Page 186 - Origin and Prediction of Abnormal Formation Pressures

DRILLING PARAMETERS 161

based on the principle that the degree of absorption of gamma rays by a material is a
function of the density of that material. With proper calibration, radioactive densometers
provide density measurements with an accuracy of +0.1 lb/gal for muds in the 7-20
lb/gal specific weight range.

Pressure kicks

Balanced drilling techniques require a very narrow margin between the effective
pressure control and threatened blowout. Differential pressures are frequently reduced
considerably below 500 psi. Improper pressure balance, therefore, may cause well kicks,
which can be a direct indication of the presence of overpressured formations. Whereas
pressure kicks have occurred at pressure differentials as high as 9 lb/gal, the control of
most kicks requires less than 2 lb/gal mud weight increase.
Pressure kicks in high-pressure wells are influenced by the following factors: (1)
difference between pressure due to the hydrostatic mud-column weight and formation
pressure; (2) thermodynamic behavior of the gas; (3) interaction of gas with drilling
fluids (especially oil-base); (4) downhole pressures and temperatures; and (5) time
required for the circulation of mud (which is a function of depth) and recording of
the transmitted pressures. Further considerations include: (a) well location, including
onshore and offshore remote areas; (b) deep-water drilling; and (c) pressure kicks
associated with drilling shallow surface holes and zones below the protection pipe.
Excellent discussions and detailed reviews of well control methods are available in
the literature. The articles by O'Brien and Goins (1960), Goins and O'Brien (1962),
Goins (1968, 1969), Rehm (1969), Moore (1974), West (1976), Bourgoyne (1976),
Nance (1977) and Adams (1977) are notable examples.
In general, however, proper pressure control requires installation of blowout preventor
valves, adjustable surface chokes, accurate and reliable pressure reading equipment, gas
separators, and drilling and mud analysis equipment. In addition, the presence of trained
rig personnel, with a sound understanding of the basic concepts involved and having a
planned control program to meet any emergency, is of utmost importance.

Flowline temperature

In a pressure transition zone, the formation pressure increases with depth at a
rate above the normal one. The same appears to hold true for the rate of formation
temperature increase with depth.
Inasmuch as heat conductivity varies with rock and fluid characteristics of subsurface
formations, overpressured, high-porosity shales act as 'thermal barriers', thereby locally
increasing the geothermal gradient (Jones, 1968; Fertl and Timko, 1970). Lewis and
Rose (1970) proposed a mathematical model relating overpressures and high formation
temperatures, which is based upon basic heat flow considerations (Guyod, 1946).
Changes in flowline temperature gradients of up to 10~ ft have been observed
prior to and/or when entering overpressured intervals. This pressure indicator, however,
is also affected by the lithology, circulation and penetration rates, tripping the drillstring
for bit change, long risers in deep-water drilling, and drilling through permafrost