Intermediate Petroleum Engineering interview questions

150 questions with answers (PET-151 to PET-300). Try each one aloud before reading the answer.

Petroleum Geology

PET-151 · How do temperature and time control hydrocarbon generation?

Kerogen cracking is a kinetic process, so generation depends on both temperature and time: a younger, hotter basin and an older, cooler basin can reach similar maturity. Basin models use reaction kinetics over burial history to predict generation.

PET-152 · What is vitrinite reflectance?

The percentage of light reflected from vitrinite particles in the rock, Ro, which rises irreversibly with thermal maturity. The oil window corresponds to roughly Ro ≈ 0.6–1.3%, with gas generation dominating at higher values.

PET-153 · Why do hydrocarbons migrate upward?

Oil and gas are less dense than formation water, so buoyancy pushes them up-dip through permeable carrier beds. Migration stops where capillary entry pressure of a seal exceeds the buoyancy pressure of the hydrocarbon column.

PET-154 · What limits the hydrocarbon column a seal can hold?

The seal's capillary entry pressure. The maximum column height is h = Pc,entry/[(ρw − ρh)·g]; in field units h (ft) = Pc (psi)/(water gradient − hydrocarbon gradient, psi/ft).

PET-155 · What is a spill point?

The lowest structural point at which a trap can still hold hydrocarbons. If more hydrocarbons arrive once the trap is filled to the spill point, they spill out and migrate further.

PET-156 · What is the difference between the free water level and the oil-water contact?

The free water level is where capillary pressure is zero and oil and water pressures are equal. The oil-water contact is the depth where oil saturation effectively reaches zero on logs, which lies above the FWL by the threshold capillary pressure height.

PET-157 · How does diagenesis affect reservoir quality?

Compaction and cementation by quartz, calcite or clays reduce porosity and permeability. Dissolution of grains or cement and dolomitisation can create secondary porosity. Early hydrocarbon charge can slow cementation.

PET-158 · Why do depositional environments matter to reservoir engineers?

They control reservoir geometry, continuity and heterogeneity. Fluvial channels give narrow, sinuous sand bodies, deltas give laterally extensive sands, turbidites give stacked lobes, and reefs give mound-shaped carbonates, each with different flow behaviour.

PET-159 · What are acoustic impedance and reflection coefficient?

Acoustic impedance is Z = ρ·V, density times velocity. At an interface the normal-incidence reflection coefficient is R = (Z₂ − Z₁)/(Z₂ + Z₁), so larger impedance contrasts give stronger seismic reflections.

PET-160 · What are direct hydrocarbon indicators?

Seismic features that may indicate hydrocarbons, such as bright spots (strong amplitudes from low-impedance gas sands), flat spots at fluid contacts, polarity reversals and amplitude variation with offset (AVO) anomalies.

PET-161 · What is the difference between 2D, 3D and 4D seismic?

2D seismic gives individual cross-sections along lines. 3D seismic records a dense grid to produce a volume image. 4D (time-lapse) seismic repeats 3D surveys over time to track fluid movement and pressure changes during production.

PET-162 · How do salt domes form traps?

Mobile salt rises as diapirs and deforms the overlying and adjacent strata, creating traps in domed beds above the salt and against its flanks. Salt is an excellent seal because it is nearly impermeable and deforms plastically without fracturing.

PET-163 · What is the geothermal gradient and why does it matter?

The rate at which temperature increases with depth, typically around 25–30 °C/km. It controls the depth of the oil and gas windows, fluid properties and the temperature rating of downhole equipment.

Petrophysics & Well Logging

PET-164 · How is shale volume estimated from a gamma ray log?

First compute the gamma ray index IGR = (GR − GRclean)/(GRshale − GRclean). The linear model takes Vsh = IGR; nonlinear corrections such as Larionov or Clavier usually give lower Vsh for the same index.

PET-165 · Calculate Sw for φ = 0.20, Rw = 0.05 Ω·m and Rt = 5 Ω·m (a = 1, m = n = 2).

Sw² = a·Rw/(φᵐ·Rt) = 0.05/(0.04 × 5) = 0.25, so Sw = 0.50. Hydrocarbon saturation is therefore 50%.

PET-166 · Calculate density porosity for ρb = 2.32 g/cm³ in sandstone with fresh mud filtrate.

φ = (ρma − ρb)/(ρma − ρf) = (2.65 − 2.32)/(2.65 − 1.00) = 0.33/1.65 = 0.20, or 20%.

PET-167 · Why does gas cause neutron-density crossover?

Gas has low hydrogen density, so neutron porosity reads too low, and low bulk density, so density porosity reads too high. On a compatible scale the curves cross over, which flags gas.

PET-168 · What is the formation resistivity factor?

