GATE Petroleum Engineering Latest trends in Petroleum Engineering Syllabus

What is oil shale?

Oil shale is a fine-grained sedimentary rock containing solid, immature organic matter called kerogen that has not yet been converted to oil; on heating (pyrolysis) it yields liquid shale oil and combustible gas.

What is the critical distinction between oil shale and shale oil?

Oil shale is a rock containing solid kerogen (immature organic matter) that must be artificially heated to produce oil. Shale oil (tight oil) is already-formed liquid crude trapped in low-permeability shale/tight formations, produced by fracturing. (Note: 'shale oil' can also refer to the synthetic oil from retorting oil shale.)

What is kerogen?

Kerogen is the solid, insoluble, high-molecular-weight organic matter disseminated in sedimentary rocks that is the precursor to oil and gas; when heated it decomposes to produce hydrocarbons. It is insoluble in common organic solvents (unlike bitumen).

What is retorting in the context of oil shale extraction?

Retorting is the thermal process of heating oil shale to roughly $450$–$500\,^{\circ}\mathrm{C}$ in the absence of oxygen (pyrolysis) to decompose (crack) the solid kerogen into liquid shale oil, gas, and a solid carbonaceous residue (spent shale/char).

What are the two broad categories of oil shale processing technologies?

(1) Ex-situ (surface/above-ground) retorting — the shale is mined, crushed, and heated in a surface retort; and (2) In-situ retorting — the shale is heated underground in place and the liberated oil is pumped to the surface.

Name the four kerogen types and which is most oil-prone.

Type I (lacustrine, algal — most oil-prone, highest H/C, highest oil yield), Type II (marine — oil and gas prone), Type III (terrestrial, woody — gas-prone), and Type IV (inert/residual — little to no yield).

What laboratory method/assay is used to determine the oil yield of an oil shale?

The Fischer Assay, a standardized retorting test that heats a crushed sample to about $500\,^{\circ}\mathrm{C}$ and measures the volume of oil produced, usually expressed in litres per tonne (L/t) or gallons per ton.

What is the advantage of in-situ retorting over surface (ex-situ) retorting of oil shale?

In-situ retorting avoids mining and crushing, eliminates the disposal problem of large volumes of spent shale, can access deeper deposits, and generally has a smaller surface footprint — though it is slower and harder to control.

Why does shale oil from retorting require upgrading before refining?

Raw shale oil is high in nitrogen, sulfur, oxygen, and arsenic, has high pour point and viscosity, and contains olefins/diolefins. It must be hydrotreated (hydrogenation to remove heteroatoms and stabilize) before it can be processed in conventional refineries.

What is a major environmental drawback of ex-situ oil shale processing related to volume?

Spent shale expands (popcorn effect) so the processed waste occupies a greater volume than the original mined rock, creating large solid-waste disposal problems; the process is also water-intensive and energy-intensive with high $\ce{CO2}$ emissions.

What is a gas hydrate (clathrate hydrate)?

A gas hydrate is a crystalline, ice-like solid in which gas molecules (typically methane) are physically trapped within cages formed by hydrogen-bonded water molecules; it is a clathrate compound, not a chemical bond between gas and water.

Under what general conditions do methane hydrates form and remain stable?

Under high pressure and low temperature, typically found in permafrost regions and in deep marine sediments along continental margins (water depths greater than about $300$–$500$ m where the seafloor is cold enough).

What are the three main crystal structures of gas hydrates?

Structure I (sI) — cubic, formed by small molecules like $\ce{CH4}$ and $\ce{CO2}$; Structure II (sII) — cubic, formed with larger molecules like propane and isobutane; and Structure H (sH) — hexagonal, requiring both small and large guest molecules.

Approximately how much gas (at STP) does one volume of methane hydrate release on dissociation?

About 160–180 volumes of methane gas at standard temperature and pressure per unit volume of solid hydrate, reflecting its extremely high gas storage density.

What is the hydrate number, and what is the ideal value for methane hydrate (Structure I)?

The hydrate number is the ratio of water molecules to gas molecules in the hydrate. For ideal (fully occupied) methane Structure I hydrate it is $\ce{CH4 \cdot 5.75 H2O}$, i.e. a hydrate number of 5.75 (often approximated as 6).

Name the three main methods proposed for producing gas from methane hydrate deposits.

(1) Depressurization — lowering pressure below the hydrate stability point; (2) Thermal stimulation — injecting heat/hot water/steam to raise temperature above stability; and (3) Chemical inhibitor injection — adding inhibitors (e.g., methanol, glycol) to shift the equilibrium curve. A fourth is $\ce{CO2}$–$\ce{CH4}$ swapping/exchange.

