Page 5 - Offshore Well Construction

CHAPTER 1
The Role of Well Construction in the Evaluation and Development
Petroleum Extension-The University of Texas at Austin
of Oil and Gas Reserves

INTRODUCTION
The evaluation and development of oil and gas reserves is a complex process requiring the interaction of numerous dif-
ferent disciplines. Well construction forms a pivotal role in this process as it is responsible for constructing the conduit
from the reservoir to the surface.

The process of exploring for oil and gas can be broken down into a number of successive operations, each more expen-
sive and complex than the previous and each generating higher quality data. In addition, at the end of each operation,
the data is reviewed and the process amended or terminated as required.

The main components are:
• Geological appraisal
• Geophysical prospecting
• Exploration drilling
• Appraisal drilling
• Development drilling

Geological Appraisal
The known geological data of a region is reviewed. Government bodies have an interest in the economic geology of its
sovereign territory and usually enforce laws, which maintain a database of all geological activity within the territory.
This means that data which may have been determined during exploration for a particular mineral resource is available
to other explorationists. It is obvious that regions of the earth with easy access have been explored in greater detail.

The objective is to identify the types of rocks in which oil and gas may have accumulated. These are sedimentary rocks
and the sequence of occurrence of the rocks can be related to other sequences in which oil and gas have already been
found. Unfortunately, the patterns in deposition are unreliable and, although patterns in one area may be similar to
those where oil and gas have been found, little confidence can be placed on them containing the correct structures to
trap oil and gas and also actually containing oil or gas.

Onshore, a geological survey of surface features may be conducted to confirm the geological prognosis, or to fill in
details which may be missing from existing surveys.
Offshore, this can be done with shallow drilling.

Geophysical Prospecting
Geophysical prospecting is the application of the principle of physics to the study of subsurface geology.
Geophysical prospecting enhances the geological information already known about a formation. The objective is to
separate the basement rocks (those which were formed first and on which sedimentary basins may have subsequently
formed) from the sedimentary rocks since oil and gas form in sedimentary rocks. Geophysical methods can be used to
measure the thickness of sediments and to measure the shape of structures within the sediments.

Geophysical surveys can be divided into two broad categories:
1. Reconnaissance surveys to outline possible areas of interest where there are thick sediments and the possibil-
ity of structural traps
2. Detailed surveys to define well locations to test specific structures

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