Mr. Erik Milito - The House Committee on Natural Resources ...

ISOLATING POTENTIAL FLOW ZONES DURING WELL CONSTRUCTION 31
to the setting of liner top packer or annular wellhead seals can lead to predictions which overestimate or
underestimate the actual potential for gas flow.
5.8 Wellbore preparation and conditioning
5.8.1 General
Well preparation, particularly circulating and conditioning fluids in the wellbore, enhances cementing success. Many
primary cementing failures are the result of fluids that are difficult to displace and/or of inadequate wellbore
conditioning. Drilling fluid with low fluid loss (thin, tough filter cake) and rheological properties that provide low, flat gel
strengths are generally more conducive to proper displacement.
Even when good well preparation is planned, contingencies in the cementing operation should be provided in case
well conditions prevent the planned well conditioning program from being performed.
Well preparation may include the following:
— adjusting drilling fluid rheological properties to aid in its removal during cementing;
— ensuring the well is static;
— curing losses;
— conditioning of fluids prior to cementing to ensure that static gel strength is broken, that cuttings and gas are
removed and that the well is cooled for cementing.
<strong>The</strong> pre-cementing considerations that are included in this summary are based on sound cementing best practices
that are known to enhance the probability of success. Primary cement job failures are predominately due to a
breakdown in the “displacement process” which leads to channeling of the cement through the drilling fluid. <strong>The</strong>se
guidelines, when applied in conjunction with a simulation software program will enhance the displacement process
and improve the probability of successful primary cementing. It is not possible to predict the exact behavior of a fluid
in a complex wellbore, but simulation software can be used to gain a qualitative understanding of the impact of design
and displacement options.
5.8.2 Lost Circulation Control
If losses have occurred or are expected, impact of the loss on the well objectives should be assessed. In some cases,
it may not be necessary to treat the loss, such as when the loss zone is shallower than the potential flow zone which
will be covered regardless of losses. When treatment is necessary there are a plethora of options. <strong>The</strong>se include:
maintaining the downhole circulating pressures below the pressure at which losses occur by reducing the density of
the cement slurry, minimizing the height of the cement column and/or limiting friction pressure during the cementing
operation, using lost circulation materials, running a liner rather than a long string, using stage tools, using diverter
tools, etc.
5.8.3 Conditioning the Drilling Fluid
<strong>The</strong> condition of the drilling fluid is one of the most important variables in achieving good displacement during a
cement job. Regaining and maintaining good mobility of the drilling fluid is a key parameter. <strong>The</strong> drilling fluid should be
conditioned in preparation for cementing.
Drilling fluids with low gel strength, low rheology and low fluid loss are more easily displaced. Pockets of gelled fluid,
which commonly exist following drilling, make displacement difficult. Drilling fluid is conditioned by adjusting
properties to those which will be favorable for drilling fluid removal during cementing.