Advantages and Disadvantages

There are some very basic advantages and disadvantages to mud drilling and air drilling operations. The earliest recognized advantage of air and gas drilling technology was the increase in drilling penetration rate relative to mud drilling operations. In general，the lighter the drilling fluid the greater the drilling penetration rate. The lighter the fluid column in the annulus ( with entrained rock cuttings) the lower the confining pressure on the rock bit cutting face. This lower confining pressure allows the rock bit to be more easily advanced into the rock.

Formation damage is an important issue in fluid resource recovery ( e. g. ，water well， environmental monitoring， well drilling operations， oil and natural gas， and geothermal fluids) . The lighter the fluid column in the annulus ( with entrained rock cuttings) ，the lower the potential for formation damage. Formation damage occurs when the fluid column pressure at the bottom of borehole is higher than the pore pressure of the resource fluid ( oil，gas，or water) in the rock formations. This higher bottom hole pressure forces the drilling fluid ( with entrained rock cutting fines) into the exposed fractures and pore passages in the drilled rock formations. These fines plug the immediate region around the borehole. This damage is called a“skin effect”. This skin effect damage restricts later formation fluid flows to the borehole， thus，reducing the productivity of the well.

Loss of circulation occurs when drilling with drilling muds or treated water through rock formations that have fractures or large interconnected pores. If these features are sufficiently large and are not already filled with formation fluids，then as drilling progresses the drilling fluid that had been flowing to the surface in the annulus can be diverted into these fractures or pore structures. This diversion can result in no drilling fluid ( with entrained rock cuttings) returning to the surface. The rock cuttings are left in the borehole and consolidate around the lower portion of the drill string and the drill bit. If this situation is not identified quickly，the drill string will begin to torque-up in the borehole and mechanical damage to the drill string will occur. Such damage can sever the drill string and result in a fishing job to retrieve the portion of the drill string remaining in the borehole.

For deep oil and natural gas recovery wells，loss of circulation can result in even more catastrophic situations. If drilling fluids are lost to thief formations，the fluid column in the annulus can be reduced resulting in a lower bottom hole pressure. This lowbottom hole pressure can cause a high pressure oil and / or natural gas kick，or geothermal fluid kick ( a slug of formation fluid) to enter the annulus. Such kicks must be immediately and carefully circulated out of the annulus ( to the surface) otherwise an uncontrolled blowout of the well could occur. Here again heavier drilling fluids are generally more prone to loss of circulation.

High pore pressures are encountered in oil， natural gas， and geothermal drilling operations. Newdiscoveries of oil，natural gas，or geothermal fluid deposits are usually highly pressured. In order to safely drill boreholes to these deposits heavily weighted drilling muds are utilized. The heavy fluid column in the annulus provides the high bottom hole pressure needed to balance ( or overbalance) the high pore pressure of the deposit.

Someone's study shows that the heavier the drilling fluid column in the annulus the more useful the drilling fluid is for controlling high pore pressure . There are limits to howheavy a drilling mud can be. As was discussed above，too heavy a drilling mud results in overbalanced drilling and this can result in formation damage. But there is a greater risk to overbalanced drilling. If the drilling mud is too heavy the rock formations in the open hole section can fracture. These fractures could result in a loss of the circulating mud which could result in a blowout.

In the past decade it has been observed that drilling with a circulation fluid that has a bottom hole pressure slightly belowthat of the pore pressure of the fluid deposit gives near optimum results. This type of drilling is denoted as underbalanced drilling. Underbalanced drilling allows the formation to produce fluid as the drilling progresses. This lowers or eliminates the risk of formation damage and eliminates the possibility of formation fracture and loss of circulation. In general，if the pore pressure of a deposit is high，an engineered adjustment to the drilling mud weight ( with additives) can yield the appropriate drilling fluid to assure underbalanced drilling. However，if the pore pressure is not unusually high then air and gas drilling techniques are required to lighten the drilling fluid column in the annulus.

Formation water is often encountered when drilling to a subsurface target depth. This water can be in fracture and pore structures of the rock formations above the target depth. If drilling mud is used as the circulating fluid，the pressure of the mud column in the annulus is usually sufficient to keep formation water from flowing out of the exposed rock formations in the borehole. The lighter drilling fluids have lower bottom hole pressure，thus，the lower the pressure on any water in the exposed fracture or pore structures in the drilled rock formations.