Q.2:
State briefly the principles of measurements and uses
for
the following wire-line well logs:
(10 Marks)
1-
Resistivity (R) log.
•
Uses induction and measures
resistivity
•
Distinguishes type of
fluid; hydrocarbon, fresh water and brine.
•
Many types and names;
differ largely in depth of penetration.
•
Short penetration reflects
drilling mud; longer is due to formation water.
Usually
used in combination and can be used to make quantitative estimates of porosity,
permeability, etc. Rocks, fresh water, and hydrocarbons are resistive; current
can only flow through rocks containing salt water.
2-
Dipmeter log.
Dipmeter logs determine
the orientations of sandstone and shale beds in the well, as well as the
orientations of faults and fractures in these rocks. The original dipmeters did
this by measuring the resisitivity of rocks on at least four sides of the well
hole. Modern dipmeters actually make a detailed image of the rocks on all sides
of the well hole. Borehole scanners do this with sonic (sound) waves, whereas FMS (formation microscanner) and FMI (formation
micro-imager) logs do this by measuring
the resisitivity.
Q.3:
Briefly, compare between the following logs: (12 Marks)
1-
Gamma ray and SP.
Gamma ray
•
Measures natural radioactivity
•
Sandstone and carbonate usually low
•
Shale high
•
Sandstones (or carbonates) with feldspar,
glauconite, or mica also high due to potassium.
•
Some gamma ray logs may distinguish between
potassium, thorium, or uranium.
•
Can be run in cased hole.
•
Spontaneous potential (SP)
•
Measures voltage difference
between electrode in well and at surface
•
Shale positive; sand
negative
•
Identifies permeable zones
(usually sandstone) and boundaries.
2-Neutron
and Formation Density Logs.
CNL (compensated neutron) logs,
also called neutron logs, determine porosity by assuming that the reservoir
pore spaces are filled with either water or oil and then measuring the amount
of hydrogen atoms (neutrons) in the pores. Because these logs underestimate the
porosity of rocks that contain gas they result in "crossover" of the
log curves when paired with FDC logs (described below).
FDC (formation density compensated) logs, also called density logs, determine porosity by
measuring the density of the rocks. Because these logs overestimate the
porosity of rocks that contain gas they result in "crossover" of the
log curves when paired with Neutron logs (described under CNL logs Above).
Q4:
Write short notes on:
(11 Marks)
1)
ROP
Measures
and evaluates formation characteristics and drilling efficiency. The
formation’s lithology (rock type and hardness), porosity, and pressure affect
the ROP. The drilling parameters that affect ROP include the weight on the bit,
the bit’s speed (rpm), the drilling configuration, the type of the bit selected
and its conduction, and hydraulics.
2) Clean and Shaly
Formation.
The term clean formation refers to those that are shale
free, while shaly formations describe
formations where some of the formation
void space (porosity) is filled with shale.
3) Gas Effect
There are two states of matter that can occupy the pores of
a subsurface rock:
A) Liquid (water & oil)
B) Gas (natural gas)
Natural gas can be detected in subsurface reservoir rocks
by running both formation and neutron porosity logs, side by side on the same
subsurface rock.
If natural gas is present, the neutron porosity log will
read low, whereas the formation density log will read high.
The divergence of the logs is called the Gas Effect.
A correction can be applied to the formation density log to
calculate the accurate porosity of the natural gas reservoir rock.
4) Information that
geologist can get from running a set of logs on a well:
1)
What depths are the formation tops?
2)
Is the environment suitable for accumulation of
hydrocarbons?
3)
Is there evidence of hydrocarbons in this well?
4)
What type of hydrocarbons?
5)
Are hydrocarbons present in commercial quantities?
6)
How good a well is it?
7)
What are the reserves?
8)
Could the
formation be commercial in an offset well?