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

62 API STANDARD 65-2
In some higher pressure wells with a small margin between the drilling fluid weight and the fracture pressure, the
recommended kick tolerance is nearly impossible to achieve. This is particularly true for many wells drilled in the Gulf
of Mexico.
<strong>The</strong>re are numerous other factors that affect the design of shoe depths. <strong>The</strong>se factors include the following.
— Regulatory Requirements—Applicable local regulations should be obtained before beginning the design.
— Hole Stability—This can be a function of drilling fluid weight, deviation and stress at the wellbore wall, or it can be
chemical. Hole stability problems often exhibit time-dependent behavior, making shoe selection a function of
penetration rate. <strong>The</strong> plastic flowing behavior of salt zones should also be considered.
— Differential Sticking—<strong>The</strong> probability of becoming differentially stuck increases with increasing differential
pressure between the wellbore and formation, increasing permeability of the formation, and increasing fluid loss
of the drilling fluid (i.e. thicker drilling fluid cake).
— Shallow Zones with Potential for Flow—Any potential flow zone should be isolated.
— Zonal Isolation—Shallow fresh water sands need to be isolated to prevent contamination. Lost circulation zones
should be isolated before a higher-pressure formation is penetrated to avoid downhole cross flow.
— Directional Drilling Concerns—A casing string is often run after an angle-building section has been drilled. This
avoids drillstring key seating problems in the curved portion of the wellbore due to the increased normal force
between the wall and the drill pipe while drilling deeper sections of the well.
— Uncertainty in Predicted Formation Properties—Exploration wells often require additional strings to compensate
for the uncertainty in the pore pressure and fracture gradient predictions.
— Hole and Pipe Diameters—<strong>The</strong> selection of pipe diameters has the largest impact on well costs in both design
base and detailed casing design. In general, hole and pipe diameters should be designed to be the smallest
possible, which meet all design requirements, well objectives, safety, and environmental requirements. In
exploratory wells, hole diameters may be larger to allow for contingency casing string(s). <strong>The</strong> final hole or casing
diameter is generally determined by evaluation, completion, and production requirements. Because of this,
casing sizes should be determined from the inside outward.
Hole and casing diameters are based on the following requirements.
— Drilling—Bit diameter (hole size) should be minimized to aid in maintaining the required “reflection point” when
directionally drilling, available downhole equipment, rig specifications, and available BOP equipment.
— Cementing—See B.2.4 for more information.
— Production—Production equipment requirements including tubing, subsurface safety valve, submersible pump
and gas lift mandrel size, completion requirements (e.g. gravel packing), and weighing the benefits of increased
performance of larger tubing against the higher cost of larger casing over the life of the well.
— Evaluation-logging requirements and tool diameters.