Tricky Petroleum Engineering interview questions

75 questions with answers (PET-426 to PET-500). Try each one aloud before reading the answer.

Petroleum Geology

PET-426 · Is oil found in underground lakes or caverns?

Trap: The "oil lake" picture is wrong. Oil and gas fill the tiny pore spaces between grains and in fractures of solid rock, like water in a sponge, and must flow through those pores to reach the well.

PET-427 · Does every anticline contain oil or gas?

Trap: A trap is not an accumulation. It also needs a mature source rock, a migration path, an effective seal and the right timing, with the trap formed before migration. Many drilled anticlines are dry because one of these failed.

PET-428 · Does oil come from dinosaurs?

Trap: This is a popular myth. Most oil comes from marine and lake plankton and algae, and most gas from these and from land plants, buried in fine sediments. Dinosaurs contributed essentially nothing.

PET-429 · Does drilling deeper always find more oil?

Trap: Deeper is not always better. Below the oil window, temperatures crack oil to gas and eventually overcook source rocks, and porosity usually falls with burial. Each basin has a depth range where oil is most likely.

PET-430 · Does a seismic bright spot always mean commercial gas?

Trap: Bright spot does not equal pay. Low gas saturation ("fizz gas") gives almost the same amplitude as commercial gas, and coals, tuning effects and hard or soft lithologies can also create bright amplitudes. AVO, calibration and other evidence are needed.

PET-431 · Is shale always a seal and never a reservoir?

Trap: Shale is not only a seal. Organic-rich shales are major reservoirs when stimulated, and fractured or silty shales can leak. Whether shale seals depends on its capillary entry pressure and whether faults or fractures cut it.

Petrophysics & Well Logging

PET-432 · Does high porosity always mean high permeability?

Trap: Storage is not the same as flow. Shale and chalk can be highly porous yet have very low permeability because pores and throats are tiny and poorly connected. A coarse, well-sorted sandstone with lower porosity can be far more permeable.

PET-433 · Does high resistivity always mean hydrocarbons?

Trap: Resistivity alone does not prove hydrocarbons. Tight low-porosity rock, fresh formation water, salt, anhydrite and coal also read high. Hydrocarbons are only confirmed by combining resistivity with porosity, Rw and other evidence.

PET-434 · Does a low gamma ray reading always mean clean sand, and high GR always shale?

Trap: GR measures radioactivity, not shale. Feldspar- or mica-rich sands and uranium-bearing zones read high while being reservoir quality, and clean carbonates and salt read low. Spectral gamma ray helps separate potassium, thorium and uranium sources.

PET-435 · Does Archie's equation apply to all reservoirs?

Trap: Archie is not universal. It assumes clean rock where only brine conducts. In shaly sands, rocks with conductive minerals or complex carbonate pores, it gives wrong Sw unless shaly-sand models or calibrated m and n are used.

PET-436 · Does the neutron log measure porosity directly?

Trap: It measures hydrogen index, not porosity. Bound water in shale makes it read too high, gas makes it read too low, and the result depends on the lithology the tool is calibrated for.

PET-437 · Is core-measured permeability the same as reservoir permeability?

Trap: Core and reservoir perm differ. Plugs are small, may be unstressed or gas-measured (Klinkenberg), miss fractures and vugs, and come from selected intervals. Well-test permeability averages a large volume and can be much higher or lower.

Reservoir Engineering

PET-438 · Why can't we recover all the oil in a reservoir?

Trap: Displacing oil is not like draining a tank. Capillary forces trap residual oil in pores, heterogeneity leaves unswept zones, and economic limits end production early. Typical primary plus secondary recovery is often only about 30–50%.

PET-439 · Does producing a well faster always increase ultimate recovery?

Trap: Speed is not free. High rates can cause coning, bypassed oil, early gas cap or water breakthrough, sand production and wasted reservoir energy. Rate-sensitive reservoirs often recover more when produced more slowly.

PET-440 · If a reservoir is above bubble point, is no gas produced?

Trap: No free gas in the reservoir does not mean no gas at surface. The dissolved gas comes out of solution in the tubing and separator, so the well produces gas at a GOR equal to the initial solution GOR, Rsi.

PET-441 · Does a water drive reservoir keep its pressure constant?

Trap: Only a very large, permeable aquifer gives near-constant pressure. Many aquifers are limited or slow to respond, so pressure still falls, sometimes significantly, especially at high withdrawal rates.

PET-442 · Do gas reservoirs have lower recovery factors than oil reservoirs?

Trap: The opposite is usually true. Gas is very compressible and mobile, so depletion-drive gas reservoirs often recover about 70–90%. Strong water drive can actually lower gas recovery because water traps gas at high pressure.

