Page 12 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 7

1.1.3 The geology of hydrocarbon reservoirs
An understanding of some very basic hydrocarbon reservoir properties is
useful for anyone working with wells. Permeability, formation fracture
pressure, and formation pore pressure all have a direct influence on how
well control is managed. With a few notable exceptions, hydrocarbon
reservoirs are found in sedimentary rocks.

1.1.3.1 Sedimentary rock
Formed by the deposition of sediments settling in layers over long periods
of geological time, sedimentary rocks can be classified as clastics, evapor-
ites, and organics.
Clastics, from the Greek word Klastos meaning “broken” are formed
from the compacted fragments of other rocks. Sandstone is a common
type of clastic rock. Individual grains of sandstone are fragments of older
rocks that have been weathered, crushed, and broken down until they are
small enough to be transported by wind and water to a place of deposi-
tion. If they remain in place for long enough, the deposited sand grains
become covered by layer upon layer of other sediments. The buried sedi-
ment is then consolidated by heat, chemical action, and the pressure
(overburden) of the overlying formations. Many hydrocarbon accumula-
tions are in sandstone formations (Fig. 1.2).

Figure 1.2 Horizontally bedded sandstone formation: analogous to that found in
many hydrocarbon reservoirs. Isle of Skye, Scotland.