F = Ro/Rw = a/φᵐ, the ratio of the resistivity of rock fully saturated with brine to the brine resistivity. It depends on porosity and pore geometry through the cementation exponent m.

PET-169 · What is the invasion profile?

Mud filtrate displaces fluids near the borehole, creating a flushed zone (Rxo), a transition zone and the uninvaded zone (Rt). Shallow, medium and deep resistivity readings are combined to correct for invasion and estimate Rt.

PET-170 · How is formation water resistivity (Rw) found?

From produced water samples, from the SP log, from apparent Rw (Rwa = Rt·φᵐ/a) in clean water-bearing zones, or from a Pickett plot. Rw must be corrected to formation temperature.

PET-171 · What is a Pickett plot?

A log-log crossplot of Rt against porosity. Water-bearing points lie on a line of slope −1/m, from which m and Rw are read; lines of constant Sw are parallel to it and give saturation directly.

PET-172 · What is the Klinkenberg effect?

Gas permeability measured in the lab exceeds liquid permeability at low pressure because gas molecules slip along pore walls. Measurements at several pressures are extrapolated to infinite pressure to get the equivalent liquid permeability.

PET-173 · What does the Kozeny–Carman equation express?

It relates permeability to rock texture: k ∝ φ³/[(1 − φ)²·S²], where S is the specific surface area of grains. Permeability rises sharply with porosity and with grain size, since S falls as grains get larger.

PET-174 · What does NMR logging provide?

It measures the T2 relaxation of hydrogen nuclei in pore fluids. The result is porosity largely independent of lithology, pore-size distribution, free versus bound fluid volumes and permeability estimates.

PET-175 · How is a wireline formation tester used to identify fluids?

It measures formation pressure at many depths. The pressure gradient shows the fluid: water about 0.43–0.45 psi/ft, oil about 0.3–0.38 psi/ft and gas about 0.05–0.15 psi/ft. Gradient intersections locate fluid contacts, and the tool can also take fluid samples.

PET-176 · What is the photoelectric factor (Pe) used for?

Pe, measured by the density tool, is a strong lithology indicator: about 1.8 barns/electron for quartz sandstone, 3.1 for dolomite and 5.1 for limestone. It is little affected by porosity.

Reservoir Engineering

PET-177 · State Darcy's law.

For linear, laminar, single-phase flow, q = (k·A/μ)·(ΔP/L). In field units q (bbl/d) = 0.001127·k·A·ΔP/(μ·L), with k in mD, A in ft², ΔP in psi, μ in cP and L in ft.

PET-178 · Calculate linear flow rate for k = 100 mD, A = 1000 ft², ΔP = 500 psi, μ = 2 cP and L = 1000 ft.

q = 0.001127 × 100 × 1000 × 500/(2 × 1000) = 56,350/2000 ≈ 28.2 bbl/d.

PET-179 · What is the steady-state radial flow equation?

q = 0.00708·k·h·(Pe − Pwf)/[μ·Bo·(ln(re/rw) + s)] in STB/d, with k in mD, h in ft, pressures in psi and μ in cP. Skin s adds an extra near-wellbore pressure drop.

PET-180 · Estimate OOIP for A = 640 acres, h = 50 ft, φ = 0.20, Sw = 0.25 and Boi = 1.25 rb/STB.

N = 7758 × 640 × 50 × 0.20 × 0.75/1.25 ≈ 29.8 million STB.

PET-181 · What is the volumetric equation for gas in place?

G = 43,560·A·h·φ·(1 − Sw)/Bgi, giving scf, with A in acres, h in ft and Bgi in ft³/scf. 43,560 is the number of ft² in one acre.

PET-182 · How is the gas formation volume factor calculated?

Bg = 0.02827·z·T/P in ft³/scf, with T in °R and P in psia. Gas volume falls strongly as pressure rises.

PET-183 · How is the gas deviation factor z found?

From the real gas law PV = znRT. z is read from the Standing–Katz chart or calculated by correlations using pseudo-reduced pressure and temperature (P/Ppc and T/Tpc), with corrections for H₂S and CO₂.

PET-184 · What is total compressibility?

ct = co·So + cw·Sw + cg·Sg + cf, the saturation-weighted sum of fluid compressibilities plus pore compressibility. Compressibility is c = −(1/V)(∂V/∂P), and ct controls how fast pressure transients spread.

PET-185 · What is the principle of material balance?

The reservoir is treated as a tank: underground withdrawal equals the expansion of oil, gas cap, connate water and rock plus water influx. In Havlena–Odeh form F = N·Et + We.

PET-186 · What is the fractional flow equation?

