally with shaped explosive charges in a perforating gun conveyed on wireline, tubing or coiled tubing.

PET-080 · What is a production packer?

A downhole device that seals the annulus between tubing and casing. It isolates zones, keeps well fluids off the casing and lets the annulus be filled with an inhibited packer fluid.

PET-081 · Why is production tubing used?

Tubing is a replaceable conduit inside the casing that carries produced fluid to surface. Its size controls flow velocity, and it protects the casing from pressure, erosion and corrosion.

PET-082 · What is a Christmas tree?

The assembly of valves on top of the wellhead, including master, wing and swab valves and a choke. It controls flow from the well and gives access for intervention.

PET-083 · What is the difference between a wellhead and a Christmas tree?

The wellhead is the pressure-containing base that suspends and seals the casing and tubing strings. The tree is installed on top of it and controls the produced flow.

PET-084 · What is a subsurface safety valve?

A fail-safe valve in the tubing, usually surface-controlled through a hydraulic line, that closes automatically if control pressure is lost. It shuts the well in below surface during an emergency.

PET-085 · What is acidizing?

Pumping acid into the well to remove near-wellbore damage or create new flow paths. HCl is used in carbonates, and HCl–HF mud acid is used to dissolve clays and fines in sandstones.

PET-086 · What is the difference between matrix acidizing and acid fracturing?

Matrix acidizing injects acid below fracture pressure so it flows into the pores. Acid fracturing injects above fracture pressure and the acid etches the fracture faces of carbonate rock so the fracture stays conductive without proppant.

PET-087 · What is proppant?

Granular material, such as sand, resin-coated sand or ceramic beads, carried by fracturing fluid into a hydraulic fracture to hold it open after pumping stops.

PET-088 · What is sand control?

Methods to stop formation sand from entering the well, such as standalone screens, gravel packs, frac packs, expandable screens and chemical consolidation. Sand production erodes equipment and can plug the well.

Production Engineering & Artificial Lift

PET-089 · What is natural flow?

Production in which reservoir pressure alone is enough to push fluid from the formation up the tubing and through the surface facilities.

PET-090 · What is artificial lift and when is it needed?

Any method that adds energy to lift fluid to surface. It is needed when reservoir pressure is too low for natural flow, or to raise rates by lowering flowing bottomhole pressure, often as water cut rises.

PET-091 · What are the main types of artificial lift?

Sucker rod (beam) pumping, electrical submersible pumps (ESP), gas lift, progressive cavity pumps (PCP), hydraulic jet or piston pumps and plunger lift.

PET-092 · How does a sucker rod pump work?

A surface pumping unit moves a rod string up and down, driving a downhole plunger pump. On the upstroke the travelling valve closes and the standing valve opens to fill the barrel; on the downstroke the valves reverse.

PET-093 · What is an electrical submersible pump (ESP)?

A multistage centrifugal pump driven by a downhole electric motor, powered through a cable from surface. It suits high liquid rates but is sensitive to free gas, sand and high temperature.

PET-094 · How does gas lift work?

Gas is injected through valves in the tubing into the produced fluid. This lowers the density of the fluid column and its hydrostatic pressure, so the reservoir can push fluid to surface.

PET-095 · What is a progressive cavity pump?

A pump with a helical steel rotor turning inside an elastomer stator, forming sealed cavities that move fluid upward. It handles viscous oil and sand well but the elastomer limits temperature and some chemistries.

PET-096 · What is the productivity index?

J = q/(P̄r − Pwf), in STB/d/psi, the rate produced per unit of pressure drawdown. It is constant for single-phase liquid flow above bubble point.

PET-097 · What is an inflow performance relationship (IPR)?

A curve of production rate against flowing bottomhole pressure that describes how the reservoir delivers fluid to the well. It is a straight line above bubble point and curved below it.

PET-098 · What is water cut?

The fraction of produced liquid that is water: WC = qw/(qo + qw), usually expressed as a percentage.

PET-099 · What is producing gas-oil ratio?

The volume of gas produced per stock-tank barrel of oil, in scf/STB. A rising GOR may indicate falling pressure below bubble point or gas coning.

PET-100 · What is a production choke?

A fixed or adjustable restriction at the wellhead that controls flow rate and protects the reservoir and equipment. At critical flow the rate becomes independent of downstream pressure.

PET-101 · What is wax (paraffin) deposition?

Precipitation of high-molecular-weight paraffins when oil cools below its wax appearance temperature. Wax builds up on tubing and pipeline walls and is controlled by insulation, heating, inhibitors, scraping or pigging.

Well Testing

PET-102 · What is a well test?

