Page 55 - Primer on Enhanced Oil Recovery

46 Primer on Enhanced Oil Recovery

The wells are drilled on land and at sea using drilling rigs that use conventional
rotary drilling technology. The drill bit is connected to either drill pipes, which are
joined using threaded box-and-socket joints, or to the flexible continuous pipes
wound on a drum of pipe (up to 5000 m and more in lengths) the so-called
“coiled tubing technology”.
Well drilling is a complex process consisting of the following basic processes:

deepening of wells in rock destruction process by drilling tools;
removal of drilled rock from the well;
borehole fastening during casing drilling;
undertaking a complex of geological and geophysical studies with the view to uncover
rock properties and identify oil productive horizon(s) (strata);
descent to the design depth and cementing of the last (operational) column.
When drilling for oil and gas, the rock is destroyed by drill bits, and the bottom
hole is usually cleaned of the drilled rock with a stream of continuously circulating
washing fluid (drilling mud). In some rare cases the bottom hole debris is blown
with gaseous working agent.
A well is a cylindrical mine workings, with a diameter that is many times smal-
ler than the total length of its trunk (depth). A well shape and arrangement excludes
access to it by a person. The beginning of the well is called the mouth, the cylindri-
cal surface the wall or the barrel, the bottom the bottom. The distance from
the mouth to the bottom along the axis of the wellbore determines the length of the
well, and the projection of the axis on the vertical its depth.
The altitude of the well (absolute height) of the wellhead is the distance from
the point of the earth’s surface to the level of the surface adopted in the geodetic
network as the original.
The maximum initial diameter of oil and gas wells usually does not exceed
900 mm, and the final diameter is rarely exceeds 165 mm.
When drilling a well, the rock is destroyed over the entire area of the face (solid
bottom) or along its peripheral part (ring face). With the later method of drilling in
the center of the well remains an intact column of rock a core, which is periodi-
cally extracted to the surface for an inspection and laboratory study. The depths of
oil and gas wells vary from the first tens to several thousand meters.
To date, the deepest oil well Z-42 (with a depth of 12,700 m) has been drilled at
the Chayvo field in the Russian Federation under the Sakhalin-1 project.

6.1 Well construction elements

During the drilling process, it often becomes necessary to strengthen the walls of
the well for various reasons. For instance, while drilling through unstable rocks, to
prevent fluids cross-flow, etc.
If drilling of the following section (named interval) of the wellbore without wall
strengthening of the previous section becomes impossible, then these sections of the
wellbore are called “intervals with incompatible drilling conditions”.