Page 325 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 301

2.9.1
Drilling Problems--Avoidance Planning [ ]

the shakers are clean; he slugs the pipe, racks the kelly and starts to
pull out of the hole. Now instead of brief—if fairly severe—pressure
peaks and troughs from the connections, there is a long, steady drop in
pressure as each stand is pulled out. That whole mud that was forced
into the cracks the driller just created in the shale now wants to get
back to that lower pressure wellbore but this small lump of shale, by
now cracked nearly all the way around, is in the way. It is not difficult
to finish the job and push the lump out too, helped by the formation
stresses behind it. As it exits, more small cracks are starting to appear
in the freshly exposed formation face because the supporting hydro-
static no longer exceeds the heightened pore pressure.
Running back in the hole, the piston effect of the BHA is increas-
ing wellbore pressure again. Those cracks now make a convenient
escape route for tiny amounts of mud and filtrate. The crack faces are
forced slightly apart, lubricating mud slides between the fracture faces.
The driller starts to drill again. On bottoms up he notices a lot of
cavings on the shakers but probably does not bother to mention it to
anyone. After all, that always happens after a wiper trip, doesn’t it?
If the driller is well briefed on this mechanism, he will take a lot
more care when making connections. The drilling supervisor must
understand the destabilizing effect of wiper trips by listening to the
hole. He should think carefully about the cost and benefits of those
wiper trips. So, what are the answers they need?

Use good connection practices. Start and stop the pumps slowly.
Take a minute or two. Refer to the procedure in Section 3.3.7,
“Making Connections to Minimize Wellbore Instability and Losses.”
Do not wiper trip unless it can be justified. Will the time and cost
of the trip be repaid with better conditions?
Listen to the hole. Cavings are a warning that something is not
working. An increase in cavings after trips is saying that things are
becoming unbalanced. If oil-based mud is in use, cavings indicate
that the shales may be naturally fractured and possibly that the
mud properties, plus the drilling and connection practices, are
causing fractures.
Ensure that drillers especially understand the mechanisms of shale
instability. How they to their job has the greatest effect.
Use mud systems that are tailored to address the mechanisms of

301