A controlled change in a well's rate, with downhole pressure and rates measured over time. Analysing the pressure response gives reservoir properties such as permeability, skin, pressure and boundaries.

PET-103 · What is a drawdown test?

A test in which a shut-in, stabilised well is produced at a constant rate while the flowing bottomhole pressure is recorded as it falls.

PET-104 · What is a buildup test?

A test in which a producing well is shut in and the rise in bottomhole pressure is recorded. Zero rate during the shut-in makes it easier to control than a drawdown.

PET-105 · What is a drill stem test (DST)?

A temporary completion using the drill string, packers and valves to flow and shut in a formation during drilling. It gives early pressure, rate, permeability and fluid samples.

PET-106 · What is an injection falloff test?

An injection well is shut in after injecting at a steady rate and the falling pressure is analysed, much like a buildup test in a producer.

PET-107 · What is an interference test?

The rate is changed in an active well and the pressure response is measured in one or more observation wells. It shows connectivity and gives interwell permeability and storativity.

PET-108 · What is a production (well rate) test?

Routing a well through a test separator to measure its oil, gas and water rates. It is done periodically for allocation, surveillance and checking well performance.

PET-109 · What is wellbore storage?

The effect where the sandface rate lags behind a surface rate change because fluid in the wellbore expands or compresses, or the liquid level moves. It dominates early-time test data.

PET-110 · What is radius of investigation?

The distance into the reservoir that a pressure transient has noticeably moved by a given test time. It grows with permeability and time.

PET-111 · What is average reservoir pressure?

The pressure the drainage area would reach if all wells were shut in long enough for pressure to equalise. It is a key input to material balance and productivity index.

PET-112 · What gauges are used in well testing?

Downhole quartz or strain-gauge pressure and temperature gauges, either memory gauges retrieved later or surface-readout gauges, plus surface separators and flow meters for rates.

PET-113 · What are the main flow regimes in a well test?

Early-time wellbore storage, then infinite-acting radial flow (used for permeability and skin), then late-time boundary effects or pseudo-steady state when the whole drainage area is felt.

Enhanced Oil Recovery

PET-114 · What is enhanced oil recovery (EOR)?

Recovery by injecting energy or substances not normally present in the reservoir, such as steam, CO₂, miscible gas, polymers or surfactants, to change fluid or rock behaviour and recover oil that primary and waterflooding leave behind.

PET-115 · What is the difference between IOR and EOR?

Improved oil recovery (IOR) is any action that raises recovery, including infill drilling, waterflooding and better well management. EOR is the narrower set of thermal, gas and chemical injection methods.

PET-116 · What is waterflooding?

Injecting water into the reservoir to maintain pressure and physically sweep oil toward producers. It is the most common secondary recovery method.

PET-117 · What is thermal EOR?

Methods that heat the reservoir, mainly steam injection and in-situ combustion, to cut the viscosity of heavy oil and bitumen so it can flow.

PET-118 · What is cyclic steam stimulation?

Also called huff-and-puff: steam is injected into a well, the well is shut in to soak, and then the same well produces the heated oil. The cycle is repeated.

PET-119 · What is steamflooding?

Continuous steam injection into injector wells, forming a heated zone that drives oil toward separate producers.

PET-120 · What is miscible gas injection?

Injecting gas such as CO₂, enriched hydrocarbon gas or nitrogen at pressures where it becomes miscible with oil. With no interface between the fluids, capillary trapping is removed and residual oil can be displaced.

PET-121 · What is polymer flooding?

Adding water-soluble polymer to injected water to raise its viscosity. This lowers the mobility ratio and improves sweep efficiency.

PET-122 · What is surfactant flooding?

Injecting surfactants that cut oil-water interfacial tension to very low values, so capillary-trapped residual oil can be mobilised.

PET-123 · What is alkaline flooding?

Injecting an alkali such as sodium carbonate, which reacts with acidic components in the crude to form natural surfactants in place. It also reduces surfactant adsorption on rock.

PET-124 · What is mobility ratio?

M = (krw/μw)/(kro/μo), the mobility of the displacing fluid divided by that of the displaced oil. M ≤ 1 is favourable; M > 1 causes fingering and early breakthrough.

PET-125 · What is sweep efficiency?

The fraction of the reservoir contacted by the injected fluid. Volumetric sweep is areal times vertical sweep, and overall recovery efficiency is displacement efficiency times volumetric sweep.

Unconventional Resources

PET-126 · What are unconventional resources?

Hydrocarbon accumulations that cannot be produced economically without large-scale stimulation or special recovery methods. Examples are shale gas and oil, tight gas, coalbed methane, oil sands and gas hydrates.

PET-127 · What is shale gas?

