Notes

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Introduction

Introduction

© Schlumberger 1999

Notes

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Introduction

Log InterpretationInterpretation is defined as the action of explaining the meaning of something.

Log Interpretation is the explanation of logs ρb, GR, Resistivity, etc. in terms of well and reservoir parameters, zones, porosity, oil saturation, etc.

Log interpretation can provide answers to questions on:

In a well evaluation the questions asked are simple, where is the oil and how much is there. Effectively the question is where will we perforate and how much will come out, will it produce. These answers are available (usually) from log evaluation.

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Introduction

Why Run LogsUses of logs has advanced over the 60+ years since the technique was pioneered. Simple correlation and hydrocarbon indication has advanced to geochemistry and resistivity profiling. Logs are employed to give information about the reservoir, from formation tops and marker beds to porosity and permeability of layers, to porosity and fluids and their types.The data used depends on the needs and the type of wells being evaluated. An exploration well needs more data than a simple development well.

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Introduction

The Reservoir

The elements of gas oil and water are not always present at the same time. Any combination is possible.

To have a reservoir all the elements are needed:

A reservoir rock

A source rock (but it may be far away from the actual reservoir).

The cap rock has to be on top.

The structure must be there.

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Introduction

Requirements of a reservoir

To form a reservoir needs

- source of organic material (terrestrial or marine)

- a suitable combination of heat, pressure and time

- an oxygen free environment

- a suitable basin

Organic material is needed as the source of the hydrocarbons. This material is “cooked” at high temperatures and pressures to give the liquid hydrocarbons. The process takes a very long time. The oxygen-free environment is needed otherwise the organic material cannot become hydrocarbons.

The basin is a stable zone on a plate where the process has time to be completed. Unstable areas do not leave enough time for the necessary reactions.

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Introduction

Reservoir Information

Reservoirs are represented differently by geologists with rocks and reservoir engineers with the fluids.The reservoir is pictured in two forms

The cross sectionGeological column

The Cross section shows the structure and the fluidsThe geological column shows only the rocks making up the reservoir and the depths of each layer

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Introduction

Reservoir elements

The major elements of a reservoir are

• permeable rock stores the hydrocarbon• source rock produces hydrocarbon• impermeable rock traps hydrocarbon• trap captures fluids

The permeable rock is the reservoir rock. The rock must have porosity if it has permeability.

The source rock is the origin of the hydrocarbons.

The impermeable rock is the cap rock and the structure makes up the trap.