Page 479 - Air and Gas Drilling Manual

10-4 Air and Gas Drilling Manual
created by the process can be later used to control the drilling operations as the actual
operations progress.
10.1 Deep Well Drilling Planning
Stable foam drilling operations can use a variety of incompressible fluids and
compressed gases to develop a stable foam. The majority of the operations use fresh
water and a commercial surfactant with injected compressed air. Commercial
surfactants for drilling operations can be obtained for drilling service companies [7].
Recently developed inert atmosphere filter machines have been used to generate
injected gas to reduce the corrosion of the drill string and the borehole casing. In
this chapter fresh water, surfactant and atmospheric air will be used as the example of
a stable foam drilling fluid.
The basic planning steps for a deep well are as follows:
1. Determine the geometry of the borehole section or sections to be drilled
with the stable foam drilling fluids (i.e., openhole diameters, the casing
inside diameters, and maximum depths).
2. Determine the geometry of the associated drill string for the sections to be
drilled with stable foam drilling fluids (i.e., drill bit size and type, the drill
collar size, drill pipe size and description, and maximum depth).
3. Determine the type of rock formations to be drilled in each section and
estimate the anticipated drilling rate of penetration.
4. Determine the elevation of the drilling site above sea level, the temperature
of the air during the drilling operation, and the approximate geothermal
temperature gradient.
5. Establish the objective of the stable foam drilling fluids operation:
• To drill through loss of circulation formations,
• To counter formation water entering the annulus (by injecting additional
surfactant to foam the formation water in the annulus),
• To maintain low bottom hole pressures to either preclude fracturing of
the rock formations, or to allow underbalanced drilling operations.
6. If underbalanced drilling is the objective, it should be understood that
stable foam drilling operations cannot maintain near constant bottomhole
annulus pressures.
7. For either of the above objectives, determine the required approximate
volumetric flow rate of the mixture of incompressible fluid (with surfactant)
and the compressed air (or other gas) to be used to create the stable foam
drilling fluid. This required mixture volumetric flow rate is governed by,
a) the foam quality at the top of annulus (i.e., return flow line back
pressure) and, b) the rock cuttings carrying capacity of the flowing mixture
in the critical annulus cross-sectional area (usually the largest cross-sectional
area of the annulus). The rock cuttings carrying capability of the stable
foam can be estimated using a minimum kinetic energy per unit volume
value in the critical annulus cross-sectional.
8. Using the incompressible fluid and air volumetric flow rates to be injected
into the well, determine the bottomhole pressure and the surface injection
pressure as a function of drilling depth (over the openhole interval to be
drilled).