GATE Petroleum Engineering Petroleum Production Operations Syllabus

Define water cut (WC).

$$WC=\frac{q_{w}}{q_{w}+q_{o}}\times 100\%$$ the fraction (or percentage) of water in the total produced liquid stream.

What is the purpose of a separator in surface facilities?

To physically separate the produced wellstream into its gas, oil and water phases using gravity and momentum so each phase can be measured, treated and transported separately.

List the four mechanisms/sections of a typical oilfield separator.

Primary separation (momentum/inlet diverter), gravity settling section, coalescing/mist-extraction section, and liquid collection (with level control); they remove gas from liquid and liquid mist from gas.

Compare two-phase and three-phase separators.

A two-phase separator splits the stream into gas and total liquid; a three-phase separator additionally separates the liquid into oil and water, providing separate oil, water and gas outlets.

Compare vertical and horizontal separators.

Vertical separators handle high GOR/low liquid and surging flow, occupy small footprint, and handle sand well; horizontal separators give more gas-liquid interface area, are best for high-liquid/foaming and three-phase service, but need more plot space.

What is retention (residence) time in a separator?

The average time the liquid spends in the separator, allowing gas to break out and phases to separate: $$t_{r}=\frac{V_{liquid}}{q_{liquid}}$$ typically 1–3 minutes for oil/water separation.

Define an emulsion in production and name its two types.

An emulsion is a stable dispersion of one immiscible liquid as droplets in another. Water-in-oil (W/O, most common in oilfields) and oil-in-water (O/W); they are stabilized by emulsifying agents at the interface.

What is the role of demulsifiers and heater-treaters?

Demulsifiers are chemicals that neutralize emulsifying agents so water droplets coalesce; heater-treaters apply heat (and sometimes electrostatic fields) to reduce viscosity and break the emulsion, separating water from crude.

State the Stokes' law settling velocity used for droplet separation.

$$v_{t}=\frac{g\,d^{2}(\rho_{w}-\rho_{o})}{18\,\mu}$$ the terminal settling velocity of a droplet of diameter $d$ and density difference $(\rho_w-\rho_o)$ in a fluid of viscosity $\mu$.

What is BS&W and its typical sales specification?

Basic Sediment and Water: the combined volume fraction of water and sediment in crude oil. Pipeline/sales crude is typically required to be below about $0.5\%$–$1\%$ BS&W.

Why is crude oil stabilization/sweetening sometimes required before sale?

Stabilization removes light ends ($\ce{C1}$–$\ce{C4}$, $\ce{H2S}$) to control vapor pressure (e.g., Reid vapor pressure) for safe storage/transport; sweetening removes $\ce{H2S}$ to meet corrosion and toxicity limits.

What is the difference between primary, secondary and tertiary recovery?

Primary uses natural reservoir energy; secondary injects water or gas to maintain pressure and displace oil (pressure maintenance/sweep); tertiary (EOR) uses thermal, chemical or miscible methods to mobilize residual oil beyond secondary.

Name the natural reservoir drive mechanisms.

Solution (dissolved) gas drive, gas-cap drive, water drive, gravity drainage, and combination drive; they differ in recovery factor and pressure-decline behavior.

Rank common drive mechanisms by typical recovery factor.

Water drive (about $35$–$75\%$) > gas-cap drive (about $20$–$40\%$) > gravity drainage (variable, can be high) > solution-gas drive (about $5$–$30\%$), which is the least efficient.

What is the wellhead / Christmas tree?

The assembly of valves, spools and fittings at surface that provides pressure control of the well, allows access for interventions, and routes produced fluids into the flowline; the 'Christmas tree' is the valve stack mounted on the wellhead.

Define a choke and its function in production.

A choke is a restriction (fixed or adjustable bean) installed in the flow stream to control flow rate and protect downstream equipment by creating a controlled pressure drop, often maintaining critical (sonic) flow.

What is critical (sonic) flow through a wellhead choke?

Flow where the downstream pressure is low enough (typically $p_{down}\le 0.5\,p_{up}$ for gas) that the rate depends only on upstream pressure and choke size, not on downstream pressure; it isolates the well from downstream fluctuations.

What is formation damage and list common causes.

A reduction in near-wellbore permeability (positive skin). Causes: clay swelling/migration, solids and mud-filtrate invasion during drilling, scale and paraffin/asphaltene deposition, fines migration, and emulsion blocking.

What is matrix acidizing and which acids are used?

Injecting acid below fracturing pressure to dissolve damage and increase near-wellbore permeability. Hydrochloric acid ($\ce{HCl}$) for carbonates; hydrofluoric/HCl mud acid ($\ce{HF}$/$\ce{HCl}$) for sandstones.

Write the reaction of HCl acidizing a limestone reservoir.

$$\ce{CaCO3 + 2HCl -> CaCl2 + H2O + CO2}$$

What is hydraulic fracturing and how does it improve productivity?

Pumping fluid above the formation parting (fracture) pressure to create conductive fractures held open by proppant, creating a high-conductivity path that bypasses damage and increases the effective wellbore radius, lowering skin.

Define proppant and its role in a fracture.

Proppant (sand, resin-coated sand, or ceramic beads) is the granular material carried by the frac fluid that props the fracture open after pumping stops, maintaining fracture conductivity against closure stress.

What is sand control and name common methods.

Techniques to prevent formation sand production in unconsolidated reservoirs. Methods: gravel packing, screens (wire-wrapped, premium), frac-pack, and chemical consolidation; they retain formation sand while allowing fluid flow.

What is scale in production operations and give two common types.

Mineral deposits precipitating from produced water as conditions change. Common scales: calcium carbonate ($\ce{CaCO3}$) and barium sulfate ($\ce{BaSO4}$); they plug tubing, perforations and equipment and are controlled with scale inhibitors.

What causes paraffin (wax) and asphaltene deposition?

Cooling below the wax appearance temperature causes paraffin crystallization; pressure/composition changes (e.g., $\ce{CO2}$, light-end loss) destabilize asphaltenes. Both deposit in tubing/flowlines and are managed by heat, solvents, inhibitors or scrapers/pigs.

What is well testing (pressure transient testing) used to determine?

By measuring pressure response to a rate change (e.g., buildup or drawdown), it yields reservoir permeability ($k$), skin ($s$), average reservoir pressure, and boundary/heterogeneity information.

What is the difference between a drawdown test and a buildup test?

In a drawdown test the well is produced at constant rate and the declining $p_{wf}$ is recorded; in a buildup test a producing well is shut in and the recovering bottomhole pressure is recorded—buildup is preferred for stable rate control.

Define total liquid flow rate and gas rate measurement at a test/group separator.

After separation, oil and water rates are metered separately (turbine/Coriolis/positive-displacement meters) and gas through an orifice or flow meter; combining gives well GOR, water cut and total production for allocation.

What is a pig and why is pigging done in flowlines?

A pig is a device pushed through a pipeline by the flow to clean (remove wax/scale/water), batch-separate fluids, or inspect (smart/intelligent pig) the line; pigging maintains throughput and integrity.

Define recovery factor (RF) and write its expression.

$$RF=\frac{\text{cumulative oil produced}}{\text{original oil in place (OOIP)}}\times100\%$$ the fraction of original hydrocarbons in place that is ultimately recovered; product of displacement and sweep efficiencies.

What is the difference between volumetric and material-balance estimates of oil in place?

The volumetric method computes OOIP from rock/fluid properties: $$N=\frac{7758\,A\,h\,\phi(1-S_{w})}{B_{o}}$$ (STB); the material-balance method infers in-place volume and drive from observed production and pressure history.