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GeoscienceCase studyOil and GasGeologyGeophysics
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Calculating Original Oil in Place with
Geologic Uncertainties
Learning Objectives
1. Calculate original oil in place
for a hydrocarbon
accumulation
2. Explain why you made the
decisions based on the
information
3. Explain the value of having
more data that could have
made your decisions easier
4. Explain what you might have
done differently if you had the
option
You are a reservoir engineer in a
deep water business unit, tasked
with calculating OOIP for a
prospect with very little data. Your
choices at each step in the
calculation may dramatically
change your outcome. A little
geologic insight will help reduce
the uncertainty of your estimate.
CASE STUDY
Problem Statement
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Geoscience Case Study
Engineering Academy 2014-2015
Project Summary
In this exercise, you will calculate OOIP using real experimental data from an offshore deepwater prospect.
Carefully consider the materials given to you, to find values for reservoir thickness, reservoir area, porosity,
and water saturation. Once you have them, plug and chug!
Objective
Calculate OOIP for an oil reservoir by using the following equation:
Use the following table to record your answers:
Key: Notes: Answer:
OOIP
A
H
Phi
Swi
Boi Given (You’re welcome!)
1.646 rb/stb
OOIP = [7758 A h phi (1-Sw)]/Boi
Original Oil in Place
Reservoir Area
Reservoir Thickness Reservoir Porosity
Water Saturation
Initial formation volume
factor (rb/stb)
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Geoscience Case Study
Engineering Academy 2014-2015
Background Information
Shown is a 2D seismic line through an offshore deepwater prospect. The black and white lines are seismic
“reflectors”, or strata in the Earth off of which seismic energy bounces. The black and white lines appear fuzzy
in some parts of the image, largely because of the salt bodies (blue) that cover them. It is very difficult to get
clear seismic images, below salt.
The colored lines (pink, red, green, yellow) represent Miocene-aged geologic horizons that are correlated
regionally across this basin. Two wells were drilled in this prospect. For this case study, you are concerned
with the first, straight well targeting just above the yellow horizon. This is the ADRIAN ANGOVE-ROGERS EA-1.
As you can see, the prospect itself is some kind of structural “high”, either a ridge or dome.
Gulf of Mexico – 2D Seismic
SALT
SALT
cLccc
cT
Seafloor Bottom
ADRIAN ANGOVE-ROGERS EA-1
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Geoscience Case Study
Engineering Academy 2014-2015
Select the Areal Extent of the Reservoir
The image below is the prospect shown in 3D. 3D seismic volumes are commonly viewed using a red/blue
color bar to show changes in seismic amplitude. Those amplitude values reflect lithology changes as you pass
from one layer (or type) of rock to another. In an area with pancake geology, you would expect a time slice
taken horizontally through the rock to be pretty boring: since it would pass through just one rock type, and
one seismic amplitude, the time slice would look all one color. Here you can see that the region surrounding
our prospect is much more complex: the time slice looks very splotchy, indicating that there are many lumps
and bumps. Our prospect in particular shows a circular pattern, indicating that this prospect may be more of a
dome (“anticline”) than a ridge. (Think about it: what would a dome look like, if you sliced through the top of
it? What would a ridge look like?) Sometimes, you will hear anticlines referred to as “4-way” or “3-way”
closure, depending on whether a fault breaches the structure. Obviously, the more closure, the better, for the
purpose of retaining oil.
1. Use this picture to determine the areal extent of your prospect. Is the pink, white, green or yellow
circle the best estimate? Why?
Areas for each of the choices:
Pink: A = 300 acres White: A = 500 acres Green: A = 750 acres Yellow: A = 1200 acres
3D Seismic Image,
W. Steven Holbrook, n.d
ADRIAN ANGOVE-ROGERS EA-1
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Geoscience Case Study
Engineering Academy 2014-2015
Calculate the Height (h)
The sidetracked well was logged through the pay interval, which is marked by the red box on the bottom right
of the gamma log. The reservoir occupies just a tiny fraction of the wellbore. The operator reported $118
million in costs, associated with appraisal of this prospect: just think about how much oil must be lurking
there, in order to make this well pay out! The exact location of the horizontal time slice is also indicated on
this slide.
2. Calculate the height of the pay from the logs below:
Sonic Density
Time slice
Gamma Ray Log
Open Hole Logs ADRIAN-ANGOVE-ROGERS EA-1
Gamma Ray Resistivity
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Geoscience Case Study
Engineering Academy 2014-2015
Identify the Porosity (phi)
Here is a photomicrograph of a thin section taken from the reservoir unit. Thin sections are slices of rock that
are polished down to about 30 microns thick, so they are thin enough for light to pass through. In this slide,
white and black are grains of rock; blue is the epoxy used to fill in the pore spaces. Sedimentary rocks are
commonly prepared using a blue epoxy to fill in pore spaces. The epoxy serves two purposes: first, to protect
the fragile rock as it is polished and second, make the pores more visible.
ADRIAN ANGOVE-ROGERS EA-1
Image retrieved from CoreLab
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Geoscience Case Study
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This is a visual aid to help
with estimating porosity
from the thin section. If the
black dots represented pores
in this chart, then the upper
left box would have 1%
porosity, the bottom right
square would have 75%
porosity, etc. Of course,
there are more precise tools
for estimating porosity
(including a variety of
wireline logs), but it is also
useful to train your eye for
visual estimates.
**Use this chart alongside
the thin section to arrive at a
porosity estimate for your
reservoir. Remember that in
the thin section, pore space
is blue.
3. What is your porosity estimate from the thin section on page 5?
Image retrieved from external site - Stevenson, 2012
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Calculate the Water Saturation
The final value you will need to calculate to determine the OOIP will be water saturation (Sw). This is
commonly calculated using Archie’s Equation, as follows:
The trick for you will be deciding what is the wet rock (water or brine only), and what is the rock with the
mixture of oil and brine. You should use the logs given previously to determine this.
HINT: Brine is considered conductive, whereas oil is considered resistive.
4. What is the water saturation (Sw)? Refer to page 4 to find Ro and Rt.
Sw = (Ro/Rt)^(1/N)
Sw = water saturation
Ro = Resistivity of a Rock with only water or brine in its pores (ohm-m)
Rt = Resistivity of a rock with a mix of oil and brine in its pores (ohm-m)
N = Saturation Exponent, Constant = 2
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