PET-443 · Does one barrel of reservoir oil give more than one barrel at the surface?

Trap: It gives less. Oil shrinks as dissolved gas comes out and it cools, so Bo > 1 rb/STB means each stock-tank barrel occupied more than one barrel in the reservoir.

PET-444 · Does higher API gravity mean heavier oil?

Trap: API is inverse to density. Higher API means lighter, less dense crude: water is 10 °API, light crude is typically above about 31 °API and heavy crude below about 22 °API.

Drilling Engineering

PET-445 · Does more weight on bit always mean faster drilling?

Trap: ROP is not linear in WOB. Beyond the founder point, extra weight brings bit balling, poor bottom cleaning, vibration and cutter damage, so ROP flattens or falls. Hydraulics and rotary speed also limit ROP.

PET-446 · Is a horizontal well always better than a vertical well?

Trap: Not always. In thick reservoirs with low vertical permeability, or where many stacked zones need access, a vertical or deviated well may do as well at lower cost and risk. Horizontals pay off in thin, low-permeability or coning-prone reservoirs.

PET-447 · Is drill pipe in compression because it pushes the bit down?

Trap: In vertical wells drill pipe should stay in tension. Weight on bit comes from part of the buoyed weight of the drill collars, with the neutral point in the BHA. In high-angle wells, heavy-weight pipe may carry compression within buckling limits.

PET-448 · Are measured depth and true vertical depth the same?

Trap: Only in a vertical well. In deviated and horizontal wells MD exceeds TVD, sometimes by many thousands of feet. Pressure calculations use TVD, while friction, volumes and pipe length use MD.

PET-449 · Does hook load equal the weight of the drill string in air?

Trap: Hook load is the buoyed string weight minus weight on bit, plus or minus drag, plus the travelling equipment. Buoyancy alone cuts it by about 15% in 10 ppg mud.

PET-450 · Is tripping faster always better because it saves rig time?

Trap: Fast trips cause high swab pressures that can draw in kicks and high surge pressures that can fracture the formation. Trip speeds are limited based on mud properties, clearances and the pressure window.

Drilling Fluids & Cementing

PET-451 · Is heavier drilling mud always safer?

Trap: Overbalance has costs. Too heavy a mud can fracture weak formations and cause lost circulation, which drops the mud level and can lead to a kick. It also slows ROP, raises differential sticking and damages the reservoir.

PET-452 · Does thicker mud always clean the hole better?

Trap: In vertical holes moderate viscosity helps, but too much raises ECD and pressure losses. In high-angle holes cuttings beds are better removed by high flow rate, pipe rotation and low-viscosity turbulent sweeps.

PET-453 · Since oil-based mud is oil, can it damage an oil reservoir?

Trap: Yes, it can. OBM can form emulsion blocks, alter wettability toward oil-wet through surfactants, and invade with solids, all of which reduce near-wellbore permeability.

PET-454 · Is set cement impermeable forever?

Trap: Cement can lose integrity. Shrinkage, pressure and temperature cycling cause micro-annuli and cracks, and CO₂ or H₂S can chemically degrade it, opening leak paths over the life of the well.

PET-455 · Is a denser cement slurry always better for isolation?

Trap: Density is not quality. A slurry too heavy for the formation causes losses and leaves cement short of the planned top. Good isolation depends on mud removal, centralisation and correct density and rheology hierarchy.

PET-456 · Does barite dissolve in mud to make it heavier?

Trap: Barite is an insoluble solid suspended in the mud. The mud must have enough gel strength and suspension to keep it from settling (sag), especially in deviated wells.

PET-457 · Does cement set by drying out?

Trap: Cement sets by hydration, a chemical reaction with water that forms calcium silicate hydrates. That is why it sets downhole while fully surrounded by fluid.

Well Control

PET-458 · Does a gas kick only expand when it reaches the surface?

Trap: Gas expands progressively as it rises and hydrostatic pressure falls, with the largest expansion near surface. If it is not allowed to expand in a shut-in well, it carries its high pressure upward instead.

PET-459 · Once the well is shut in, is the problem solved?

Trap: Shut-in is only a pause. Gas can migrate and raise pressures toward fracture limits, so pressures must be monitored and the influx circulated out or handled with the volumetric method.

PET-460 · Is SICP equal to formation pressure?

Trap: Neither shut-in pressure equals formation pressure. Formation pressure = SIDPP + hydrostatic pressure of the mud in the drill pipe. SICP is usually higher than SIDPP because the annulus contains light influx.

PET-461 · Can a kick only happen if mud weight is too low?

