Page 152 - Geothermal Energy Renewable Energy and The Environment

138 Geothermal Energy: Renewable Energy and the Environment

Pipe string

Mud filling Drilling mud
pores and
fractures

Cuttings

Bit

FIGUre 8.3 Rotary drill string and mud flow. Light gray zone outside the well indicates penetration of the
drilling mud into regions with contrasting permeability. Dashed arrows indicate clean drilling mud flowing to
the bit, solid wiggly arrows indicate mud flowing up the well bore carrying cuttings to the surface.

inside the drill pipe or outside the drill pipe) at high pressure and it circulates back to the surface,
bringing the cuttings with it. Drilling mud also performs a lubrication function, reducing the fric-
tion between the drill string and hole wall as well as lubricating the bit. This reduces wear on the
drill pipe and bit while simultaneously reducing the amount of energy needed to drive the rotary
motion of the drill string.
Drilling muds also stabilize the hole and act as a barrier to isolate the hole from the environment
it penetrates. These functions are particularly important and require careful consideration of the
type of mud to use. The importance of this issue can be appreciated if we consider the physical and
geological context within which the drilling process occurs.
The rocks that are found at any location on the surface of the Earth represent the result of millions
of years of geological processes. Rocks of different chemical compositions and physical properties
are intimately interlayered with each other. Over time, stresses from a variety of directions may be
imposed on that heterogeneous collection of materials, resulting in complex stress fields being stored
in the rock, with different orientations of maximum and minimum stresses at different depths. As
water infiltrates from the surface, a hydrological regime will evolve that can lead to the formation of
one or more aquifers, each with its own flow rates, recharge rates, and direction of flow.
Drilling a well into that system perturbs the local balance of forces and processes in complex
ways. One possible effect is that poorly consolidated material may collapse into the hole, defeating
the drilling effort. Another process, which can have the same effect, is that rock that is under stress
may spall off pieces from the wall into the borehole, destabilizing it or jamming the drill string.
In instances where multiple aquifers occur, it is likely they will have different water compositions.
Drilling a hole provides a potential pathway for cross-communication between the aquifers that, in
cases where one aquifer is used as a source for potable water, may result in unwanted effects.
Finally, for those instances in which the purpose of the hole is to obtain a controlled volume or mass
of geothermal fluid, sealing the hole from the surrounding environment prevents unwanted water
from entering the hole or geothermal fluid from escaping from the hole. The use of drilling mud is