Pneumatic fluids in drilling

In today’s environment where increased efficiency is crucial to drilling fast and smart, it is worthwhile reviewing the role that can be played by pneumatic fluids. I believe these fluids are many times overlooked because they have been around for so long and may be viewed as ‘old technology’. It is nevertheless true that these fluids, old or not, can be the most efficient fluids available in the proper application. Beyond that, the potential for increasing their effective use is being revived by incorporating a pneumatic phase in new systems that allow the fluid to cover a broader range of drilling problems. These new fluids have the potential for almost universal use in one form or another to prevent drilling and production problems at the same time increasing efficiency.

Air drilling, or ‘dusting’, has been used for decades for improving drilling efficiency in specialized applications where the rock is hard and stable. In areas like west Texas and eastern Oklahoma, air is used as the circulating media, and is supplied by large compressors. High air volumes are required for velocity to carry cuttings out of the hole. This method allows high penetration rates and prevents damage to water sensitive rock. Because it is a minimum density fluid, any formation fluids will move into the wellbore readily with no impairment due to the drilling process. In some cases, it is an ideal drilling fluid except for a few drawbacks.

When air drilling, since there is no carrying or suspension capacity, the air volume must be sufficient to blow the cuttings out of the hole. This typically limits the depth of utilization since the deeper the hole, the more difficult to bring to the surface. The velocity also creates hole enlargement through erosion, making hole cleaning even more difficult. Because this process allows fluid to enter the borehole, high volume gas or other flows may present a well control problem. This can become even more hazardous when H2S or CO2 are present.

The entry of liquids can quickly cause problems due to the development of ‘mud rings’ as water coats the solids and causes a buildup around the drill-string where packing off and stuck drill-string can occur. The increasing presence of water with the high oxygen environment also increases the tendency for corrosion of the drill-string. Entry of any condensate can cause downhole fires that will burn off the drill pipe or collars and result in fishing jobs or sidetracks.

In order to mitigate some of these potential problems, air/mist drilling is sometimes used where a small volume of surfactant/polymer slurry is injected into the airstream. This will aid in allowing the solids to be coated and kept from forming the mudrings, decrease the potential of downhole fires, and aid in hole cleaning. Some corrosion inhibitor is also typically added to alleviate corrosion tendencies. This method still requires high air volume and has limited hole cleaning capacity.
Foam drilling is the next step in the evolution of the underbalanced air drilling. It is characterized by the creation of a foam slurry to which is added from 50-90 percent by volume air. This allows better hole cleaning due to the combination of the foam and air velocity, handles solids better, and lends more stability to the borehole. The foam is non-circulateable so the slurry is circulated to the surface while new foam slurry is continually injected. Some slurries are partially recoverable and can be regenerated by addition s of polymer and surfactant

Underbalanced drilling (UBD) is carried out by aerating a fluid system with air generated by compressors or by nitrogen sources. Air is injected at a rate so as to maintain the downhole fluid so that it is underbalanced to the formation. This is done to prevent fluid movement into the producing zone so the formation will not be damaged by the drilling fluid. This typically requires a high air to mud ratio so that most of the air is broken out at the surface to allow recirculation. This means that the invasion control properties are minimal and any instances of temporary invasion can result in formation damage. One of the problems with UBD is the loss of borehole integrity as the equivalent fluid density is reduced. It is not unusual for sloughing or even borehole collapse to occur when density is reduced to near balance.

A new form of pneumatic fluids can create drilling efficiency while preventing lost circulation and providing optimum hole conditions by utilizing aphron technology. Aphrons are micro-bubbles that can be maintained as a stable phase in the fluids. Because of their unique shell structure, the aphrons are stable and can exist downhole without losing their integrity. This allows them to create a micro-environment in the near wellbore rock that mitigates the differential pressure between wellbore and formation. These at-balance fluids create a condition where the formation and wellbore remain in balance and there is no differential to cause movement and wallcake buildup. With no differential there is no tendency for differential sticking or tight hole. A unique rheological profile gives excellent hole cleaning and cuttings suspension. This assures a hole free of cuttings buildup even in horizontal or high angle wells.

The at-balance condition assures that no movement of fluid from the borehole to the formation will occur and even filtrate movement is prevented. The micro-environment is held in place by the differential pressure across the zone while the wells is being drilled and completed. Clean up is accomplished by swabbing or blowing down the annulus to reduce the hydrostatic pressure below that of the formation. This will allow the aphrons to expand and begin to move to the borehole. As they expand and are diluted by formation fluids, they break down and clean up readily. The air escapes and any residue of polymer or surfactant is not tenacious and is readily removed. It is typical for cleanup to be rapid and production is established quickly in most cases without any remediation.

About the author

Tom Brookey is CEO of MASI Technologes, LLC. He studied civil engineering at Texas Tech before joining Humble Oil in the drilling department. He has held positions with several operating companies and drilling fluids companies. He has authored numerous technical papers and presented at SPE and IADC conferences. He holds many US and International patents.

Brookey is a member of SPE and AADE. He has served as chairman of the IADC Drilling Fluids Subcommitte, past president of the Association of Independent Mud Companies, past member of the API Committee 13, past member of the SPE/IADC Program Committee, and SPE/IADC Session Chairman.