Page 164 - Introduction to Petroleum Engineering

THE DRILLING PROCESS 151
diameter ranges from 18 in. to more than 3 ft; its length is 40–80 ft. Two other smaller
holes, the rathole and the mousehole, are often drilled within 10 ft of the conductor
hole. These holes are for temporary storage of pipe during the drilling process.

8.3.3 Drilling
After the location is fully prepared, the drilling rig, associated equipment, housing, and
materials are moved onto the location and “rigged up,” or MIRU for move in and rig
up. For remote areas, this move (or mobilization) may be 5–10% of the total well cost.
The first task for the drilling rig is to drill to the depth required for the surface
casing, usually 500–2000 ft as specified by the agency that permitted the well.
The surface casing has two functions: first, it protects water in aquifers near the
surface from contamination; second, it provides mechanical support for the well. The
drilling fluid for the surface hole is typically freshwater. When the required depth is
reached, the surface casing is lowered into place and cemented by pumping cement
down the casing and up the annulus between the casing and the surrounding forma-
tions to the surface. The last step in cementing is to push a cement plug with drilling
fluid down the casing until it reaches the bottom of the casing.
After the cement has cured, a casing head is attached to the top of the surface
casing, and the BOP is attached to the top of the casing head. The BOP is used to shut
the well in emergencies.
To continue drilling, the drilling crew feeds the BHA and drill pipe through the
top of the BOP and into the surface casing, tripping down to the top of the cement
plug. The bit on the BHA must be small enough to enter the surface casing. After
starting circulation of drilling mud, the crew can drill through the plug and cement
and past the bottom of the surface casing.
For many wells, the next drilling objective is the depth of the target formation.
While drilling to this depth, the crew will adjust the composition of the mud as
needed to clean the hole and maintain pressure control. Throughout the drilling pro-
cess, a company employee submits daily reports to management of drilling activities
and costs. In some cases, the drilling plan may need adjustment if an unanticipated
event occurs.
At some point after setting surface casing, a separate contractor arrives on loca-
tion to create a continuous tabular record, or log, of the drilling process and results
for a well. This contractor is the mud logging company and its employee on location
is a mud logger, usually a geologist. The mud logger installs hardware and software
to monitor operations as requested by the production company. The hardware usually
includes computer displays of the log in the doghouse on the drilling rig, in the office
of the company employee, and in the mud logger’s workspace.
The mud log consists of four to six columns of information. In the first column of
the table, the ROP of the bit is recorded, typically in minutes per foot. Other operating
parameters (such as rotation rate, WOB, pumping rate, mud weight and viscosity,
and mud composition) are included as notes in this column. In a second column,
lithology (sandstone, shale, limestone, etc.) of cuttings are recorded by the on‐site