Trap: Kicks also come from swabbing, failing to keep the hole full on trips, lost circulation lowering the mud level, gas-cut mud and ECD loss when pumps stop. Correct mud weight alone does not prevent kicks.

PET-462 · During a kill, is opening the choke a safe way to reduce high casing pressure?

Trap: Opening the choke lowers bottomhole pressure and lets in more influx. The rule is to keep bottomhole pressure constant, using drill pipe pressure, and let casing pressure rise as gas expands, within MAASP.

PET-463 · Can blind-shear rams cut anything in the BOP?

Trap: Shear rams have limits. They may not cut tool joints, drill collars, heavy-wall pipe or off-centre or buckled pipe, so space-out and pipe position matter when shearing.

Well Completion & Stimulation

PET-464 · Do more perforations always mean more production?

Trap: Returns diminish once perforation skin is small, and extra shots add cost and can weaken casing. Limited-entry fracturing deliberately uses few perforations to spread fluid between clusters.

PET-465 · Do hydraulic fractures always grow vertically upward?

Trap: Fractures grow perpendicular to the minimum stress, which in shallow formations may be vertical, giving horizontal fractures. Height growth is limited by stress contrasts and layering, and fractures can grow down as well as up.

PET-466 · Does acid always improve a sandstone well?

Trap: Acid can harm sandstones. Wrong acid or volume can precipitate fluorides and silicates, release fines, damage acid-sensitive clays such as chlorite, or loosen the rock and cause sand production. Damage type must be diagnosed first.

PET-467 · Does bigger tubing always mean more production?

Trap: At low rates larger tubing lowers velocity, so liquids slip back and gas wells load up and die. Tubing must be sized for the rate range over the well's life.

PET-468 · Does hydraulic fracturing always create a single planar crack?

Trap: The bi-wing planar model is a simplification. In naturally fractured or laminated rock such as shale, fractures can branch, turn and interact with natural fractures, forming complex networks.

PET-469 · Does negative skin mean flow faster than Darcy's law allows?

Trap: Negative skin does not break any law. It means the well behaves as if its radius were larger: rw′ = rw·e^(−s). A fracture or acid wormholes enlarge the effective drainage surface.

Production Engineering & Artificial Lift

PET-470 · Is the productivity index constant over the life of a well?

Trap: PI is constant only for single-phase liquid flow. Below bubble point, gas cuts oil relative permeability and the IPR curves, as in Vogel's equation. Rising water cut, damage and depletion also change it.

PET-471 · Does injecting more gas in gas lift always produce more oil?

Trap: The gas lift performance curve has a maximum. Beyond it extra gas adds friction more than it lightens the column, so oil rate falls, and the economic optimum is usually below the maximum.

PET-472 · Can an ESP pump gassy fluid as easily as liquid?

Trap: Centrifugal pumps lose head with free gas and can gas-lock. Gas separators, gas handlers, deeper setting or alternative lift are needed in gassy wells.

PET-473 · Does choking a well back always waste reserves?

Trap: Choking cuts current rate but can delay coning, control sand and protect equipment, and so can increase ultimate recovery. It also manages facility constraints.

PET-474 · If a well stops flowing, is the reservoir depleted?

Trap: A dead well is not necessarily depleted. Liquid loading, rising water cut or higher backpressure can kill a well with plenty of reserves remaining, which artificial lift or deliquification can recover.

PET-475 · Does a rising water cut always mean aquifer water has broken through?

Trap: Water can also come from casing leaks, channels behind pipe from poor cement, injected water through high-permeability streaks, or coning. Diagnosis must precede treatment.

Well Testing

PET-476 · If buildup pressure flattens out, has average reservoir pressure been reached?

Trap: A flat curve is not proof. In low-permeability reservoirs pressure may still be rising below gauge resolution or on a log time scale. Average pressure is estimated by methods such as MBH or Dietz, or from extrapolation.

PET-477 · Does wellbore storage tell you about reservoir properties?

Trap: It reflects wellbore volume and fluid compressibility, not the reservoir, and it masks early reservoir response. Downhole shut-in reduces it.

PET-478 · Does positive skin always mean formation damage?

Trap: Total skin includes partial penetration, perforation, turbulence (non-Darcy) and other effects. Positive skin may come from limited perforations or high gas rate rather than damage.

PET-479 · Does a dip in the pressure derivative always indicate dual porosity?

Trap: Similar dips come from changing wellbore storage, phase segregation, layered reservoirs, partial penetration or gauge noise. Dual porosity must be supported by geology and consistency checks.

PET-480 · Do drawdown and buildup tests always give equally reliable results?

