SPE Drilling & Completion
The authors presented the first industry SPE paper on wellbore
strengthening, which proposed a new concept for lost-circulation prevention
while drilling (Morita et al. 1988; Fuh et al. 1992; Morita et al. 1996).
Recently, the authors were invited to present at the 2010 SPE Forum held at
Park City, Utah, USA. This presentation focused on wellbore-strengthening
methods currently used in the industry, including well cooling, stress cage,
and tip screening of induced fractures. During the forum, participants
requested a parametric analysis of these methods using rock-mechanics equations
and principles. This paper presents a set of analytical equations developed for
parametric analysis of three typical wellbore-strengthening methods (Fuh et al.
2007; Alberty and McLean 2004) and provides information about the strengths and
limitations of each method. In addition, an updated set of equations developed
on the basis of previous work completed by the authors has been provided to
make the analysis of wellbore-strengthening methods easier to implement.
The equations can be classified by the length of the cracks to be
stabilized, as follows:
- Borehole strengthening by heating to stabilize 0.0- to 0.1-in. cracks. (Not
included in this parametric study.)
- Borehole strengthening to stabilize microcracks (with mudcake or fine
particles) for 0.1- to 1-in. cracks.
- Borehole strengthening to stabilize macrocracks (stress-cage method) for
1-in. to 2-ft cracks.
- Borehole strengthening to stabilize a large fracture with the tip-screening
method for cracks longer than 10 ft.
Parametric studies were conducted on three wellbore-strengthening methods,
and the following observations were made:
- Borehole-stabilization method: Water-based mud mixed with 25/40-mesh
particles stabilizes the borehole by plugging the microcracks with mudcake.
Borehole stability is enhanced by mixing 25/40-mesh crushed nut shells into the
mud. Some drilling engineers enhance borehole stability by intentionally
returning a proper range of cutting particles from the shale shakers.
- Stress-cage method: This method is applicable if the formation permeability
is not too low. However, if the permeability is low, a high-fluid-loss pill is
required to reduce propagation of the induced fracture and adequately place the
granular materials. After wellbore strengthening, ultralow-fluid-loss mud is
required during drilling to reduce the pressure buildup in the fracture section
behind the seal.
- Tip-screening method: This method does not require the squeezing of
particles to induce a fracture. Mixing particles with drilling fluid prevents
fracture initiation, and if a fracture is induced, this mixture prevents
further fracture propagation by tip screening. Tip screening is effective if a
lost-circulation zone has some permeability, but is less effective if the
lost-circulation zone has no permeability.
© 2012. Society of Petroleum Engineers
View full textPDF
- Original manuscript received:
13 November 2011
- Meeting paper published:
31 October 2011
- Revised manuscript received:
27 February 2012
- Manuscript approved:
2 March 2012
- Published online:
24 May 2012
- Version of record:
11 June 2012