Page 36 - Drilling Technology in Nontechnical Language

Chapter 2 – OIL AND GAS RESERVOIR FORMATION 27

Reservoir rocks often contain other materials within the pore spaces.
For instance, varying amounts of clay minerals may exist within a
sandstone reservoir. These can cause problems while drilling through the

reservoir. The clays can react with part of the drilling fluid and expand
to plug pores and passages, reducing permeability in the zone around the
wellbore. This can seriously reduce or even prevent production of oil and
gas from the well.
Reservoirs are rarely uniform throughout. They may consist of layers
of material with slightly different characteristics, leading to directional
permeability—the permeability differs depending on the direction of

flow. Permeability might be better horizontally than vertically. Within
the reservoir, there may be faults or distortions. Other rock types may be

present. These things all form barriers to the free flow of hydrocarbons,
and they can make the reservoir structure extremely complex. In fractured
limestones, the fractures containing oil may be vertical, requiring a
wellbore to be drilled horizontally in order to intersect many fractures

for efficient production. Clearly, selection of where to place a wellbore
in order to hit the best (most permeable) parts of the reservoir can
get complicated!
Modern 3-D seismic techniques can obtain detailed knowledge of
the reservoir structure, and 3-D seismic is now an indispensable tool in
modern well planning. This data, plus data from other wells drilled in the
reservoir during exploration, are used to create computer simulations that
model reservoir structure and behavior. This in turn allows the operator
to exploit the reservoir with the minimum number of wells and surface
facilities—maximizing return on the money invested in developing

the field.

Seal Rock

While a reservoir rock must be permeable, there must also be an
impermeable rock seal above it that prevents further upward migration of
the oil and gas. Often this seal is formed by a layer of “clean” shale (shale
with little or no sand within it). Other impermeable seal rocks may be
formed from salt or unfractured limestone.

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