Selecting A Drilling Technique

were drilled, a mixture of water and liquid soap was added to the compressed air to enhance lifting of cuttings and cleaning of the hole. For more information on removing drilling cuttings, refer to step 7, Designing the Hydraulics of the Drillstring.

Figure 2-5

Conventional Rotary and Rotary-Percussion Drilling Techniques

In most cases, you can achieve the greatest penetration rate in hard formations by using a percussion bit with an air hammer. However, if you encounter a particularly hard formation when drilling with a tri-cone roller bit, you may switch from air to water to better cool the bit. All of the wells at the Rock Creek site were drilled using only air or air mist as the circulating fluid.

The main benefits and limitations of drilling with air circulation are:

Benefits

❖ Eliminates possible filtration damage to coal

❖ Reduces loss-of-circulation problems

❖ Provides straighter holes because of less weight-on-bit

❖ Lower cost because no mud is used

❖ Faster drilling rate Limitations

❖ Unable to effectively lift large volumes of water

❖ Bit gauge can degrade appreciably during drilling

❖ Drillpipe can wear excessively from sandblasting effect

❖ Air compressor packages may not be available in some areas When drilling in some parts of the Black Warrior Basin, you may encounter permeable faults and fracture systems that produce large volumes of water. Because state and federal environmental regulations prohibit overflow of drilling pits, you must stop air drilling if a well produces water faster than it can be hauled away. This problem can severely jeopardize projects with economics based on the lower cost of air drilling.

Water producing zones can also cause loss of circulation problems with wellbores that are rotary drilled with fluid. Using conventional lost circulation materials to control fluid loss has sometimes proven ineffec-tive and expensive. In addition, lost circulation materials may greatly reduce the effective permeability and the gas producing potential of coal formations. Similarly, squeeze cementing to control water influx and loss of returns can be prohibitively expensive.

To solve these water problems, a drilling contractor in the Black Warrior Basin has successfully used a system of alternately drilling with air mist and water. The contractor has successfully demon-strated that if the wellbore is generally competent, you can drill with air mist until all surface recovery tanks are full of produced formation water. You can then continue drilling by switching to water circulation until the surface storage tanks are pumped dry.

By continuing this process of alternating air mist and water drill-ing, you can drill to the total depth of the well. This technique of alternating drilling fluids can minimize excess water production and allow you to reach target depths without pumping potentially damaging lost circulation materials or expensive squeeze cement treatments. For more information on this technique, refer to Additional References at the end of this chapter.

To optimize the alternating fluid technique, you should strive to circulate a mixture of air and water that will balance the pressure in the hole. That is, the mixture should neither permit a large influx of water into the wellbore nor a large loss of fluid to formations.

This balance requires carefully monitoring the drilling pits and adjusting the water/air mixture. When you achieve the proper mixture, the pits will neither lose nor gain large amounts of water.

If you use the alternating fluid technique, you should use bits that do not contain jets. (Air bits usually do not have jets installed.) If you must use jets, they should be large enough to keep standpipe pressure below maximum compressor pressure. (For more infor-mation on drillbits, refer to step 8, Selecting the Drillbit and Drillstring).

When drilling with a rotary-percussion assembly, you cannot use the technique of alternating air mist and water. Percussion hammers operate pneumatically and will not tolerate large amounts of water.

Alternating Drilling with Air Mist and Water

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▲ Caution

Designing a hydraulics program for the drillstring involves select-ing the proper combination of drillselect-ing fluids and drillbits. An optimum drilling hydraulics program can accelerate drilling rate and lower rig cost. A poorly designed program can slow penetra-tion, increase cost, and possibly damage the formation.

The design of the hydraulics program for deep coalbed methane wells can be complex. If you plan to drill in an area where drilling fluids are needed to control formation pressure and maintain wellbore integrity, you should consult with experienced drilling engineers. They can use hydraulics software to determine the optimum design for your application. Service companies can also assist you in designing an effective hydraulics program.

Fortunately, most coalbed methane wells in the Black Warrior Basin can be drilled with air and thus require a relatively simple hydraulics program. The three main considerations in designing the hydraulics program are:

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Minimizing Damage to Coal Formations

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Effectively Cleaning the Hole

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Cooling and Lubricating the Bit

By minimizing damage caused by invasion of drilling fluids into prospective coal intervals, you can help ensure optimum gas pro-duction rates. You should drill holes using air, air mist, or water instead of drilling muds, when possible, to minimize formation damage. Air circulation exposes the coal to less solids and chemi-cal additives, and it exerts minimal hydrostatic pressure on the coal.

If you need to use a drilling fluid to control formation pressures, you should carefully select the type of fluid and additives. If formation pressures permit, the safest and most economical fluid to use is fresh water with a small amount of bentonite to add viscosity. Using heavy muds could plug or even fracture the coal.

You should use them only as a last resort. You should also avoid chemicals that could damage the coals.

Minimizing Damage to Coal Formations

In document A Guide to Coal Bed Methane Operations (Page 46-50)