Ignoring gravity and capillary pressure, fw = 1/[1 + (kro·μw)/(krw·μo)]. Low water viscosity or high oil viscosity raises fw, so water reaches producers earlier and in larger amounts.

PET-187 · What does the Welge tangent construction give?

A tangent from the initial water saturation to the fractional flow curve gives the water saturation at the flood front and at breakthrough, and the average saturation behind the front, which sets displacement efficiency at breakthrough.

PET-188 · What are Arps decline curves?

Empirical rate-time relationships: q = qi/(1 + b·Di·t)^(1/b). b = 0 gives exponential decline, 0 < b < 1 hyperbolic, and b = 1 harmonic. They are used to forecast production and estimate reserves.

PET-189 · A well declines exponentially from 1000 STB/d with D = 0.2 per year. What is its rate after 3 years?

q = qi·e^(−Dt) = 1000 × e^(−0.6) ≈ 549 STB/d.

Drilling Engineering

PET-190 · Calculate hydrostatic pressure for 10 ppg mud at 10,000 ft TVD.

P = 0.052 × 10 × 10,000 = 5200 psi.

PET-191 · What is equivalent circulating density (ECD)?

The effective mud density at a depth while circulating, including annular friction: ECD = MW + ΔPannulus/(0.052 × TVD), in ppg. It must stay below the fracture gradient.

PET-192 · How are bit hydraulics optimised?

Nozzles are chosen to maximise either bit hydraulic horsepower, when the bit pressure drop is about 65% of total system pressure, or jet impact force, at about 48%. Both aim to clean the bit and the hole bottom.

PET-193 · What loads are considered in drill string design?

Tension (with an overpull margin for stuck pipe), torsion, collapse and burst, buckling and fatigue. The neutral point should lie in the drill collars so that drill pipe stays in tension.

PET-194 · What is the buoyancy factor?

BF = 1 − MW/65.5 for steel in ppg, where 65.5 ppg is the density of steel. In 10 ppg mud, BF = 0.847, so the string weighs about 85% of its weight in air.

PET-195 · What is differential sticking?

Pipe resting against a permeable zone is pressed into the filter cake by the overbalance between mud and pore pressure. The force needed to free it is about ΔP × contact area × friction coefficient, and it grows with time.

PET-196 · What loads and safety factors are used in casing design?

Casing is checked for burst, collapse and tension, plus compression and triaxial loads. Company design factors vary, typically around 1.1 for burst, 1.0–1.125 for collapse and 1.6–1.8 for tension.

PET-197 · How are casing setting depths chosen?

A pore pressure and fracture gradient plot is used, usually working from the bottom up. Each string is set where the mud weight needed below would fracture the weakest formation above, with margins for kicks, trips and ECD.

PET-198 · What is dogleg severity?

The overall change in wellbore angle (inclination and azimuth combined) per unit length, usually in degrees per 100 ft or per 30 m. High dogleg severity raises torque, drag, casing wear and fatigue.

PET-199 · What are the common directional well profiles?

Build-and-hold (J-type), S-type (build, hold and drop back toward vertical), and horizontal wells that build to about 90° and drill a lateral. Choice depends on target, geology and completion.

PET-200 · What are the advantages of rotary steerable systems over steerable motors?

An RSS steers while the whole string rotates, so there is no sliding. This gives better hole cleaning, higher ROP, smoother boreholes, less sticking risk and the reach needed for long laterals and extended-reach wells.

PET-201 · What is the difference between MWD and LWD?

MWD measures directional and drilling data such as inclination, azimuth, toolface and vibration. LWD measures formation properties such as gamma ray, resistivity, density and neutron. Data are commonly sent to surface by mud-pulse telemetry.

PET-202 · What controls hole cleaning?

Annular velocity, mud rheology, pipe rotation, cuttings size and hole inclination. Inclinations of about 30–60° are hardest because cuttings beds form and can slide down the low side.

Drilling Fluids & Cementing

PET-203 · What are the functions of drilling mud?

It cools and lubricates the bit, carries cuttings to surface, suspends them when circulation stops, controls formation pressure, stabilises the wellbore with a filter cake, transmits hydraulic power to the bit and motors, and carries MWD signals.

PET-204 · What is the Bingham plastic model for mud?

τ = YP + PV·γ̇. From a viscometer, PV (cP) = θ600 − θ300 and YP (lbf/100 ft²) = θ300 − PV. PV reflects solids and base fluid; YP reflects attractive forces that help carry cuttings.

PET-205 · Calculate PV and YP for θ600 = 50 and θ300 = 30.

PV = 50 − 30 = 20 cP and YP = 30 − 20 = 10 lbf/100 ft².

PET-206 · What is the power-law model?

