Revisiting Hard Rock Reservoir using Air Drilling Technique

Abstract

Presence of hard rock in the drilling region causes quite a percentage in the overall
cost of a well where such formations are present. Bits number required followed
with low penetration rate and tripping time are the driving factor contributing in the
overall cost increase. Under such conditions can be useful Air Drilling which in itself is
no new technology. The first use in records was in early 1860. With some
advancement in technology, it reestablished again in late 1940s becoming a popular
alternative to rotary drilling. However, it remained limited to shallow wells of
maximum 6000 ft depth. With refined method and advancement in technology today
making us accessible to high energy air hammer and diamond-enhanced hammer
pulverizing the rock, using a rapid-action pneumatic hammer tool and hammer bit.
These bits and hammers are capable of increasing the penetration and footage in
such air drilling areas as the Appalachian and Arkoma Basins and Areas of middleeast,
thereby reducing drilling costs.
Air Drilling can be defined as drilling with the hydrostatic head of the drilling fluid
intentionally designed to be lower than the pressure of the formations being drilled.
Large water-bearing formations however could be the biggest enemies of air drilling.
Air drilling cannot effectively handle wellbore fluid influxes, those influxes will wet
cuttings and result in mud rings in the annulus, and there is a risk of a down-hole fire
if mud rings are not eliminated. Switching to mist or foam drilling would allow
continued air drilling in the presence of water. The “lightest” form of drilling, using
only gas (air, nitrogen, or natural gas) allows circulating with results in an equivalent
specific gravity (s.g.) of close to zero.
Using it with high-frequency impacts to overcome the compressive strength of the
formation, could achieve very high rates of penetration. Applications though are
primarily in straight vertical wells in high compressive strength formations that have
little to no water or oil content. Along with the time and resultant dollar savings,
other advantages include immediate and continuous hydrocarbon detection,
minimum damage to liquid sensitive pay zones thus no formation damage, better
control of lost circulation, and cleaner cores are obtained and thus hard rock
reserves which were inaccessible once are now within reach.