GATE Petroleum Engineering Enhanced Oil Recovery Techniques Syllabus

In alkaline flooding, how is in-situ surfactant produced?

Injected alkali (e.g., $\ce{NaOH}$, $\ce{Na2CO3}$) reacts with naturally occurring organic acids (naphthenic acids) in acidic crude oils to form soaps (in-situ surfactants) that lower interfacial tension. Hence alkaline flooding works best with high-acid-number crudes.

Define miscible flooding and contrast first-contact vs multiple-contact miscibility.

Miscible flooding injects a solvent that mixes with reservoir oil in all proportions, eliminating the interface (zero interfacial tension) so residual oil $\to 0$. First-contact miscible: solvent and oil mix immediately on contact (e.g., LPG). Multiple-contact (dynamic) miscible: miscibility develops after repeated mass transfer between oil and gas (vaporizing or condensing gas drive).

What is the Minimum Miscibility Pressure (MMP)?

MMP is the lowest reservoir pressure at which a given injection gas becomes (dynamically) miscible with a given crude oil, achieving very high displacement efficiency. The reservoir must be operated at or above MMP for a miscible flood to work.

Why is $\ce{CO2}$ a favored miscible flooding agent, and what is its typical MMP range?

$\ce{CO2}$ achieves miscibility (multiple-contact, vaporizing/condensing) at relatively low pressures, swells the oil, and reduces its viscosity. Typical MMP is about $\sim 1100\text{--}1500\,\text{psi}$ ($\sim 7\text{--}10\,\text{MPa}$), much lower than $\ce{N2}$ or flue gas, making it widely applicable.

What is the main problem with miscible gas floods despite their high microscopic efficiency, and one remedy?

Low gas viscosity and density give a very unfavorable mobility ratio and gravity override, causing viscous fingering and poor sweep (low macroscopic efficiency). Remedy: Water-Alternating-Gas (WAG) injection, which alternates water and gas slugs to control mobility and improve sweep.

List the three major thermal EOR processes.

1. Steam (cyclic) stimulation — huff and puff. 2. Steam flooding (steam drive) and hot-water flooding — a continuous-drive process. 3. In-situ combustion (fireflooding). All target heavy/viscous oils by reducing oil viscosity with heat.

Why are thermal methods primarily applied to heavy oils, and what is the dominant recovery mechanism?

Heavy oils have very high viscosity that makes flow nearly impossible at reservoir temperature. Heat dramatically lowers oil viscosity (viscosity falls steeply with temperature), so viscosity reduction is the dominant mechanism, aided by thermal expansion, steam distillation, and gas drive.

Describe the three stages of steam (cyclic) stimulation, a.k.a. 'huff and puff'.

1. Injection (huff): steam is injected into the well for a period to heat the surrounding oil. 2. Soak: the well is shut in for days/weeks so heat spreads and oil viscosity drops. 3. Production (puff): the well is produced; the heated, mobilized oil flows out. The cycle is repeated.

Contrast steam stimulation with steam flooding in terms of well usage.

Steam stimulation is a single-well process: the same well is used for both injection and production in cycles. Steam flooding (steam drive) is a multi-well/pattern process: steam is injected continuously into injection wells to drive oil toward separate producing wells.

What is steam quality and why does it matter in steam flooding?

Steam quality is the mass fraction of vapor in a wet-steam mixture: $$x = \frac{m_{vapor}}{m_{vapor}+m_{liquid}}$$ Higher quality carries more latent heat per unit mass, delivering more thermal energy to the reservoir for viscosity reduction and steam distillation.

In a steam drive, name the zones formed between injector and producer (in order of decreasing temperature).

From injector outward: (1) steam zone (highest T, steam distillation), (2) condensation/hot-water zone (steam condenses, hot-water drive), (3) oil bank (displaced oil accumulating), (4) cold/native reservoir zone ahead of the front.

Why is hot-water flooding generally less effective than steam flooding?

Hot water carries only sensible heat (no large latent heat of vaporization), so it transports far less energy per unit mass and cools quickly. Steam additionally provides latent heat, steam distillation of light ends, and a gas-drive effect, giving better heating and higher recovery.

What is in-situ combustion (fireflooding) and what is injected?

In-situ combustion ignites a portion of the crude oil in the reservoir and injects air (or oxygen-enriched air) to sustain a combustion front. The burning fuel (coke) generates heat that reduces oil viscosity and drives a combination of thermal, gas, and steam drives toward producers.

Distinguish forward and reverse in-situ combustion.

Forward combustion: the combustion front moves in the same direction as the injected air, from injector toward producer; oil is displaced ahead of the heated zone. Reverse combustion: the front moves opposite to the air flow (ignited near the producer), so oil flows through the heated zone — useful for very viscous oils but rarely successful in practice.

In forward in-situ combustion, what acts as the fuel for the burning front?

The heaviest fraction of the crude — the coke (carbon residue) deposited by thermal cracking of the oil as the front advances. The lighter components are vaporized and displaced ahead, leaving coke that burns to sustain the combustion.

What is the air-fuel ratio's role and the air requirement concept in in-situ combustion design?

The fuel concentration (coke deposited per unit volume of rock) and the air-fuel ratio determine the air requirement — the volume of air needed to burn through a unit volume of reservoir. High fuel deposition needs more air (higher cost) but sustains the front; it must be balanced for an economic burn.

What is wet combustion (COFCAW) and its advantage?

Wet combustion injects water alternately or simultaneously with air. The water vaporizes in the hot burned zone and carries heat forward (as steam) that would otherwise be left behind, improving heat utilization and recovery while reducing air (fuel) requirements compared with dry forward combustion.