τ = K·γ̇ⁿ, where n is the flow behaviour index and K the consistency index. From viscometer readings, n = 3.32·log(θ600/θ300). n < 1 means shear-thinning, which suits drilling muds.

PET-207 · What is gel strength?

The shear stress needed to start a mud moving after it has rested, usually measured at 10 seconds and 10 minutes. It shows how well the mud suspends cuttings and barite when pumps stop; very high gels cause surge and swab problems.

PET-208 · How much barite is needed to raise mud from 10 to 12 ppg?

Barite (lb/bbl of original mud) = 1470(W₂ − W₁)/(35 − W₂) = 1470 × 2/23 ≈ 128 lb/bbl. Here 35 ppg is the density of barite, and volume increases.

PET-209 · How is fluid loss measured?

The API filter press test measures filtrate collected through filter paper in 30 minutes at 100 psi and room temperature. The HPHT test uses a 500 psi differential at elevated temperature to mimic downhole conditions.

PET-210 · How are reactive shales inhibited?

With KCl or other salts that suppress clay hydration, encapsulating polymers such as PHPA, glycols, silicates, or by switching to oil- or synthetic-based mud with balanced water-phase salinity.

PET-211 · How is lost circulation classified and treated?

By loss rate: seepage, partial or total losses. Treatments include reducing mud weight and ECD, adding lost circulation material sized to the openings, cross-linked pills, cement plugs, or setting casing.

PET-212 · What are the main cement additive groups?

Accelerators (such as CaCl₂), retarders (such as lignosulfonates), extenders (bentonite, pozzolans, microspheres), weighting agents (hematite, barite), fluid loss additives, dispersants and gas migration control additives.

PET-213 · What is cement thickening time?

The time a slurry stays pumpable under simulated downhole temperature and pressure, measured in an HPHT consistometer until it reaches a set consistency, typically 70 Bc. It must exceed job time with a safety margin.

PET-214 · How is a cement bond log interpreted?

A sonic tool measures the amplitude of the casing arrival. Low amplitude means casing is well bonded to cement; high amplitude means free pipe. The variable-density log (VDL) also shows whether formation signals arrive, indicating bond to formation.

Well Control

PET-215 · What is a blowout preventer (BOP)?

A stack of valves on the wellhead, including annular and ram preventers, that can seal the well around pipe or across an open hole, or shear pipe. It stops uncontrolled flow and allows kicks to be circulated out under control.

PET-216 · Calculate kill mud weight for OMW = 10 ppg, SIDPP = 520 psi and TVD = 10,000 ft.

KMW = OMW + SIDPP/(0.052 × TVD) = 10 + 520/520 = 11.0 ppg.

PET-217 · Why is SIDPP used instead of SICP to calculate kill weight?

The drill pipe is full of mud of known density and is not contaminated by influx, so SIDPP plus drill pipe hydrostatic gives formation pressure directly. The annulus contains influx of unknown volume and density.

PET-218 · How do the Driller's method and the Wait-and-Weight method differ?

The Driller's method circulates the kick out with original mud first, then circulates kill mud in a second circulation. The Wait-and-Weight method weights up first and kills in one circulation, giving lower casing shoe pressures but a delay before starting.

PET-219 · How are initial and final circulating pressures calculated?

ICP = slow circulating rate pressure + SIDPP. FCP = SCR pressure × KMW/OMW. Drill pipe pressure is reduced from ICP to FCP as kill mud travels down the string.

PET-220 · What is MAASP?

Maximum allowable annular surface pressure, the surface casing pressure at which the weakest point, usually the shoe, would fracture: MAASP = (fracture EMW − current MW) × 0.052 × shoe TVD.

PET-221 · What are leak-off tests and formation integrity tests?

After drilling a few metres below a casing shoe, the well is closed and pressured up. A leak-off test continues until the pressure-volume trend departs from linear, showing fracture initiation; a FIT stops at a predetermined pressure without breaking down the formation.

PET-222 · What is kick tolerance?

The largest kick volume, at a given intensity, that can be shut in and circulated out without fracturing the weakest formation, usually the casing shoe. It is used to choose casing depths and to decide whether to continue drilling.

PET-223 · What happens when gas migrates in a shut-in well?

The gas bubble rises without expanding and carries its original pressure up, so bottomhole and surface pressures both rise. The volumetric method bleeds mud in controlled steps to keep bottomhole pressure constant.

PET-224 · How is influx type estimated from shut-in pressures?

Influx gradient = mud gradient − (SICP − SIDPP)/Hinflux, where Hinflux = pit gain/annular capacity. Roughly, gas reads below about 0.25 psi/ft, oil or mixtures 0.25–0.4, and saltwater above about 0.4 psi/ft.

PET-225 · What is a BOP accumulator unit?

