Page 176 - Origin and Prediction of Abnormal Formation Pressures

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Chapter 6

DRILLING PARAMETERS

W.H. FERTL, G.V. CHILINGAR and J.O. ROBERTSON JR.

DRILLING RATE (PENETRATION)

Drilling rate is a function of weight on the bit, rotary speed, bit type and size,
hydraulics, bottom-hole cleaning properties of drilling fluid, and formation character-
istics. Under controlled conditions of constant bit weight, rotary speed, bit type, and
hydraulics, the drilling rate in shales decreases uniformly with depth. This is due to
increase in degree of compaction of shales with depth; however, in pressure transition
zones and highly overpressured zones the penetration rate often increases. Slower pene-
tration rate is frequently observed in the sealing pressure barrier (caprock) overlying this
transition zone. Any other major lithological changes in the shales (silty and/or limey
shales, mudstones, etc.) will also cause penetration rate variations.
Penetration rate should be plotted at proper depth increments (5- to 10-ft increments
in slow-drilling formations or in 30- to 50-ft increments in fast-drilling intervals).
Plotting such data points, however, should not lag too much behind the total drilling
depth (not more than twice the plotted depth increment behind the total well depth
reached by the bit). Drilling rate recorders automatically plot rate in feet per hour versus
depth.
Simple rules of thumb, such as the one proposed by Forgotson (1969) that a twofold
penetration rate increase indicates the onset of overpressures, do not always apply. For
example, an increase in mud weight to 12 lb/gal upon encountering the transition zone,
may partially mask any further pressure increase with depth. It is also of interest to
note that the first unit of mud weight (lb/gal) in excess of formation pore pressure will
reduce the drilling rate more than each subsequent unit of mud weight (lb/gal) increase
(Moore, 1974).
Complications may also arise during bit drilling, which may mask any penetration
rate change due to overpressure. Penetration rate may even decrease due to fluctuating
rotary torque and erratic action of the drill bit on the bottom of the borehole.

Normalized rate of penetration (d-exponent)

Inasmuch as it is not always possible and/or feasible to maintain the bit weight
and rotary speed constant, the concept of the d-exponent was developed by Jorden and
Shirley (1966). Data required to calculate the d-exponent, a dimensionless number, are
the penetration rate (R, in ft/h), bit size (diameter D, in inches), weight on bit (W, in