Natural gas held in organic-rich shale that acts as both source and reservoir. Matrix permeability is in the nanodarcy range, so wells need horizontal drilling and multistage hydraulic fracturing.

PET-128 · What is tight gas?

Gas in low-permeability sandstones or carbonates, often defined as having in-situ permeability below about 0.1 mD. It usually needs hydraulic fracturing to flow at economic rates.

PET-129 · What is coalbed methane?

Methane stored mainly by adsorption on the internal surfaces of coal. Wells are produced by pumping off water to lower pressure, which makes the gas desorb and flow through natural fractures called cleats.

PET-130 · What are oil sands?

Sands saturated with bitumen too viscous to flow at reservoir conditions. Shallow deposits are mined; deeper ones are produced in situ with thermal methods such as SAGD or cyclic steam stimulation.

PET-131 · What are gas hydrates?

Ice-like crystalline solids in which water molecules form cages trapping gas, mostly methane. They occur in permafrost and deep-water sediments and are also a flow assurance problem in pipelines.

PET-132 · Why is horizontal drilling important for unconventionals?

A horizontal lateral running through a thin, low-permeability formation contacts far more rock than a vertical well, and it allows many fracture stages from one wellbore.

PET-133 · What is multistage fracturing?

Fracturing a horizontal lateral in many separate stages, each isolated by plugs or sliding sleeves, so fractures are spread along the whole lateral.

PET-134 · What is plug-and-perf?

A multistage method in which a wireline string pumped down the casing sets a frac plug and fires perforating guns, the stage is fractured, and the cycle is repeated. The plugs are milled out at the end.

PET-135 · What is pad drilling?

Drilling several wells from one surface location. It reduces surface footprint, rig moves and facility cost, and allows simultaneous or zipper fracturing.

PET-136 · What is total organic carbon (TOC)?

The weight percentage of organic carbon in a rock. It measures source-rock richness and is a key screening parameter for shale plays.

PET-137 · What is slickwater?

A fracturing fluid that is mostly water with a friction reducer added. It is pumped at high rate and carries fine proppant, creating long, complex fractures at low chemical cost.

Surface Facilities, Pipelines, Refining & HSE

PET-138 · What are upstream, midstream and downstream?

Upstream covers exploration and production. Midstream covers processing, transport by pipeline, ship or rail, and storage. Downstream covers refining, petrochemicals and marketing of products.

PET-139 · What does a production separator do?

It separates well fluids into gas and liquid (two-phase) or gas, oil and water (three-phase) using gravity and residence time. Separators may be horizontal or vertical.

PET-140 · What is crude oil dehydration?

Removing emulsified water from crude to meet sales specifications, using heat, demulsifier chemicals, residence time and electrostatic coalescers, often in a heater-treater.

PET-141 · Why is natural gas dehydrated?

Water in gas can form hydrates and cause corrosion. It is removed by absorption in triethylene glycol (TEG) or by adsorption on molecular sieves.

PET-142 · What is gas sweetening?

Removing acid gases, H₂S and CO₂, from sour gas, most often by absorption in amine solutions. The H₂S is then usually converted to sulfur.

PET-143 · What is LNG?

Liquefied natural gas: mainly methane cooled to about −162 °C at atmospheric pressure. It occupies roughly 1/600 of its gas volume, which makes long-distance shipping practical.

PET-144 · What is pipeline pigging?

Sending a device called a pig through a pipeline to clean it, remove liquids or wax, separate batches, or inspect the wall (intelligent or smart pigs).

PET-145 · What is cathodic protection?

Corrosion control that makes the pipeline the cathode of an electrochemical cell, using sacrificial anodes (such as zinc or magnesium) or an impressed-current system. It is used along with coatings.

PET-146 · What is petroleum refining?

Converting crude oil into fuels and other products through separation (distillation), conversion (cracking, reforming) and treating (removing sulfur and other impurities), followed by blending.

PET-147 · What does a crude distillation unit do?

It heats crude and separates it by boiling range in an atmospheric column into gas, naphtha, kerosene, diesel and atmospheric residue. The residue goes to a vacuum column to recover gas oils at lower temperature.

PET-148 · What is cracking?

Breaking large hydrocarbon molecules into smaller, more valuable ones. Types are thermal cracking, fluid catalytic cracking (FCC) and hydrocracking with hydrogen.

PET-149 · Why is H₂S so dangerous?

It is highly toxic, flammable, heavier than air and collects in low areas. It paralyses the sense of smell at higher concentrations, and NIOSH sets its IDLH at 100 ppm.

PET-150 · What is a permit-to-work system?

A formal written system for authorising hazardous work such as hot work, confined space entry or breaking containment. It defines hazards, isolations, precautions and responsibilities before work starts.

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