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

32 API STANDARD 65-2
To condition the drilling fluid in preparation for a cement job, the following should be considered.
a) Drilling fluid displacement is generally more effective if yield point and gel strength are minimized. However, other
competing priorities may prevent this, such as hole cleaning requirements. <strong>The</strong> manner in which cuttings are
transported, and the ideal rheological properties, vary between low, intermediate and high angles. Hydraulic and
hole cleaning software may be used to conduct sensitivity analysis to aid the rheological design of the drilling fluid
in balancing hole cleaning and cement placement.
b) <strong>The</strong> gel strength profile of the drilling fluid should be determined as per methods defined in the following
publications:
1) API RP 13B-1/ISO 10414-1, for water-based drilling fluids,
2) API RP 13B-2/ISO 10414-2, for oil-based drilling fluids.
Gel strength should be as low as possible within the constraints of cuttings transport. <strong>The</strong> gel strength profile
should be non-progressive. <strong>The</strong> API standard time periods for measuring gel strength are at 10 seconds and 10
minutes. Longer time periods are allowed by the API procedures such as measurements at 30 minutes or longer.
For the purpose of conditioning the drilling fluid prior to cementing, a minimum of three measurements
(10 seconds, 10 and 30 minutes) are recommended to plot a gel strength profile showing whether or not a flat
profile exists.
c) Maintain filtrate loss control. Filtrate loss into a permeable zone enhances the creation of a filter cake. A high fluid
loss creates a thick or high viscosity, drilling fluid layer immediately adjacent to the formation wall that is difficult to
displace prior to or during cementing. <strong>The</strong> fluid loss recommended is dependent on the hole section being drilled.
Fluid loss control should be maintained while conditioning the hole and running casing and cementing. Note that a
thick, gelled filter cake deposited while drilling using high fluid loss drilling fluid cannot easily be removed by later
lowering the fluid loss of the drilling fluid.
5.8.4 Rathole
Rathole beneath the casing shoe can lead to contamination of cement during placement, or drilling fluid can swap
with the cement after placement. <strong>The</strong>se can result in poor strength development, pockets of drilling fluid, or a wet
shoe. Rathole length should be minimized or filled with densified drilling fluid.
5.8.5 Surge Pressures while Running Casing
Surge pressures while running casing may cause lost returns if the pressure exceeds formation integrity, or in some
situations, loss of well control. When casing is run into the hole, the drilling fluid flow rate (and the friction pressure) is
proportional to the casing running speed. Running casing with conventional float equipment causes the drilling fluid to
flow at a higher rate up the annulus. Since the well has been static for a prolonged period, the drilling fluid will be
gelled, also increasing the surge pressure. Surge pressures can be reduced by decreasing running speed, using
auto-fill float equipment, lowering the rheology and gel strength of the drilling fluid and/or staging in hole and breaking
circulation while running casing.
Surge prediction software may be used for job planning to predict running speeds that maintain wellbore pressure
below formation integrity. However, the lowest integrity in the open hole is often unknown and the surge calculation
itself is affected by many other factors that are not precisely known. If losses occur at the planned running speeds, it
may be possible to regain circulation by lowering the running speed.
5.8.6 Centralizer Program
Centralizing the casing across the intervals to be isolated helps optimize drilling fluid displacement. In poorly
centralized casing, cement will follow the path of least resistance; as a result, the cement flows on the wide side of the