Page 182 - Origin and Prediction of Abnormal Formation Pressures

DRILLING PARAMETERS 157

rate as follows:

R=GNm(W-Wf)K (6-4)

where a reciprocal drilling constant (K), the weight intercept value (Wf), and the bit
rotary speed exponent (m) are determined by a five-spot drilling test. G is a function of
the normalized bit tooth height.
Drilling performance optimization and identification of overpressured formations
have been also investigated by Wardlaw (1969).
Several other empirical equations for drilling rate determination have been developed
by different investigators. Consequently, field data from (1) previously drilled adjacent
wells, and/or (2) feedback data from short-interval testing in the subject well are usually
needed in order to determine input parameters.
Unfortunately, drilling rates change with lithologic variations, even though the
pressure gradient remains constant. Deviated holes, severe dog legs, drilling from
floating vessels, and frequently occurring water-sensitive and sloughing shales can make
these indicators questionable.
Development of fully automated pressure-detection techniques is difficult, except for
local geographic areas where the lithology is well known. Other pressure indicators,
such as chemistry of produced water, drilling mud properties, density of formation
cuttings, and wireline and related tools must still be used meticulously.

POROSITY AND FORMATION PRESSURE LOGS
Several models establish the relationship between the formation characteristics and
drilling parameters, and provide an early indication of formation type and porosity and
pore pressure variations (Zoeller, 1970; Boone, 1972). Several service companies have
made similar 'data units' commercially available, and many oil companies and drilling
contractors have developed their own drilling program models.
For example, a field example of formation porosity and pressure logs in an offshore
5 inch
Louisiana wildcat (U.S.A.) is shown in Fig. 6-5 (Fertl, 1976, p. 131). There is a 9g
casing seat at a depth of 14,401 ft, which is followed by a fast pressure increase over the
next 600 ft.
Bourgoyne (1971) proposed a general equation relating various controllable drilling
variables and drilling performance, which can be expressed as follows:

R = K. fl(W/D), f2(N)- f3(H), f4(Ap) (6-5)

where R is rate of penetration (ROP); K is drillability constant or normalized ROP;
fl (W/D) is a function describing the effect of bit weight, W, per inch of bit diameter,
D, on ROP; fz(N) is a function defining the effect of rotary speed, N, on ROP; f3(H)
is a function defining the effect of tooth dullness, H, on ROP; and f4(Ap) is a function
defining the effect of the differential pressure across the hole, A p, on ROE
Normalized penetration rate, K, is related to the bulk density by the following