A bank of bottles holding hydraulic fluid under pressure from a nitrogen precharge. It stores enough energy to close and open BOP functions quickly, even if rig power is lost.

PET-226 · What is the difference between hard and soft shut-in?

In a hard shut-in the BOP is closed with the choke already closed, which minimises influx volume. In a soft shut-in the BOP is closed with the choke open, and the choke is then closed, which reduces water-hammer pressure but lets in more influx.

Well Completion & Stimulation

PET-227 · What are the key perforation design parameters?

Shot density (shots per foot), phasing (angle between shots), penetration depth, entry hole diameter and charge type. Deep penetration beats damage; large holes suit gravel packing and fracturing.

PET-228 · Why perforate underbalanced?

With wellbore pressure below reservoir pressure at the moment of firing, the sudden surge of reservoir fluid into the well cleans crushed rock and debris out of the perforation tunnels, giving lower perforation skin.

PET-229 · What is the difference between wireline and tubing-conveyed perforating?

Wireline guns are quick and accurate in depth but limited in gun length and in underbalance. Tubing-conveyed perforating runs long guns on the completion string, allows large underbalance and lets the well flow straight after firing.

PET-230 · How is gravel size selected for a gravel pack?

The common Saucier criterion sizes gravel median diameter at 5–6 times the formation sand median diameter, so formation sand bridges at the gravel face without plugging it. The screen opening is then chosen to retain the gravel.

PET-231 · What fracturing fluid types are used?

Slickwater, linear gels (guar or derivatives), crosslinked gels (borate or metal crosslinked) for higher viscosity and proppant transport, foams and energised fluids, and viscoelastic surfactant fluids. Breakers reduce viscosity after the job for cleanup.

PET-232 · What are ISIP, closure pressure and net pressure?

ISIP is the instantaneous shut-in pressure right after pumping stops. Closure pressure is the pressure at which the fracture closes, about equal to the minimum horizontal stress. Net pressure is fracture pressure minus closure pressure and controls fracture width and growth.

PET-233 · Which direction does a hydraulic fracture grow?

Perpendicular to the minimum principal stress. In most deep reservoirs the minimum stress is horizontal, so fractures are vertical and grow along the direction of the maximum horizontal stress.

PET-234 · What is dimensionless fracture conductivity?

FCD = kf·wf/(k·xf), where kf·wf is fracture conductivity (mD·ft), k is formation permeability and xf is fracture half-length. It compares how easily the fracture delivers fluid with how easily the formation feeds it.

PET-235 · What is the reaction of HCl with limestone?

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Products are soluble, so carbonate acidizing leaves no harmful precipitates; 15% HCl is a common strength.

PET-236 · What is an intelligent completion?

A completion with downhole interval control valves, gauges and control lines that let operators monitor and adjust flow from each zone from surface without intervention.

PET-237 · What are dual and commingled completions?

A dual completion produces two zones separately through two tubing strings or tubing and annulus. A commingled completion produces several zones through one string, which is simpler but makes allocation and crossflow control harder.

PET-238 · What do inflow control devices do in horizontal wells?

ICDs add a calibrated pressure drop at intervals along the completion to even out inflow from heel to toe and across permeability contrasts. This delays water or gas breakthrough at high-flow points.

Production Engineering & Artificial Lift

PET-239 · What is the Vogel IPR?

For solution-gas-drive wells below bubble point, q/qmax = 1 − 0.2(Pwf/P̄r) − 0.8(Pwf/P̄r)². It gives a curved inflow relationship because gas reduces oil relative permeability as pressure falls.

PET-240 · A well makes 500 STB/d at Pwf = 2000 psi with P̄r = 3000 psi. What are PI and AOF?

J = 500/(3000 − 2000) = 0.5 STB/d/psi. With a straight-line IPR, AOF = J × P̄r = 1500 STB/d.

PET-241 · What is nodal analysis?

A systems method that splits the production system at a node, usually bottomhole. The inflow curve (IPR) and outflow curve (vertical lift performance) are plotted, and their intersection gives the operating rate and pressure.

PET-242 · What are the flow regimes in vertical multiphase flow?

As gas rate rises: bubble flow, slug flow, churn flow and annular (mist) flow. Each regime has different holdup and friction, which correlations such as Hagedorn–Brown or Beggs–Brill account for.

PET-243 · How does a gas lift well unload?

Gas is injected into the annulus and enters through the top valve, lightening the fluid above it. As pressure drops each lower valve is uncovered and the upper valves close in turn, until gas enters only at the deepest operating valve.

PET-244 · How is the number of ESP stages chosen?

Total dynamic head TDH = net vertical lift + tubing friction loss + wellhead pressure head. Stages = TDH/head per stage at the design rate, from the pump curve.

