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

ISOLATING POTENTIAL FLOW ZONES DURING WELL CONSTRUCTION 27
Free fluid can result in a channel or a void in the cement into and through which formation fluid or gas can easily flow.
It may also result in a severely underbalanced condition (through the water channel) initiating the flow. Control of free
fluid is imperative in situations where there is the potential for flow.
Not only is the presence of a free fluid channel and the resulting underbalanced condition critical, but the condition of
the remaining slurry is key as well. When water is lost from the slurry (by free fluid separation), the solids
concentration is increased. This can result in uncontrolled gelation, changing other properties of the cement (such as
ability to transmit hydrostatic pressure).
Additionally, sedimentation (which results in concentration of solid particles in lower sections and reduced
concentrations in upper sections of the well) should be controlled to the extent that the slurry properties both at the
top and the bottom of the column are sufficient for controlling flow zones in the well. <strong>The</strong> properties of the slurry will be
changed by sedimentation, leading to greater gelation where the solids are concentrated and low strength and high
permeability where they are reduced.
5.7.7 Rheology
Rheology affects fluid displacement and friction pressure generated during placement. <strong>The</strong> temperatures to which a
fluid is exposed, and to a lesser extent the pressure, will alter its rheological properties. In some cases, slurry stability
may be dependent on rheology. Gel strength development may also be affected by components of the cement which
are used to control rheology. <strong>The</strong> design of the fluids used in cementing should take these parameters into account.
5.7.8 Static Gel Strength
Static Gel Strength (SGS) development is one of many factors that contribute to decay of hydrostatic pressure. As
gelled fluid interacts with the casing and the borehole wall it loses its ability to transmit hydrostatic pressure. It also
contributes to the ability of slurries to suspend solids under static conditions. One method to evaluate the impact of
gel strength development on wellbore fluid influx is to calculate the CSGS and then to measure the CGSP.
CSGS is defined as the static gel strength of the cement that results in the decay of hydrostatic pressure to the point
that pressure is balanced (hydrostatic equals pore pressure) across the potential flowing formation(s).
<strong>The</strong> CSGS is calculated by:
where
CSGS = (OBP)(300) ÷ (L/D eff )
OBP is the initial calculated overbalance pressure, i.e. hydrostatic pressure minus the pore pressure, (psi);
300 is a conversion factor;
L
is the length of the cement column above the flow zone (ft);
D eff is the effective diameter (in.) = D OH – D c ;
D c
D OH
is the outside diameter of the casing (in.);
is the diameter of the open hole (in.).
Wellbores have variable hole diameters and contain multiple fluids (drilling fluid, spacer, lead cement, tail cement) in
the annulus. Many wellbore sections have more than one potential flow zone to be evaluated. For these reasons, it is
recommended that a computer program be used to accurately calculate CSGS for all potential flow zones.