GATE Petroleum Engineering Petroleum Formation Evaluation: Evaluation of petrophysical of sub-surface formations Syllabus

Petroleum Formation Evaluation: Evaluation of petrophysical of sub-surface formations syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Petroleum Formation Evaluation: Evaluation of petrophysical of sub-surface formations in GATE Petroleum Engineering, not a summary of it.

1. 1

   Principles applications, advantages and disadvantages of SP, resistivity, radioactive, acoustic logs and types of tools used

   4 topics

   • SP Log
   • Resistivity Log
   • Radioactive Log
   • Acoustic Log
2. 2

   Evaluation of CBL/VDL, USIT, SFT, RFT

   5 topics

   • Cement Bond Log (CBL)
   • Variable Density Log (VDL)
   • USIT
   • SFT
   • RFT
3. 3

   Production logging tools, principles, limitations and applications

   1 topic

   • Production Logging Tools
4. 4

   Special type of logging tools

   1 topic

   • Special Logging Tools
5. 5

   Casing inspection tools (principles, applications and limitations)

   1 topic

   • Casing Inspection Tools
6. 6

   Formations micro scanner (FMS), NMR logging principles

   2 topics

   • Formations Micro Scanner (FMS)
   • NMR Logging Principles
7. 7

   Standard log interpretation methods

   1 topic

   • Log Interpretation Methods
8. 8

   Cross-plotting methods

   1 topic

   • Cross-plotting Methods

Petroleum Formation Evaluation: Evaluation of petrophysical of sub-surface formations flashcards for GATE Petroleum Engineering

24 of 64 cards from the Petroleum Formation Evaluation: Evaluation of petrophysical of sub-surface formations deck — real questions with worked answers.

1. What is the fundamental purpose of formation evaluation in petroleum engineering?

   To quantitatively assess the petrophysical properties of subsurface formations — porosity, permeability, water/hydrocarbon saturation, lithology and net pay — using well logs, cores and tests to determine whether a reservoir is commercially productive.

2. What physical phenomenon does the SP (Spontaneous Potential) log measure?

   It measures the naturally occurring electrical potential (in millivolts) between a moving electrode in the borehole and a fixed surface electrode, arising from electrochemical and electrokinetic potentials between the drilling mud filtrate and the formation water.

3. Write the formula for the Static SP (SSP) deflection opposite a thick, clean, water-bearing sand.

   $$SSP = -K \log\frac{R_{mf}}{R_{w}}$$ where $K = 61 + 0.133\,T$ (T in $^{\circ}F$), $R_{mf}$ is mud-filtrate resistivity and $R_{w}$ is formation-water resistivity.

4. In SP logging, what is the temperature coefficient K at 25°C (77°F)?

   $K = 61 + 0.133(77) \approx 71$, commonly rounded to about $71\,mV$ per decade of resistivity ratio at $77^{\circ}F$.

5. What are the two components that make up the Spontaneous Potential?

   The electrochemical potential (sum of membrane potential across shale plus liquid-junction/diffusion potential at the invaded zone boundary) and the electrokinetic (streaming) potential caused by filtrate moving through the mudcake.

6. What is the 'shale baseline' on an SP log and which way does the SP deflect opposite a permeable clean sand when $R_{mf} > R_{w}$?

   The shale baseline is the relatively constant SP reading opposite thick shales. Opposite a permeable clean sand with $R_{mf} > R_{w}$ (fresh mud, salty formation water), the SP deflects to the left (negative).

7. Why does the SP curve give no deflection when $R_{mf} \approx R_{w}$?

   Because the SP deflection is proportional to $\log(R_{mf}/R_{w})$; when the two are equal the logarithm is zero, so the curve stays on the shale baseline (a 'flat' SP), as occurs in salt-saturated muds.

8. How can the SP log be used to estimate formation-water resistivity $R_w$?

   Read SSP from a clean water sand, obtain $R_{mf}$ at formation temperature, then solve $SSP = -K\log(R_{mfe}/R_{we})$ for $R_{we}$ and convert $R_{we}$ to $R_w$ using standard charts.

9. Name three main quantitative/qualitative uses of the SP log.

   (1) Detecting permeable beds and locating their boundaries, (2) estimating formation-water resistivity $R_w$, and (3) estimating the shaliness (volume of shale) of a formation; also useful for bed correlation between wells.

10. What rock/fluid property does a Resistivity log primarily respond to, and why is it the key to saturation?

    It responds to the formation's ability to conduct electrical current, which depends on the conductive brine in the pores. Hydrocarbons and rock matrix are non-conductive, so resistivity rises with hydrocarbon content — the basis for computing water saturation.

