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2 Air Drilling Techniques by ROY M. WON, KEN DEAKINS, and ROGER L. SUER M-l Air Drilling Fluids Company P.O. Box 42842, Houston, TX INTRODUCTION The successful drilling of a geothermal well depends on the appropriate sejection of the circulating fluid. Using a conventional mud system, loss circulation, formation damage, and high mud costs can occur. Air drilling techniques lower the downhole fluid pressure to inhibit the development of these familiar problems, and may enhance production from geothermal reservoirs. "Air drilling" refers to the use of air in the circulating system. The purpose for using an air drilling method is to drill low pressure formations. During the last 21 years, air drilling techniques have been applied worldwide, successfully drilling for geothermal energy. The four general classifications of air drilling techniques arc as follows: 1. "Air Dust" Drilling 2. "Mud Mist" Drilling 3. "Aerated Fluid" Drilling 4. "Stiff Foam" Drilling ADVANTAGES Air drilling techniques offer the following advantages, when compared to the use of conventional mud systems (M-I Air Drilling Handbook, 1990). Faster rates of penetration. Improved bit performance. Detection of low pressure zones. Effective pressure control through loss circulation zones. Lower mud material costs. * Fast return of uncontaminated cuttings for geological evaluation. Minimized formation damage. Rate of Penetration (ROP) The downhole circulating density of an air drilling system is low, compared to a typical mud system. The decreased circulating fluid pressure exerted on the wellbore increases the relief of the vertical and axial stresses residual in the formations. This results in a "reverse-pressure'' gradient that increases the drillability of the rock. As the downhole circulating fluid pressure is lowered below the formation pressure, the rock tends to "explode" at the bit tooth. This results in faster penetration rates, provided there is sufficient circulating fluid volume to clear the hole of cuttings. The increase in ROPmay be 2-5 times the drilling rate of a conventional mud drilled hole. This can reduce the number of days required to complete a well, and reduce drilling costs. Bit Performance Elevated formation temperatures are common when drilling a geothermal well. One of the main factors affecting the performance of a bit is bearing life. As high formation temperatures are encountered, bearing life can be decreased. An air drilling system supplies the bit with a cool stream of air that flows around thebearings, reducing the bearing temperature and increasing bit performance. An air drilling system provides sufficient fluid turbulence to ensure proper cleaning of the cutting structure. Abrasive cuttings are carried away from the bit and into the annulus, faster than a conventional mud system. This lessens the regrinding of drilled cuttings, increasing the removal efficiency of the solids control equipment, and improving bit performance. Decreasing the bearing temperature and reducing the re-grinding of drilled cuttings increases the footage that can be drilled for a given bit. This can result in fewer bits and less trips required to complete a well, reducing well costs (Lin, 1981). Detection of Low Pressure Zones The decrease in the bottomhole circulating pressure allows the depth of low-pressure production entries to be recorded while drilling. This information can be used to guard against the possibility of damaging these zones as drilling continues. Drilling Through Loss Zones Once "loss" or production zones are encountered, drilling may continue through and beyond these lowpressure formations. The operator may increase Page 138 Geothermal Resources Council BULLmN May 1990

3 production from each well, by drilling deeper and encountering new production zones. The existing air circulating system may or may not have to be changed to maintain full circulation. A properly engineered air drilling system will permit a rapid conversion from one technique to another, without any excessive delay. Mud Material Cost The downhole circulating fluid pressure may exceed the formation pressure, when drilling into a low pressure zone with a conventional mud system. This may induce fractures, allowing drilling fluid to be lost into the porous formation. Higher drilling costs result because of the increased amount of mud materials required to replace the lost mud (Carson and Lin, 1982). The lower circulating pressures of air drilling systems usually permit effective drilling through low pressure zones, minimizing drilling fluid losses, and permitting full returns to the surface. By decreasing loss of circulation, the number of drilling days, and the products required to service the circulating system, the mud and drilling costs can be reduced by using an air drilling system. Minimize Formation Damage The use of air drilling techniques can minimize formation damage and enhance production from lowpressure geothermal wells, when compared to a conventional mud drilled well. If the circulating fluid pressure is less than the formation pressure, there is little chance that circulating fluids will invade and damage producing zones. Some geothermal