PET-245 · What is a dynamometer card?

A plot of polished rod load against rod position over a pump stroke. Its shape diagnoses rod pump problems such as fluid pound, gas interference, leaking valves or a stuck pump.

PET-246 · What is coning?

Drawdown near a well pulls water up from an underlying aquifer or gas down from a gas cap in a cone shape. Above a critical rate the cone breaks into the perforations, raising water cut or GOR.

PET-247 · What is liquid loading and how is the critical rate found?

When gas velocity is too low to carry liquid droplets, liquids fall back and build up in the well, raising backpressure until it dies. Turner's method gives the minimum velocity from a droplet force balance, depending on surface tension and phase densities.

PET-248 · How are gas hydrates prevented?

By keeping fluids outside the hydrate-forming pressure-temperature zone: insulation or heating, dehydration, thermodynamic inhibitors like methanol or MEG, low-dose kinetic or anti-agglomerant inhibitors, or depressurisation.

PET-249 · What causes mineral scale?

Calcium carbonate forms when pressure falls and CO₂ leaves solution. Barium and strontium sulfate form when sulfate-rich injection water, such as seawater, mixes with barium-rich formation water. Scale inhibitors and sulfate removal are used to prevent it.

PET-250 · What is the Gilbert choke equation?

For critical multiphase flow, Pwh = 435·R^0.546·q/S^1.89, with Pwh in psia, R the gas-liquid ratio in Mscf/STB, q in STB/d and S the choke size in 64ths of an inch. Wellhead pressure is proportional to rate.

PET-251 · Calculate rod pump displacement for a 2 in plunger, 100 in stroke and 10 strokes/min.

PD = 0.1166 × Sp × N × D² = 0.1166 × 100 × 10 × 4 ≈ 466 bbl/d at 100% volumetric efficiency.

Well Testing

PET-252 · What is the diffusivity equation?

For radial flow of a slightly compressible fluid, ∂²p/∂r² + (1/r)(∂p/∂r) = (φ·μ·ct/0.0002637·k)(∂p/∂t), with t in hours. Its solutions underlie all pressure transient analysis.

PET-253 · What is the semilog drawdown equation?

For infinite-acting radial flow, Pwf = Pi − (162.6·q·B·μ/kh)[log t + log(k/(φ·μ·ct·rw²)) − 3.23 + 0.87s]. Pwf versus log t is a straight line with slope m = 162.6·q·B·μ/kh.

PET-254 · What is a Horner plot?

Shut-in pressure plotted against log[(tp + Δt)/Δt]. The straight-line slope m gives kh = 162.6·q·B·μ/m, and extrapolation to a Horner time of 1 gives p*, the false pressure, used to estimate average pressure.

PET-255 · Calculate permeability for q = 500 STB/d, B = 1.2, μ = 1 cP, m = 50 psi/cycle and h = 20 ft.

k = 162.6·q·B·μ/(m·h) = 162.6 × 500 × 1.2 × 1/(50 × 20) ≈ 97.6 mD.

PET-256 · How is skin calculated from a buildup test?

s = 1.151[(P1hr − Pwf)/m − log(k/(φ·μ·ct·rw²)) + 3.23], where P1hr is read from the semilog straight line at Δt = 1 hr and Pwf is the flowing pressure just before shut-in.

PET-257 · How is the pressure derivative used for diagnosis?

On a log-log plot of Δp and its derivative, wellbore storage gives a unit slope, radial flow gives a flat derivative, linear flow a half slope, bilinear flow a quarter slope, and closed boundaries a late unit slope in drawdown.

PET-258 · How is radius of investigation estimated?

ri = √[k·t/(948·φ·μ·ct)] in ft, with k in mD, t in hours, μ in cP and ct in psi⁻¹. It does not depend on rate.

PET-259 · What is pseudo-steady state?

Once the transient reaches all no-flow boundaries, pressure falls at the same constant rate everywhere in the drainage area. The rate of decline gives the connected pore volume, which is the basis of a reservoir limit test.

PET-260 · What is the gas well back-pressure equation?

q = C(P̄r² − Pwf²)ⁿ, with n between 0.5 (fully turbulent) and 1 (laminar). C and n are found from multi-rate tests, and the absolute open flow is q at Pwf = 0.

PET-261 · What are isochronal and modified isochronal tests?

Gas deliverability tests in tight gas where stabilisation takes too long. Flow periods of equal length at different rates are separated by shut-ins to stabilisation (isochronal) or by shut-ins of the same length (modified isochronal), plus one extended flow point.

PET-262 · Why is pseudo-pressure used for gas wells?

Gas viscosity and z-factor change strongly with pressure, so the liquid diffusivity equation does not apply directly. The pseudo-pressure m(p) = 2∫(p/μz)dp linearises it so liquid-style analysis can be used.

