Page 8 - Drilling a Straight Hole

INTRODUCTION

Petroleum Extension-The University of Texas at Austin

Introduction

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In this chapter:
• Problems of crooked holes
• Early wellbore survey tools
• Contract deviation clauses

• Directional equipment for vertical holes

hen rotary drilling first began, operators and drillers assumed
Wthat if they held the kelly vertical when starting the hole, the Crooked holes cost more
drill string and bit would drill a straight hole. During the two-year to drill and pose a risk of
intersecting other wells
Seminole boom in Oklahoma that began in 1928, the industry began underground.
to suspect that this hypothesis was not true. On occasion, wellbores
actually intersected. In addition, actual drilled depths did not corre-
spond to projected formation depths. Obviously, the Oklahoma rigs
were drilling holes that deviated from vertical.
These crooked holes were not just a problem for geologists. For
drilling personnel, a crooked hole required more footage compared
to a straight hole (fig. 1). Consequently, the operator had to pay for
the extra footage or extra rig time, depending on the type of drilling
contract. Furthermore, contractors occasionally drilled into existing
boreholes or producing wells on offset leases, creating serious health,
environment, safety, and legal problems. Hole deviation became such
an important consideration that operators began looking for ways to
determine the amount of downhole deviation needed.

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