What is SAGD and how do its two wells operate?

SAGD = Steam-Assisted Gravity Drainage. It uses a pair of horizontal wells, one above the other (typically $\sim 5\,\text{m}$ apart). Steam injected in the upper well forms a steam chamber; heated, mobilized oil and condensate drain by gravity to the lower producing well. Used for very heavy oils/bitumen.

Define Microbial Enhanced Oil Recovery (MEOR).

MEOR uses microorganisms (and/or their metabolic products) injected into or stimulated within the reservoir to enhance oil recovery. Microbes consume nutrients and generate byproducts (biosurfactants, biopolymers, gases, acids, solvents) that mobilize trapped oil.

List the main mechanisms by which microbes enhance oil recovery in MEOR.

1. Biosurfactant production → lowers interfacial tension (improves $E_D$). 2. Biopolymer production → viscosity/selective plugging for mobility and sweep control. 3. Gas production ($\ce{CO2}$, $\ce{CH4}$) → repressurization and oil swelling. 4. Acid/solvent production → improves permeability and reduces viscosity. 5. Selective plugging of high-perm zones → improves conformance.

Name two operational limitations of MEOR.

1. Microbial activity is constrained by reservoir conditions — high temperature ($\gtrsim 80\text{--}90^\circ\text{C}$), high salinity, high pressure, and extreme pH inhibit most bacteria. 2. Difficulty controlling/predicting in-situ growth, nutrient distribution, and possible souring (e.g., $\ce{H2S}$ from sulfate-reducing bacteria).

What is selective plugging in MEOR and why is it useful?

Microbes (or their biomass/biopolymers) preferentially grow and plug the high-permeability channels (thief zones). This diverts subsequent injected fluid into lower-permeability, unswept zones, improving conformance and volumetric sweep efficiency.

State the Buckley-Leverett fractional flow equation for water (neglecting gravity and capillary pressure).

$$f_w = \frac{1}{1 + \dfrac{k_{ro}\,\mu_w}{k_{rw}\,\mu_o}}$$ It gives the fraction of water in the total flowing stream as a function of water saturation and is central to immiscible displacement (waterflood) analysis.

In Buckley-Leverett theory, how is the velocity of a plane of constant water saturation expressed?

$$v_{S_w} = \frac{q_t}{A\,\phi}\left(\frac{df_w}{dS_w}\right)_{S_w}$$ The saturation front velocity is proportional to the slope of the fractional-flow curve, so different saturations travel at different speeds, producing the shock front.

What does the capillary number $N_c$ govern, and roughly how much must it increase to substantially reduce residual oil?

$N_c = \dfrac{v\mu}{\sigma}$ governs the balance of viscous to capillary forces and hence microscopic displacement (mobilization of trapped oil). Typical waterfloods have $N_c \sim 10^{-7}$ to $10^{-6}$; reducing residual oil substantially requires raising $N_c$ by about 3-4 orders of magnitude (to $\sim 10^{-3}$ to $10^{-2}$), usually by lowering $\sigma$.

Compare microscopic vs macroscopic displacement efficiency in one sentence each.

Microscopic ($E_D$): efficiency of oil mobilization at the pore scale — how much oil is displaced from pores actually contacted, governed by capillary/IFT forces. Macroscopic ($E_V$): efficiency of contact at the reservoir scale — what fraction of the reservoir volume is swept, governed by mobility ratio and heterogeneity.

Why does an unfavorable mobility ratio lead to viscous fingering?

When $M>1$ the displacing fluid is more mobile than the oil; small perturbations at the front grow because the displacing fluid accelerates through the path of least resistance, forming fingers that bypass oil, cause early breakthrough, and reduce sweep efficiency.

For chemical flooding, why is interfacial tension reduction insufficient on its own and usually paired with mobility control?

Lowering IFT (surfactant) raises microscopic efficiency $E_D$ by mobilizing residual oil, but the mobilized oil bank can be bypassed if sweep is poor. Adding polymer for mobility control (favorable $M$) ensures the oil bank is efficiently swept to producers, so both $E_D$ and $E_V$ improve (the basis of micellar-polymer and ASP floods).

Rank $\ce{CO2}$, $\ce{N2}$, and hydrocarbon gas (LPG) by typical minimum miscibility pressure, lowest to highest.

Lowest to highest MMP: LPG/enriched hydrocarbon gas (often first-contact miscible, low pressure) < $\ce{CO2}$ ($\sim 1100\text{--}1500\,\text{psi}$) < lean hydrocarbon/flue gas < $\ce{N2}$ (highest, often $>4000\,\text{psi}$). Lower MMP means miscibility is easier to achieve.

What is the role of a 'mobility buffer' (polymer drive) following a surfactant/micellar slug?

After the costly micellar (surfactant) slug, a polymer-thickened water (mobility buffer) is injected behind it to maintain a favorable mobility ratio between the chase water and the slug. This protects the slug from being fingered through, preserving its integrity and pushing the oil bank efficiently to producers.

Summarize which efficiency each major EOR class chiefly improves: thermal, miscible, polymer, surfactant.

Thermal: improves $E_D$ (viscosity reduction, lower $S_{or}$) and mobility; Miscible gas: improves $E_D$ (zero IFT, near-complete pore-scale displacement) but needs WAG for sweep; Polymer: improves macroscopic sweep $E_V$ (favorable $M$); Surfactant: improves microscopic $E_D$ (ultralow IFT, higher $N_c$).