Which gas hydrate production method is generally considered the most energy-efficient and economically promising?

Depressurization, because it requires no large continuous external energy input (unlike thermal stimulation) and no costly chemicals (unlike inhibitor injection); the pressure drop drives endothermic dissociation throughout the reservoir.

What is the Gas Hydrate Stability Zone (GHSZ)?

The depth/temperature interval within sediments where pressure and temperature conditions fall inside the hydrate stability field, so hydrates can exist. Its base is defined where the geothermal gradient crosses the hydrate phase boundary.

Why are gas hydrates considered both a potential energy resource and a hazard?

Resource: they hold an enormous global reserve of methane (more carbon than all conventional fossil fuels combined). Hazard: their dissociation can cause seafloor instability/submarine landslides, wellbore/pipeline blockages (flow assurance), and methane (a potent greenhouse gas) release.

Write the dissociation reaction of methane hydrate.

$$\ce{CH4 \cdot nH2O (s) -> CH4 (g) + nH2O (l)}$$ with $n \approx 5.75$; the reaction is endothermic, absorbing heat as the solid breaks down into gas and water.

What is heavy oil and how is it broadly defined?

Heavy oil is a dense, highly viscous crude oil characterized by low API gravity and high viscosity. It is broadly defined as crude with an API gravity between $10^{\circ}$ and $22.3^{\circ}$ API and viscosity (at reservoir conditions) typically in the range of $100$ to $10{,}000$ cP.

Give the formula relating API gravity to specific gravity and the API cutoff for heavy oil.

$$\text{API} = \frac{141.5}{SG_{60/60}} - 131.5$$ Heavy oil has API gravity below about $22.3^{\circ}$ (down to $\sim 10^{\circ}$); extra-heavy oil and bitumen are below $10^{\circ}$ API (specific gravity $> 1$, i.e., denser than water).

How are heavy oil, extra-heavy oil, and bitumen distinguished by API gravity and viscosity?

Heavy oil: $10^{\circ}$–$22.3^{\circ}$ API, viscosity $\sim 100$–$10{,}000$ cP. Extra-heavy oil: $< 10^{\circ}$ API but still mobile/flowing in reservoir. Bitumen (oil sands/tar sands): $< 10^{\circ}$ API and essentially immobile (viscosity $> 10{,}000$ cP) at reservoir conditions.

Why does heavy oil have such high viscosity and low API gravity?

It is rich in large, complex, high-molecular-weight molecules — asphaltenes and resins — and heteroatoms (sulfur, nitrogen, oxygen) and metals (Ni, V), with a low ratio of light/saturate fractions. Biodegradation and loss of light ends during migration further degrade it.

What are the main thermal recovery methods used to produce heavy oil?

Steam-based methods that reduce viscosity by heating: Cyclic Steam Stimulation (CSS, 'huff and puff'), Steam Flooding (continuous steam drive), Steam-Assisted Gravity Drainage (SAGD), and In-Situ Combustion (fire flooding).

Describe the principle of Steam-Assisted Gravity Drainage (SAGD).

Two parallel horizontal wells are drilled, one a few metres above the other. Steam is injected into the upper well, forming a steam chamber that heats the surrounding bitumen; the heated, less-viscous oil and condensate drain by gravity into the lower production well.

What is Cyclic Steam Stimulation (CSS) / 'huff and puff'?

A single-well thermal method with three repeating phases: (1) injection — inject high-pressure steam into the reservoir; (2) soak — shut in the well to let heat transfer and reduce oil viscosity; (3) production — produce the heated, mobilized oil. The cycle is repeated.

How does viscosity of heavy oil respond to temperature, and why is this the basis of thermal recovery?

Viscosity decreases exponentially (very strongly) with increasing temperature — roughly following an Arrhenius-type relation $\mu \propto e^{E/RT}$. A modest temperature rise via steam can cut viscosity by orders of magnitude, making the oil mobile enough to flow to the well.

What is the SARA analysis used to characterize heavy oil?

SARA fractionates crude into four solubility/polarity classes: Saturates, Aromatics, Resins, and Asphaltenes. Heavy oils are rich in Resins and Asphaltenes (the heaviest, most polar fractions), which govern their high viscosity, density, and processing/upgrading challenges.

What is heavy oil upgrading and what are its two main approaches?

Upgrading converts heavy, low-API crude into lighter, higher-value synthetic crude by increasing the H/C ratio. The two approaches are carbon rejection (removing carbon, e.g., coking, visbreaking, solvent deasphalting) and hydrogen addition (e.g., hydrocracking, hydrotreating).