operators have indicated that wells completed with conventional drilling fluid systems have less geothermal production, when compared to wells drilled with air drilling systems in the same area. These operators feel that low bottomhole circulating pressures decrease invasion, and "baking off" in the high temperature fractures of drilling fluid and cuttings. "AIR DUST" DRILLING Compressed air is injected into the stand-pipe and circulated through the drillstring in much the same way as a conventional mud. The "air dust" technique is used when drilling dry formations, or where any water-influx is slight enough to be absorbed by the air stream. The name "air dust" was chosen because the cuttings return to the surface as a cloud of dust. "Air dust" drilling is the ultimate progression from a high to a low density drilling fluid. Bottom-hole pressures slightly exceed the value of the air column pressure head plus the weight of the entrained cuttings. This allows for maximum relief of the vertical and axial stresses residual in the formations. This procedure offers the fastest drilling rates and best economy. Hole Cleaning The lifting power of an air drilling system is proportional to the circulating density, and to the square of the velocity. The density, and thus the suspension properties, of an air stream is much lower than a conventional mud system. Therefore, the annular velocity is the primary factor in transporting the cuttings to the surface. Air volumes that generate annular velocities of 3,000 ft/min are normally adequate to "air dust" drill. However, when penetration rates exceed 60 ft/hr, or when cuttings are large or wet, higher annular velocities may be required to effectively clean the hole. As a well deepens, more air volume injected at higher pressure is necessary to maintain the required annular velocity. This stems primarily from higher down-hole frictional losses, caused by increased air density and quantity of cuttings in the annulus. The air drilling equipment must have adequate volume and pressure capabilities to effectively clean the hole. A particle falling under the influence of gravity will accelerate until the drag force on the particle just balances the gravitational force. The particle will continue to fall at a constant rate known as the "terminal velocity". The terminal velocity of a spherical particle can be estimated as follows (Ikoku, C.U. and others, 1980): V, = terminal velocity... (feet/=) g = acceleration of gravity.. (feet/=2) d, = diameter of particle... (9 q,, = density of particle... (p"d/r,,t3) qf = fluid density... rwd/reet3) cd = drag coefficient... ( dimensides SI The above formula shows that the terminal velocity of a cutting is inversely proportional to the fluid density. As the relative fluid density decreases, the terminal velocity will increase. Using this logic, the terminal velocity of a typical cutting in air will be very high compared to the terminal velocity of the same cutting in mud. The velocity of the air must exceed the terminal velocity of the cutting to move upward in the annulus. To use air as the circulating fluid requires a high annular velocity to successfully clear the hole of cuttings. Geothermal Resources Council BULLmN May 1990 Page 139

4 Erosion A high annularvelocity may cause erosion of the hole in soft formations. If the use of an air drilling technique causes erosion of the wellbore, the addition of a stabilizing agent or changing air drilling techniques may be required to minimize this problem. Erosion of the drillstring can be caused by the higher annular velocities and temperatures generated when steam zones are encountered. Some people estimate that the vclocity may exceed 10,OOO ft/min in the annulus. The injection of barrier type chemicals will inhibit this type of erosion. Corrosion Control Corrosion should be seriously considered before beginning the use of an air drilling technique (Bannerman and others, 1978). When drilling through formations with acid contamination (COZ and W), the problem could be a lot worse. Mixtures of hydrogen peroxide (14202) and caustic soda (NaOH) can be used to solubilize and precipitate the H2S contamination. An organic, phosphate, scale inhibitor prevents the deposition of alkaline earth metal scale. A blended amine designed to reduce erosion and corrosion of metal surfaces in high temperature geothermal environments has increased the life of drillpipe at The Geysers. The inhibitor is premixed with water and injected down-hole with the air, normally after steam has been encountered. The amine coats the pipe and creates a protective barrier on rock and metal surfaces at high temperatures. Drilling in The Geysers Air drilling techniques have been used in most of the 450 plus steam wells at The Geysers, California, the world's largest commercial geothermal field. The usual drilling program consists of mud drilling a 17 hole to approximately 2,500 feet, and cementing 13 3/S" casing. A 12 1/4" hole is either "air dust," "mud mist," or mud drilled, dependent upon hole conditions, to about 6,000 feet. A 9 s / liner ~ is hung and cemented inside the 133/~" casing. An 8 3/4" hole is then "air dust" drilled until dry-steam production is encountered in fractures directly