Page 398 - Air and Gas Drilling Manual

9-2 Air and Gas Drilling Manual
recovery operations. Over the past two decades practical field research has
demonstrated that most oil and natural gas bearing rock formations can be more
efficiently produced if they are drilled with a drilling fluids that have hydrostatic
flowing bottomhole pressures that are slightly less than the pore pressures of the
potential producing rock formations being drilled. Underbalanced drilling operations
allow the oil or natural gas to be produced into the annulus as the drilling operation
progresses. The underbalanced drilling operation allows the natural fracture and pore
systems to be keep clear of rock cutting fines and drilling mud filter cake, thereby,
avoiding formation damage. Formation damage has been a problem in oil and
natural gas recovery operations nearly since the discovery of oil and natural gas
mineral deposits. Underbalanced drilling operations are often carried out using a
variety of incompressible fluids (e.g., crude oil, formation water, or clear water) and
a variety compressible gases (e.g., air, inert atmosphere, or natural gas). Inert
atmosphere is created by a filter system (placed downstream of the primary
compressor) that strips most of the oxygen from the intake air [3]. This filter
process results in a nearly inert atmospheric gas. The success of aerated drilling
fluid drilling in underbalanced drilling operations in the oil and natural gas recovery
industry has prompted other industrial drilling uses of this technology. In
particular, aerated fluids drilling technology is being experimented with for drilling
deep water wells and for drilling environmental monitoring wells.
This chapter outlines the steps and methods used to plan a successful aerated
fluids drilling operation. This chapter also illustrates the application of these steps
and methods to typical deep drilling operations. The objective of these steps and
methods is to allow engineers and scientists to cost effectively plan their drilling
operations and ultimately select their drilling rig, compressor, and other auxiliary air
and gas equipment. The additional benefit of this planning process is that the data
created by the process can be later used to control the drilling operations as the actual
operations progress.
9.1 Deep Well Drilling Planning
Aerated drilling operations use a variety of incompressible fluids and
compressed gases to develop a gasified drilling fluid. The majority of the operations
use a standard fresh water based drilling mud with injected compressed air. More
recently inert atmosphere has been used as the injected gas to reduce the corrosion of
the drill string and the borehole casing. In this chapter a standard drilling mud and
atmospheric air will be used as the example aerated 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 aerated 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 aerated 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