Q1: Complete the following statements with the
right words:
(10 Marks)
1) A nuclear magnetic resonance (NMR) log uses magnetic
fields to orient hydrogen atom
protons in
the rock. This is used to measure both porosity and pore sizes
in the reservoir.
2) The presence of
hydrocarbons can be detected by Resitivity log;
from which Oil Saturation can be
computed.
3) Interval transit time (∆ t) is recorded by –Sonic or Acustic-log, which determines the velocity of the sound and is reported in
microsecond/foot, and plotted in Track
2.
4) The rate of
bit penetration depends upon two factors:
A- Drilling parameters, such as turns per minute, weight and
bit type, affect the rate.
B- If the drilling parameters are kept reasonably constant,
then the rock hardness is the controlling factor.
5) The
resistivity of an oil reservoir rock depends on:
1) the oil saturation
2) the resistivity of the water sharing the pore spaces; and
3) the resistivity of the reservoir rock.
Q2:
Mention reasons:
(4 Marks)
1) The drilling time log is a real time log?
Because it is recorded as the well is drilled.
2) It is necessary to know the size of the hole by logs?
A) For engineering calculations.
B) If the well is going to be cased or plugged and
abandoned, the volume of the well must be computed to order the right number of
sacks of cement.
C) Many of the other logs, called compensated logs, need to
be calibrated for well bore size to yield accurate results.
Q.3: 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.4: Briefly, compare between the
following logs: (10
Marks)
1- Gamma ray and SP.
2-Neutron and Formation Density Logs.
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).
Q5: Write short notes on: (6
Marks)
1) Clean and Shaly Formations.
2) Gas Effect
3) Information that geologist can get from running a set of logs on a well?
1) 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.
2) 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.
|