Page 150 - Geothermal Energy Systems Exploration, Development, and Utilization

126 3 Drilling into Geothermal Reservoirs
It has to

• cool the drill bit;
• transport the cuttings (drilled solids) in the annulus to surface;
• avoid settling of cuttings if circulation is stopped;
• give cuttings (and entranced gases) free at surface;
• reduce friction between drillstring and borehole wall;
• stabilize the borehole wall;
• exert hydraulic pressure (via the hydrostatic head of the mud column) to prevent
gas or fluids from entering the borehole;
• prevent mud and filtrate from entering the formation through the borehole wall;
• power DHMs;
• transport information of the formation drilled to surface (gases, cuttings, fluids).
There are several types of drill mud in use, mainly depending on the specific
characteristics of the formation to be drilled and on the function of the well. Some
types are given in the following sections.

3.3.1
Mud Types
3.3.1.1 Water-based Mud
Water-based mud is most common. It consists mainly of water. Depending on the
formation either fresh water or brine is used, for example, when drilling through
salt sections brine is more or less salt saturated.
Usually, in all water-based muds additives are added to improve mud character-
istics; for example, marble or barite to increase density, polymers to reduce filtrate
and to improve rheology, friction reducer, and others.

3.3.1.2 Oil-based Mud
Oil-based mud consist of a high percentage of oil, a lower percentage of water, and
several additives (emulsifier, weighing material, and polymers as described above).
While in the past crude oil was used it is today considered to be an environmentally
friendly oil. Oil muds are mainly used in extremely water-sensitive formations to
ensure good borehole stability even under adverse situations.

3.3.1.3 Foams
Foam type muds are used particularly if the formations to be drilled have a pressure
gradient lower than the hydrostatic gradient. It is done to avoid severe mud (filtrate)
losses into permeable formations and protect the formation against plugging of
pores.
Foams are formed of water, a gas (air, nitrogen), and foaming additives.

3.3.1.4 Air
Air is used as a ‘‘drilling fluid’’ mainly in dry hard rock where borehole stability is
not a problem. If water bearing formations are drilled high water influx into the