PET-263 · How is superposition used in well testing?

Because the diffusivity equation is linear, solutions add. Rate changes are handled by superposing solutions in time, and boundaries such as faults are handled by superposing image wells in space.

Enhanced Oil Recovery

PET-264 · What are primary, secondary and tertiary recovery?

Primary recovery uses natural reservoir energy such as solution gas, gas cap or water drive. Secondary recovery adds energy by water or gas injection for pressure maintenance. Tertiary recovery (EOR) uses thermal, gas or chemical methods to recover residual oil.

PET-265 · What is the capillary number and why does it matter?

Nc = v·μ/σ, the ratio of viscous to capillary forces. Waterfloods run at about 10⁻⁷–10⁻⁶; residual oil only falls significantly when Nc is raised by several orders of magnitude, which is why surfactant floods aim for ultra-low IFT.

PET-266 · What is minimum miscibility pressure (MMP)?

The lowest pressure at which injected gas and reservoir oil become miscible through multiple contacts at reservoir temperature. It is measured with slim-tube tests or rising-bubble tests or estimated by correlations and equation-of-state models.

PET-267 · How does CO₂ improve oil recovery?

CO₂ dissolves in oil, which swells it and cuts its viscosity, reduces interfacial tension and extracts light and intermediate components. Above MMP it becomes multicontact miscible and displaces oil very efficiently.

PET-268 · What is water-alternating-gas (WAG) injection?

Alternating slugs of water and gas. Water reduces the effective mobility of the gas, which limits viscous fingering and gravity override and improves sweep while still gaining gas displacement efficiency.

PET-269 · What polymers are used in polymer flooding?

Mainly partially hydrolysed polyacrylamide (HPAM), which is cheap but sensitive to salinity, hardness and shear, and xanthan gum, which tolerates salinity but is vulnerable to bacterial degradation. Newer polymers are designed for high temperature and salinity.

PET-270 · What are steam quality and steam-oil ratio?

Steam quality is the mass fraction of vapour in the wet steam; higher quality carries more latent heat. Steam-oil ratio is the volume of steam injected (as cold-water equivalent) per volume of oil produced, the key measure of thermal efficiency.

PET-271 · What is in-situ combustion?

Air or oxygen is injected and part of the oil, mainly the coke-like residue, is burned in the reservoir. The heat front lowers oil viscosity and the combustion gases and steam drive oil ahead. Wet combustion adds water to carry heat forward.

PET-272 · What is microbial EOR?

Injecting microbes and nutrients, or stimulating native microbes, to produce gases, biosurfactants, polymers or acids in the reservoir, or to plug high-permeability zones. Results are often hard to predict and control.

PET-273 · What is low-salinity waterflooding?

Injecting water of lower and controlled salinity than formation water. In suitable rocks it can shift wettability toward more water-wet and release extra oil; proposed mechanisms include multicomponent ion exchange and double-layer expansion.

PET-274 · What are EOR screening criteria?

Rules of thumb based on oil gravity and viscosity, depth, temperature, permeability, thickness and formation type. Thermal methods suit shallow, thick heavy-oil reservoirs; miscible gas suits deeper light oils; chemicals suit moderate temperature and salinity.

PET-275 · Calculate displacement efficiency for Soi = 0.75 and Sor = 0.30.

ED = (Soi − Sor)/Soi = (0.75 − 0.30)/0.75 = 0.60, so 60% of the oil in swept zones is displaced.

Unconventional Resources

PET-276 · Why do very low-permeability reservoirs need fracturing?

In nanodarcy rock the pressure transient moves so slowly that an unstimulated well drains only a tiny volume. Fractures create a large contact area so many short flow paths can deliver economic rates.

PET-277 · How is gas stored in shale?

As free gas in pores and natural fractures, and as gas adsorbed on organic matter and clays. Adsorbed gas follows the Langmuir isotherm V = VL·P/(PL + P) and is released as pressure falls.

PET-278 · What is rock brittleness in shale?

A measure of how readily the rock fractures and keeps fractures open. It is estimated from mineralogy (more quartz and carbonate, less clay) or from high Young's modulus and low Poisson's ratio.

PET-279 · How are sweet spots identified in shale plays?

By combining high TOC, suitable thermal maturity, thickness, porosity, overpressure, brittleness, favourable stress and natural fractures, from logs, cores, seismic and early production data.

PET-280 · What is zipper fracturing?

Alternating fracture stages between two or more adjacent wells on a pad. It keeps the frac fleet pumping while the other well is being plugged and perforated, and stress changes from one well can increase fracture complexity in the next.

PET-281 · What is flowback?

