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

26 API STANDARD 65-2
Carefully consider the design of the cement which will cover the potential flowing formations. Lead cements, although
not normally designed to cover formations which might flow, can be designed to control flows. Doing so may require
special formulations. Design criteria for lead slurries which are placed across non-productive formations having the
potential to flow are the same as for slurries placed across the hydrocarbon bearing zones.
It is important to note that if the potential flow zone is to be covered by tail cement with a lead cement above the zone,
the static gel strength development of the lead slurry may reduce the hydrostatic pressure exerted on the potential
flow zone before the tail slurry reaches 500 lbf/100 ft 2 . <strong>The</strong>se situations may require additional assessment and
adjustments of the design parameters and/or operational procedures.
5.7.3 Density
Density plays a key role in the design of cement slurries. In cases with potential for flow, there are two primary
considerations for selecting the density: (1) preventing losses to the formation and (2) preventing flow from permeable
formations. This implies the density falls between that necessary to provide enough hydrostatic pressure to control
flow from the permeable formations (well security) and that which will fracture the weak formations causing lost or
partial lost circulation (see previous discussion of pore pressure/frac gradient under engineering design). Other
considerations related to the density of the slurry are the performance related to strength development, mechanical
properties and slurry stability.
<strong>The</strong> density under placement conditions (temperature and pressure) should be considered in the design. Some
slurries are compressed by pressure while others have components which are deformed by pressure. Either of these
can lead to higher densities after placement downhole than the density at which the slurry was mixed at surface.
5.7.4 Thickening Time
<strong>The</strong> thickening time is the time the cement slurry is judged to be pumpable under conditions simulating those found
downhole during placement. Slurries are designed for the specific set of conditions found in the well and for the
designed pumping schedule (rates) to be employed during the cementing operation. Wellbore temperature simulators
are commonly used to develop schedules for conducting tests while API schedules are not appropriate (see 5.6.4).
Avoid using excessive safety factors in thickening time design. Excessive safety factors can cause delayed strength
development, long periods of gelation and increased likelihood of solids segregation. <strong>The</strong>se factors may present a
higher potential for flow from the formation before the cement has adequate strength to prevent it.
5.7.5 Fluid Loss
Control of fluid loss plays a key role in preventing flow. Loss of fluid from the slurry is a contributing factor in the loss of
the overbalance pressure controlling flow. <strong>The</strong> rate of fluid loss is dependent on the overbalance pressure, the
permeability of the formation, the condition of the drilling fluid cake (including its permeability), and the fluid loss
characteristics of the cement. <strong>The</strong>re are numerous fluid loss additives available, such as synthetic and natural
polymers, copolymers, latex, and blends thereof.
Fluid loss testing should be conducted according to API RP 10B-2/ISO 10426-2. It is not possible to make a specific
recommendation on the fluid loss rate as it depends on many factors, however a low fluid loss rate is generally
preferred where there is potential for flow.
5.7.6 Slurry Stability, Sedimentation, and Free Fluid
Stability of the slurry is an important property in preventing annular flow. Free fluid and sedimentation may occur
simultaneously or one may occur without the other.