Page 338 - Practical Well Planning and Drilling Manual

Section 3 revised 11/00/bc 1/17/01 12:00 PM Page 314

[ ] Practical Wellsite Operations
3.1.1

will eventually result, where formation fluids enter the wellbore and
blow out unconstrained into the atmosphere.
Not all formation fluid entry is a problem. For example, as perme-
able formation is drilled, the fluids contained within the cuttings can
enter the drilling fluid, causing an increase in background gas. In very
low permeability formations such as shales, gas contained in the tiny
pore spaces may be at a higher pressure than mud hydrostatic, but the
flow is very slow.
Sometimes gas-bearing cuttings can liberate enough gas to cause
serious gas cutting of the mud. In shallow tophole sections, this may
cause a shallow gas blowout due to the reduction of a few psi in hydro-
static—it could be so finely balanced. Deeper down, even serious gas
cutting is unlikely to cause a kick as the actual bottom hole pressure
(BHP) reduction is proportionally very small.
Kicks are generally prevented by ensuring that the mud hydro-
static exceeds formation pore pressures when the formation perme-
abilities can allow a significant volume to flow into the well. At first
view it might seem that the answer is simply to drill with a very heavy
mud. However, this is not practical due to the increased possibility of
losses, stuck pipe, instability in fractured shales, reduced ROP, and
reduced MAASP should be a kick result. Therefore, it is necessary to
predict what pore pressures are likely to be and to track this while
drilling in order to maintain a safe but small overbalance on perme-
able formations.
The amount of overbalance we maintain is related to what is need-
ed to eliminate influxes due to swab pressures when tripping out of the
well. It is therefore generally termed the “trip margin.” Trip margin can
be calculated by relating mud rheology to the hydraulic diameter of the
hole/drillpipe annulus by the formula:

YP
TM =
225(Dh - Dp)

where TM is trip margin in psi/ft, YP is Yield Point, Dh is hole diame-
ter, and Dp is pipe outside diameter.
The trip margin is added to the mud gradient required to balance
the formation pore pressure.
We can monitor various indicators while drilling to see whether

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