The period after fracturing when injected fluid, proppant and initial reservoir fluids return to surface. It is managed with chokes and test equipment, and the water is treated, recycled or disposed of.

PET-282 · What is the stimulated reservoir volume (SRV)?

The volume of rock around a fractured horizontal well whose effective permeability has been increased by the fracture network. It is often estimated from microseismic events and production analysis.

PET-283 · What does microseismic monitoring show?

Small seismic events caused by rock failure during fracturing, recorded by downhole or surface geophone arrays. Their locations show approximate fracture length, height, azimuth and possible interaction with faults.

PET-284 · What flow regime dominates early life of a fractured shale well?

Transient linear flow from the matrix into the fractures, which can last months to years. Rate-normalised pressure is linear with √t, and the slope gives the product of fracture area and √k.

PET-285 · How does SAGD work?

Two horizontal wells are drilled one above the other, about 5 m apart. Steam injected in the upper well forms a growing steam chamber, and heated bitumen and condensate drain by gravity to the lower production well.

PET-286 · How is coalbed methane produced?

Water is pumped from the coal to lower pressure below the critical desorption pressure. Gas then desorbs from the coal matrix, diffuses to the cleats and flows to the well, so gas rate often rises while water rate falls.

PET-287 · What are parent-child well interactions?

When a new (child) well is fractured near an older producing (parent) well, the depleted zone around the parent attracts the child's fractures. This can cause frac hits that damage the parent and lower child well performance.

Surface Facilities, Pipelines, Refining & HSE

PET-288 · What is fractional distillation of crude oil?

Crude is heated and fed to a column where vapours rise and condense at trays according to boiling range. Products from top to bottom are gases and LPG, naphtha, kerosene, diesel and gas oils, and residue.

PET-289 · How is a gas-liquid separator sized?

Gas capacity is set by the Souders–Brown equation: Vmax = K·√[(ρL − ρg)/ρg], which limits gas velocity so liquid droplets settle. Liquid capacity is set by the required residence time.

PET-290 · What are the main steps in natural gas processing?

Inlet separation, acid gas removal, dehydration, NGL recovery (often by turboexpander), and fractionation in a demethaniser, deethaniser, depropaniser and debutaniser into sales gas, ethane, propane, butane and natural gasoline.

PET-291 · How is pressure drop in a liquid pipeline calculated?

With the Darcy–Weisbach equation ΔP = f·(L/D)·(ρv²/2), plus elevation change ρ·g·Δz. The friction factor f comes from the Moody chart or Colebrook equation using Reynolds number and relative roughness.

PET-292 · Why are pipelines hydrotested?

Filling with water and pressurising above maximum operating pressure for a set period proves strength and leak-tightness. Water is used because it stores little energy, so a failure releases far less energy than with gas.

PET-293 · What does fluid catalytic cracking do?

FCC cracks heavy gas oil over a hot zeolite catalyst in a riser reactor, producing gasoline, LPG, light olefins and light cycle oil. Coked catalyst is burned clean in a regenerator, which also supplies the heat.

PET-294 · What is hydrotreating?

Reacting a feed with hydrogen over cobalt-molybdenum or nickel-molybdenum catalysts to remove sulfur (as H₂S), nitrogen (as NH₃) and metals, and to saturate olefins. It is essential for low-sulfur fuels.

PET-295 · What does catalytic reforming do?

It converts low-octane naphtha into high-octane reformate over a platinum-based catalyst, mainly by dehydrogenating naphthenes into aromatics and isomerising paraffins. It is also a major hydrogen source for the refinery.

PET-296 · What is the difference between octane and cetane numbers?

Octane number measures a gasoline's resistance to knocking (auto-ignition) in spark-ignition engines. Cetane number measures how readily diesel ignites under compression; higher cetane means shorter ignition delay.

PET-297 · How does the Claus process recover sulfur?

One third of the H₂S is burned to SO₂, then 2H₂S + SO₂ → 3S + 2H₂O over a catalyst in further stages. Tail gas treatment raises overall sulfur recovery further.

PET-298 · What is a HAZOP?

A hazard and operability study in which a team reviews each process node using guidewords (no, more, less, reverse and so on) applied to parameters like flow and pressure to find deviations, causes, consequences and safeguards.

PET-299 · How is produced water treated?

Oil is removed with skim tanks, hydrocyclones, gas flotation units and filters, and solids are removed. The water is then reinjected for pressure support, injected into disposal wells or discharged where it meets regulatory limits.

PET-300 · What is the difference between flaring and venting?

Flaring burns waste gas, converting methane mainly to CO₂ and water. Venting releases gas unburned. Venting usually has a much larger climate impact because methane is a far more potent greenhouse gas than CO₂.

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