geoscience_casestudy

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

1

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)

2



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

3



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


4



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

5



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.


Image retrieved from CoreLab

6



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

7



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

geoscience_casestudy

Documents

oil reservoir

seismic amplitude

reservoir engineer

seismic reflectors

offshore deepwater prospect

d seismic line

d seismic volumes

rbstb ooip