JNTU
JNTU B.Tech III WELL LOGGING AND FORMATION EVALUATION Previous Question Paper (March 2021 - R18)
Answer any five questions. All questions carry equal marks.
Main
Describe the conventional resistivity logging method with reference to principles, devices used, log presentation and track and applications.
Answer
Conventional resistivity logging, primarily represented by the Normal and Lateral devices, is used to measure the electrical resistivity of formation rocks to identify fluid content and lithology. These tools operate on the principle of Ohm's Law by injecting a known current into the formation and measuring the resulting potential difference.
The Normal device uses four electrodes (A, B, M, N). A current flows between electrodes A and B, while the potential is measured between M and N. The spacing, defined as the distance between the center of the current electrode and the center of the potential electrode (AM), determines the depth of investigation. Common spacings include 16-inch (Short Normal) for shallow investigation and 64-inch (Long Normal) for deeper investigation.
The Lateral device uses an asymmetrical electrode arrangement. It typically employs three electrodes (A, M, N) to measure the potential gradient, providing a deeper depth of investigation than Normal devices, which is useful for evaluating the uninvaded zone of the formation.
Log presentation is typically displayed on a standard depth scale (1:200 or 1:500). The resistivity values are plotted on a logarithmic scale across three tracks. Track 1 usually contains the SP (Spontaneous Potential) or Gamma Ray log, while Tracks 2 and 3 display the resistivity curves (Short Normal, Long Normal, and Lateral) in ohm-meters (Ωm).
Applications of these logs include:
- Differentiating between permeable and non-permeable beds.
- Estimating water saturation (Sw) using Archie's equation.
- Identifying hydrocarbon-bearing zones, as hydrocarbons exhibit high resistivity compared to saline formation water.
- Assessing the depth of mud filtrate invasion by comparing shallow and deep resistivity readings.
Write a detail note on the following:
Archie’s Principle.
Answer
Archie’s Principle is an empirical relationship used to estimate water saturation in clean, non-shaly, porous, and permeable formations. It relates the formation resistivity to the porosity and water saturation of the rock.
The relationship is defined by two fundamental equations:
Where:
- F is the formation resistivity factor.
- Ro is the resistivity of the rock when 100% saturated with water.
- Rw is the resistivity of the formation water.
- φ is the porosity.
- Sw is the water saturation.
- Rt is the true formation resistivity.
- a is the tortuosity factor, m is the cementation exponent, and n is the saturation exponent (typically assumed to be 2).
This principle assumes that the rock matrix is non-conductive and that the conductivity is entirely due to the saline formation water within the pore spaces.
Main electrochemical activities in development of SP in borehole.
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Difference between hydrostatic and lithostatic pressure environment and its relation to fluid pressure in borehole.
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Describe the principle and commonly used tools in electrical resistivity logging.
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Describe focussed micro log with reference to objectives and property measured, principle, electrode arrangement depth of investigation.
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Explain temperature logging, its objective, principle, property measured and applications.
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What is the principle and applications of Cased hole resistivity logging? Describe them.
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What is the working principle and applications of Cement bond log and Casing collar log?
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Describe the SONIC or Litho - density Log with reference to principles, tools, log representation, geological effects and applications.
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What are cross-plots? And how can water saturation tell you about a zone.
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What are the advanced tools in well logging and give its engineering applications?
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