Page 178 - Geothermal Energy Systems Exploration, Development, and Utilization

154 3 Drilling into Geothermal Reservoirs
and Somerville, 1995). Choice of high strength steel and thick casing delays
collapse but if the borehole and cement job are of poor quality, deterioration
is still inevitable. Therefore the producer is strongly recommended to focus
his efforts on drilling and cement job rather than on the choice of a casing
(Charlez, 1982). The cement job is obviously all the more difficult when heavily
weighted muds have to be displaced.
– casing burst (particularly during stimulation).
– parting of string (mainly in casing connection).
• Casing diameters too small: If the casing diameters which had been planned are
too small
– no contingency casing size may be available if needed;
– so the borehole may not reach the planned target;
– at least a high pressure drop through well during production/injection will
have to be overcome leading to high energy costs for the pumps.

3.8.2
Technical Risks

A typical technical risk is the possible failure of drilling equipment, either at surface
or downhole.

3.8.2.1 Failure of Surface Equipment
A failure of equipment at surface may cause a short interruption in the drilling
operation only but can also lead to severe problems downhole. Mainly the following
three components may be subject to failures.

• Drawworks: If the drawworks fail when the bit is on bottom the drillstring cannot
be pulled; however, this may not become critical as long as circulation and
rotation is possible until drawworks are repaired successfully. A critical situation
occurs if therepaircannot bedone at rigsite.
• Rotary table/topdrive: If thedrillstring cannot berotatedduetoafailureofeither
rotary table or top drive this may not cause a critical situation as long as the
drillstring can be pulled and mud can be circulated.
• Mud pumps: Normally a drill rig is equipped with more than one mud pump. So
a failure of one mud pump may not be critical because of availability of at least a
reduced pump rate from the remaining mud pumps. However, it is good practice
always to pull the bit off bottom as long as the desired flow rate for proper hole
cleaning cannot supplied.
• General electric system: Loss of power or failure in the electric system/Silicon-
Controlled Rectifier (SCR)-Unit.

3.8.2.2 Failure of Subsurface Equipment
Generally, a failure of downhole tools may be more critical for the well than a
failure of surface equipment. This is particularly valid for a breakage of drillstring