Page 16 - Drilling Technology in Nontechnical Language

Chapter 1 – DRILLING GEOLOGY 7

Lithology

Lithology refers to the physical character of the rock. Lithology is a
description of the rock and is based on such characteristics as mineral
composition, color, grain size, and other textures. Thus a shale could contain
some sand (sandy shale) or a rock could be mainly sand with some shale
minerals within it (shaley sand). The lithology will affect many drilling
decisions when planning and drilling the well. If the wrong decisions are
made due to a lack of detailed lithology knowledge, serious problems can
result that will increase the cost of the well and could even prevent the well
from reaching its objectives.

Shales

Shales consist of layers of clay minerals. Clay minerals are crystal
structures of various metal oxides associated with alumina silicates in
connection with varying numbers of water molecules. The metal oxides
are most commonly those of iron and magnesium but may also be
those of sodium, potassium, or other metals. Their presence together in
varying ratios results in a wide range of clay mineral types. Clay minerals
originate in sedimentary rocks by the physical and chemical breakdown
of other minerals originally present. The weathered clays are carried by
wind or water, or both, to an area of eventual deposition. They may then
undergo further physical or chemical breakdown. More water may become
associated with the clay minerals.

Eventually the clay minerals become buried under other sediments.
They will be compacted, and water will be driven off. Diagenesis will alter
their structure or composition to change the accumulation of clay minerals
into a sedimentary rock type. The water that comes out of the shale is less
saline (contains less salt) than the water left behind, so the shale gets more
salty as it dehydrates.
Some shales react very quickly with water. Hydration of these shales
leads to the crystalline layers expanding. During drilling, fluids are

pumped down the hollow drillstring and back up the hole to clean and
cool the drill bit and perform various other functions. If such a shale is

drilled using water as the circulating fluid, the shales can absorb water and
rehydrate. Millions of years of diagenesis can be reversed in under an hour.

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