11. State Archie's equation for water saturation $S_w$ in a clean formation.

    $$S_{w}^{n} = \frac{a\,R_{w}}{\phi^{m} R_{t}}$$ where $a$ = tortuosity factor, $m$ = cementation exponent, $n$ = saturation exponent, $\phi$ = porosity, $R_w$ = water resistivity, $R_t$ = true formation resistivity.

12. Define the Formation Resistivity Factor F and give Archie's first relation.

    $F = \dfrac{R_{o}}{R_{w}}$, the ratio of the resistivity of a 100% water-saturated rock ($R_o$) to that of the water itself. Archie's relation: $F = \dfrac{a}{\phi^{m}}$.

13. Distinguish the three resistivity zones radially from the borehole into a permeable bed.

    Flushed (invaded) zone with resistivity $R_{xo}$ saturated by mud filtrate; transition/annulus zone; and the uninvaded (virgin) zone with true resistivity $R_t$. Their relative magnitudes indicate moveable hydrocarbons.

14. What is the difference between Laterolog and Induction logging tools, and when is each preferred?

    Laterolog forces current into the formation through electrodes — best in salty (conductive) muds and high-resistivity formations. Induction logs use coils to induce eddy currents — best in fresh muds, oil-base muds, or air-filled holes (non-conductive borehole).

15. What does a microresistivity tool (e.g., Microlog, MicroSFL) measure and why is it useful?

    It measures $R_{xo}$, the resistivity of the shallow flushed zone, with very shallow depth of investigation. It detects mudcake/permeable beds and provides $R_{xo}$ for moveable-oil and saturation calculations.

16. Write the ratio method (Rxo/Rt) expression for water saturation when porosity is unknown.

    $$\frac{S_{w}}{S_{xo}} = \left(\frac{R_{xo}/R_{t}}{R_{mf}/R_{w}}\right)^{1/n}$$ and assuming $S_{xo}=S_{w}^{1/5}$, $S_{w} = \left(\dfrac{R_{xo}/R_{t}}{R_{mf}/R_{w}}\right)^{0.625}$.

17. On what principle do Radioactive (nuclear) logs operate, and name the three main types.

    They measure natural or induced nuclear radiation. The three main types are the Gamma Ray (natural radioactivity), Density (gamma-gamma scattering), and Neutron logs (hydrogen index).

18. What does the natural Gamma Ray log measure and what is its principal application?

    It measures the natural gamma radiation from $\ce{K}$, $\ce{Th}$ and $\ce{U}$ in the formation. Since shales are typically the most radioactive, its main use is distinguishing shale from clean reservoir rock (shale-volume estimation) and bed correlation.

19. Give the linear (Larionov older-rock) shale volume index from gamma ray.

    The gamma-ray index is $$I_{GR} = \frac{GR_{log} - GR_{min}}{GR_{max} - GR_{min}}$$ For a first linear estimate $V_{sh} = I_{GR}$; nonlinear corrections (Larionov, Steiber, Clavier) give lower values.

20. How does the Formation Density (gamma-gamma) log determine porosity?

    A source emits gamma rays that Compton-scatter off electrons; the count rate gives bulk density $\rho_b$. Porosity is $$\phi_{D} = \frac{\rho_{ma} - \rho_{b}}{\rho_{ma} - \rho_{f}}$$ with $\rho_{ma}$ matrix and $\rho_f$ fluid density.

21. What are typical matrix densities ($\rho_{ma}$) for sandstone, limestone and dolomite used in density-porosity?

    Sandstone $\rho_{ma} = 2.65\ \mathrm{g/cm^{3}}$, limestone $= 2.71\ \mathrm{g/cm^{3}}$, dolomite $= 2.87\ \mathrm{g/cm^{3}}$ (fresh-water $\rho_f \approx 1.0\ \mathrm{g/cm^{3}}$).

22. What is the photoelectric factor (Pe) from the density tool used for, and give typical values for sandstone, limestone and dolomite.

    Pe (in barns/electron) is a lithology indicator largely independent of porosity. Sandstone $Pe \approx 1.8$, dolomite $\approx 3.1$, limestone $\approx 5.1$; anhydrite $\approx 5.05$, barite very high.

23. What does the Neutron log measure and what does it actually respond to?

    A neutron source emits fast neutrons that are slowed mainly by collisions with hydrogen. The detector response reflects the hydrogen index, so the neutron log reads liquid-filled porosity (water + oil), since H is concentrated in pore fluids.

24. Explain the 'gas effect' (crossover) seen on combined Neutron-Density logs.

    Gas has low hydrogen density, so the neutron log reads too low a porosity while the density log reads too high a porosity. The curves cross over (neutron < density on compatible scales), indicating gas-bearing zones.

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