connected to the main geothermal reservoir. "Air dust" drilling techniques are a proven method of effectively drilling into low pressure, dry steam fractures without damaging the formations. Knowledgeable air service companies, with the proper air drilling equipment, can be used to obtain the maximum benefits air drilling techniques provide. While air drilling, injection pressures can range from 1% to 700 p.s.i.g., depending on steam volume, hole sizes, depth, downhole tool restrictions, type of air drilling technique, etc. Circulating pressures can change quickly, demanding the air equipment to adjust to the pressure changes without interruption of drilling. A majority of the air compressors at The Geysers are trailer-mounted, Clark CFB-4 four-stage, integral units capable of injecting up to 1,550 s.c.f.m., and having a pressure rating of 1,250 p.s.i.g. Air drilling systems must have blowout prevention assemblies (see Figure 1) (Hallmark and others, 1977). The rotating head, with rubber seals, prevents the cuttings, steam, and air mixture from reaching the rig floor. The banjo box diverts the cuttings, steam, and air mixtxm through the blooie-line to the air muffler. The air muffler is connected to the blooie line, and was designed for dust and noise emission control. "MUD MIST" DRILLING This technique is used where the amount of waterinflux is high enough to prevent "air dust" drilling, but not enough to cause hole cleaning problems. The name "mud mist" was chosen because a pre-treated drilling mud is injected with the air, and the combination returns to the surface as a mist. Essentially, the equipment for a successful "air dust" and "mud mist" drilling application is the same. The principle differences are an incase in the air volume f BLOW-DOWN LINE CEMENT 4 L! -ROTATINO BANJO BOX GATE VALVE \ HEAD FLOW LINE (NOT USED WHILE DRILLINQ WITH AIR) BLOOIE LINE CONDUCTOR PIPE URFACE CASING TYPICAL HIGH-TEMPERATURE GEOTHERMAL BOPE (AIR) STACK. Figure 1 Page 140 Geothermal Resources Council B UWN May 1990

5 requirements and the injection of a pre-treated drilling mud. Hole Cleaning Switching to a "mud mist" drilling technique requires an increase of at least 30 percent in the air volume. The additional volume is needed to overcome higher frictional losses caused by wet cuttings adhering to the drillstring and hole, higher slip velocities of larger wet cuttings, and transportation of the heavier wet air column. The mud is injected with the air stream to disperse the cuttings and inhibit them from adhering to the drillstring and hole. Lubrication of the drillstring and hole reduces the air volume requirements, and minimizes the forming of mud rings. Although injection pressures of 100 to 200 p.s.i.g. are normally enough for "air dust" drilling, pressures exceeding 350 p.s.i.g. can be encountered while "mud mist" drilling. Pressures of 1,250 p.s.i.g. may be required when large amounts of fluids are present in the annulus. The rate of fluid intrusion will dictate the amount of air and fluid that must be injected to efficiently clean the hole. Formation fluid entries of up to 150 bbl/hr have been successfully "mud mist" drilled. Corrosion Control The fluid properties required for "mud mist" drilling are lower than a conventional mud system. Chemical treatment is needed to minimize corrosion caused by the additional fluid and air. Basic corrosion control is provided by maintaining the ph of the mud system above 10.5, and treating any hardness or carbonates with the appropriate chemical. Hydrogen sulfide and carbonate scale are treated in much the same way as in a conventional mud system. Corrosion coupons should be run in the saver and (Tossover sub to monitor the type and rate of corrosion. The selection of the best chemical treatment or corrosion control product should be based on the coupon analysis. If H2S is encountexed, the first line of protection is to maintain the ph at or above 11. To precipitate out the sulfides, a source of zinc should be added based on the level of contamination and the type of mud system. To reduce carbonate corrosion, lime is used to treat out the carbonates, and some excess is maintained to buffer against this type of corrosion. Oxygen corrosion is the most difficult to combat in an air drilling system, because the air supplies large quantities of oxygen to the wet circulating system. There are several types of chemicals that can be used to minimize this type of corrosion. Scale is a common problem with some type of fluids. Using an alkaline fluid, and treating the carbonates and hardness with the appropriate chemicals will greatly reduce the tendency of scale to occur. If the fluid circulating system is correctly pre-treated, any corrosion problem can be maintained at an acceptable level. Determining Air Volume Rate Determining the required air volumetric rate to maintain "adequate" circulation can be accomplished by using an analysis developed by R.R. Angel. Air volume rates for various hole sizes, drillpipe sizes, and penetration rates can be found in his book (see Figure 2) (Angel, 1981). The Angel curves provide a good starting point to begin an "air dust" drilling program. However, there are a few assumptions that need consideration