Figure 38-Exploratory Drilling Rigs

tion reference system (Fig. 39) whereby acoustic signals from a beacon

located near the wellhead on the sea floor are received by three shipIn use, the vessel's position is determined by

board hydrophones. comparing, at each of the three shipboard hydrophones, the signals emitted by the sea floor beacon. The correct position, with reference to the wellbore, is shown on the drilling rig's console viewing screen and the rig is kept in position by adjusting mooring lines or using the rig's thrusters.

If, for some reason the rig should have to move off

location, the sea floor beacon is used to reposition it upon return.

In drilling, two distinctly important pressures must be considered. One is the pressure within the geologic formation penetrated (formation pressure), and the other is the pressure required to fracture and/or force any fluid which may be in the wellbore into the formation below the last casing string (breakdown pressure). These pressures are naturally occurring phenomena. A drilling plan calls for maintaining a hydrostatic gradient in the wellbore that will prevent formation fluids from flowing into the wellbore and, at the same time, will not exceed the breakdown pressure of any uncased formations.

This is

done by adjusting the density of the drilling fluid or "mud" that is continuously circulated through the drill string to provide pressure control, lubrication of the drill bit, and circulation of wellbore cuttings out of the hole (Fig.40).

In spite of considerable research, it is still not always possible to predetermine, particularly for wildcat wells, the formation pressure

Example of well Bore and Casing

DRILLING MUD

CONDUCTOR PIPE

SURFACE CASING

INTERMEDIATE (PROTECTIVE) CASING

DRILLING TO BOTTOM FOR

FINAL STRING OF PRODUCTION
CASING

W Cement

Figure 40. - The drilling mud circulates down through the drill pipe and up the annulus. The relation between the mud pressure gradient and the formation fracture gradient is critical.

(Adapted from Panel on Operational Safety in Offshore Resource Development, "Outer Continental Shelf Resource Development Safety," Marine Board of National Academy of Engineering, Dec., 1972.)

and breakdown pressure that will be encountered.

During drilling there

are several means of determining the trend in pressure.

They include

measurements such as formation temperature (as reflected by the temperature of the returning mud), shale density and changes in the penetration rate of the drill bit.

If the hydrostatic gradient of the drilling fluid becomes less than formation pressure, a "kick" of gas or other fluid may flow into the wellbore from the formation being drilled. The influx displaces some drilling fluid, thereby causing an additional reduction in the hydrostatic head in the annular space between the drillpipe and the borehole (Fig. 41). If the volume of the influx is not excessive, and surface indication (increased mud tank volume) is observed, the unwanted influx of fluid or gas can be circulated out of the well by careful observation and control of well conditions and adherence to

preplanned emergency procedures.

From the record of a kick, the

bottom-hole pressure can be determined accurately, and with this pressure known, the mud weight can be increased to provide a sufficient hydrostatic head for the safe continuation of drilling.

An uncontrolled kick is called a "blowout". Blowouts seldom occur but when they do, they can usually be brought back under control by implementation of preplanned emergency procedures and actuation of devices known as "blowout preventers" which are mounted on every offshore well during drilling. A simplified diagram of a blowout preventer is shown in Fig. 42. Actual blowout preventers used offshore contain at least three types of rams. A typical blowout