Page 386 - Air and Gas Drilling Manual

Chapter 8: Air, Gas, and Unstable Foam Drilling 8-69
The next higher level of water injection volumetric flow rate is the volume
needed to suppress the ignition of downhole explosions and fires due to the mixture
of circulation air with produced oil, natural gas, or coal dust and fragments as the
drill bit is advanced. When drilling into rock formations that are coal seams, or
reservoir rock containing oil or natural gas, the steel drill bit action on the rock
cutting face can easily cause a spark. If the circulation gas is air then the three
ingredients for downhole ignition are present (i.e., hydrocarbons, a spark, and an
oxygen source). Increasing the water injection volumetric flow rate (with additives)
to the borehole and creating unstable foam at the bottom of the well will suppress
most fire and explosion hazards for a vertical well [16, 17]. This tends to be
successful in vertical wells but not successful in horizontal boreholes [18]. This is
because vertical wells tend to penetrate the vertical thickness of the hydrocarbon
producing reservoir (usually a horizontal sedimentary rock formation). The vertical
thickness of these reservoirs tend to be of the order of a few hundred feet. Thus, at a
drilling rate of 60 ft/hr, the exposure time in the hazardous production zone is only a
few hours. On the other hand, horizontal boreholes require the drilling of several
thousands of feet of openhole in the hydrocarbon bearing reservoir. The drilling
rates in horizontal boreholes are usually about half the drilling rates in vertical wells.
Thus, the exposure time in a horizontal borehole is of the order of ten to near one
hundred hours. The unsuccessful record of ignition suppression with unstable foam
in horizontal boreholes is generally considered the consequence of the long exposure
(drilling time) in hydrocarbon producing rock formations. There are of course other
drilling methods that can be used to suppress or eliminate hydrocarbon ignition.
These methods require the use of circulation gases that will not support ignition.
These other circulation gases are natural gas for a gas pipeline, industrial liquid
nitrogen based gas, and inert atmospheric air (created by placing stripper/filter
equipment downstream from the compressors to remove oxygen). The use of these
other drilling gases can significantly increase drilling operation costs.
Figure 8-16 gives the ignition (ignition zone) parameters of pressure versus the
percent mixture of natural gas with atmospheric air [19, 20]. In general, natural gas
presents the somewhat greater hazard relative to exposure to oil and coal. This is
because the mixture of air and natural gas creates an explosive hazard whereas the
mixture of oil and coal creates more of a downhole fire hazard. Figure 8-16 shows
that the hazard of ignition increases with higher pressures (the wider the region of
ignition at the top of Figure 8-16). Thus, the deeper the drilling operation, the
higher the bottomhole pressures and in turn the ignition probability in the presence
of hydrocarbons.
Using the data in Figure 8-16 and applicable field case histories, Figure 8-17
has been prepared as a water injection guideline for suppressing the ignition of
downhole hydrocarbon mixtures with air. The guidelines in this figure are only
applicable for vertical boreholes in which the exposed oil and natural gas producing
reservoirs, and solid coal seam thicknesses are of the order of 200 ft or less.
There may be no absolute way to prevent downhole explosions and fires when
drilling with air in hydrocarbon bearing rock formations, but measures can be taken
to decrease the chance of ignition. As in any air drilling operation, constant
supervision is necessary. But air and gas drilling operations are unique in that the
warning signs of downhole problems are usually in the form of rapid increases of