Trap: Drawdowns rarely keep a truly constant rate, so data are noisy. Buildups are usually cleaner because rate is zero, though they cost production time.

PET-481 · Does a higher test rate increase radius of investigation?

Trap: ri depends on k, φ, μ, ct and time, not rate. Higher rate gives a larger pressure signal that is easier to measure, but the transient moves at the same speed.

Enhanced Oil Recovery

PET-482 · Does EOR always come after primary and secondary recovery?

Trap: "Tertiary" is not a time order. Heavy oil and bitumen fields often start with thermal methods because primary and waterflooding recover almost nothing, and some fields begin miscible gas injection early.

PET-483 · Does polymer flooding reduce residual oil saturation?

Trap: Polymer mainly improves sweep by lowering mobility ratio. It recovers oil from unswept zones rather than reducing residual oil in swept pores.

PET-484 · Must CO₂ be miscible to improve recovery?

Trap: Immiscible CO₂ also swells oil, cuts its viscosity and reduces IFT, so it can still add recovery, especially in heavier oils, though less efficiently than miscible flooding.

PET-485 · Is thermal EOR best in thin reservoirs because heat reaches the oil faster?

Trap: Thin reservoirs lose a large share of injected heat to the cap rock and base rock. Thermal methods work best in thick, permeable reservoirs.

PET-486 · Is waterflooding an EOR method?

Trap: Waterflooding is classed as secondary recovery or IOR. EOR refers to injecting materials or energy not normally present, such as steam, CO₂ or chemicals.

PET-487 · Do surfactants work by reducing oil viscosity?

Trap: Surfactants mainly cut oil-water interfacial tension, raising capillary number and freeing trapped oil. Viscosity reduction is the role of heat or solvents.

Unconventional Resources

PET-488 · Is shale gas a different gas from conventional natural gas?

Trap: The gas is the same, mostly methane. Only the reservoir rock differs, with very low permeability requiring horizontal wells and fracturing.

PET-489 · Do fracking fractures normally reach shallow drinking water aquifers?

Trap: Fracture heights are typically hundreds of feet, while deep shales usually sit thousands of feet below aquifers. Documented contamination is more often linked to well integrity failures and surface spills than to fractures.

PET-490 · Does doubling lateral length double production?

Trap: Gains are usually less than proportional because of friction, cleanout difficulty, toe-end stimulation quality and geological variation, though cost per foot often falls.

PET-491 · Do shale wells decline exponentially from the start?

Trap: Early shale decline is usually hyperbolic, often with b > 1 during transient linear flow. Forecasts switch to exponential only at a terminal decline rate.

PET-492 · Does tighter well spacing always increase value?

Trap: Beyond a point, wells interfere and share the same rock, so per-well EUR drops and added wells cost more than the extra oil is worth.

PET-493 · Does bitumen in oil sands flow to wells like conventional oil?

Trap: Bitumen viscosity at reservoir conditions can exceed a million cP, making it effectively immobile. It must be mined or heated in situ (SAGD, CSS) or diluted.

Surface Facilities, Pipelines, Refining & HSE

PET-494 · Is natural gas the same as LPG?

Trap: Natural gas is mostly methane, and is gaseous at ambient conditions. LPG is propane and butane, which liquefy under modest pressure and are recovered from gas processing and refining.

PET-495 · Do refineries make gasoline only by distillation?

Trap: Straight-run naphtha covers only part of demand and has low octane. Most gasoline comes from conversion and upgrading units such as FCC, reforming, alkylation and isomerisation, then blending.

PET-496 · Does higher-octane fuel give more power in any engine?

Trap: Octane measures knock resistance, not energy content. It only helps engines designed for higher compression or boost; in others it gives no benefit.

PET-497 · Will you always smell H₂S before it harms you?

Trap: At higher concentrations, around 100 ppm and above, H₂S quickly paralyses the sense of smell. Gas detectors, not your nose, must be relied on.

PET-498 · Does doubling pipeline diameter only double its capacity?

Trap: Capacity rises much faster than diameter. For gas, the Weymouth equation gives q ∝ D^(8/3), so doubling diameter raises flow about 6.3 times at the same pressures.

PET-499 · Is flaring always worse for the climate than venting?

Trap: Venting releases methane, a much more potent greenhouse gas than CO₂. Flaring converts it mostly to CO₂ and usually has a smaller climate impact, though both should be minimised.

PET-500 · Does API gravity alone set the price of a crude?

Trap: Price also depends on sulfur content (sweet or sour), acidity (TAN), metals, product yields, location and transport, and benchmark differentials. A light but high-sulfur crude may sell at a discount.

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