when using these flow nuves.ange1 assumes that the cuttings are moving at the same velocity as the air. Angel notes that this is a conservative assumption, and the analysis has been shown that the flow rates can be 20 to 30 percent low. A minimum annular velocity of 3,000 ft/min has been determined, through analysis, to be adequate in most cases to clean the hole in an "air dust" drilling application. This assumption may be high or low depending on each specific drilling situation. The downhole temperatures used for calculating these charts is assumed to be 80 F at the surface, increasing 1"F/100 ft of depth. Aconvenient way to convert to other temperature gradients is not available. Finally, the Angel charts need modification to be applied to "mud mist" drilling methods. The addition of fluid into the air stream requires increases in both the air volumetric rate and standpipe pressure, to maintain proper hole cleaning for a given penetration rate. "AERATED FLUID" DRILLING Most geothermal wells have water dominated formations. The "aerated fluid" drilling technique provides the best circulating medium to minimize formation damage, fluid loss, and well cost to effectively drill these kind of reservoirs. The following passage is taken from the article, by Wolke, Deakins, Jardiolin, and Suter, "An Aerated Drilling Fluid System Can Lower Drilling Costs And Minimize Formation Damage," GRC Bulletin, May 1990: An aerated drilling fluid system combines the advantages associated with conventional drilling fluid and air drilling techniques. A balanced pressure circulating system can effectively drill through low pressured, water dominated geothermal reservoirs with full returns to the surface (Dareing and others, 1981). Properly proportioned amounts of air and drilling fluid are combined at the stand-pipe. The circulating pressure of the aerated fluid gradually increases as it Geothermal Resources Council WUmN May 1990 Page 141

6 R.R. ANGEL'S AIR & GAS VOLUME CALCULATIONS This book presents the circulation rates that are required for air and gas drilling. These rates are the minimum necessary to provide velocities in the bottom of the annulus that are equivalent in lifting power to a standard air velocity of 3,000 feet per minute. This standard air velocity is required for best results in drilling dry formations. Each curve gives the air or gas requirements in standard cubic feet perminuteversus depthforaparticulardrjllingrate. Data for gas gravities ofl.0 (air], 0.8 and 0.6 are included. Circulation rates for intermediate gravities can be found by interpretation. Each Curve is a plot of solutions to the following equation: GAS GRAVITY 1.0 HOLE SIZE 8 3/4" DRILL PIPE 0.D 4 1/2" 4 > 120'/HR. Where a-- SQ t 288KD2,, ~ 53.3 Q 1625 i( 10 li Q' b= (DtI - Dp)' 'I' (Df, - D:,)z D,, =Hole diameter, Ft. D, = Pipe outside diameter. Ft. e =Base of natural logarithms, G = Annular temperature gradient, ORiFt. h =Depth, Ft. K = Drilling rate. Ft./Hr. P, = Pressure in the annulus at the surface, #/Ft.' Abs. Q = Required circulation rate, standard Ft."Min. (60OF and 14.7 psia) S = Specific gravity of the gas related to air, dimensionless T, = Surface temperature in the annulus, O R T,, = Average down hole temperature in the annulus, OR V, = Velocity of standard density air, Ft./Min. This equation indudes the effect of the drilled solids on downhole pressures and velocities. It was derived by applying the Weymouth fraction factor to vertical flow. This derivation was presented in the author's paper 873-C, "Volume Requirements for Air or Cas Drilling" at the annuaal fall meeting of NME in Dallas, Texas, on October 8,1951. The solutions Diat are preented in this book were obtained on a digital computer. Theuseof thiscomputer saved about six month of slide rule calculating B..*.... I I // / i /, 7-i DEPTH, THOUSAND OF FEET Figure 2 travels down the drillstring. This is due to the net increase of the combined frictional, hydrostatic, and thermal expansion pressures. This net increase in circulating fluid pressure compresses the air into small bubbles, in the two phase system. By the time the fluid reaches the bit, the air volume will compress into a small proportion of the fluid volume. After the aerated drilling fluid flows through the bit, the air expands due to the associated pressure drop. The combined pneumatic (air) and hydraulic (mud) energy improves the hole cleaning when compared to conventional mud or water systems. As the aerated fluid returns up the annulus, therc is a decrease in the hydrostatic pressure above the aerated fluid column. This allows the air to further expand within the wellbore, increasing the annular velocity for a given fluid flow rate. As the conventional fluid is displaced out of the annulus, thebottomhole circulating pressure at the bit decreases, which increases the drillability of the formation. When drilling into production zones with a conventional mud system, the higher pressured circulating fluid and cuttings can be forced into the formation where sealing of the fractures may occur. In a geothermal reservoir, high temperatures may bake this trapped drilling mud causing further damage. These sealed fractures inhibit part OF all the flow of the formation fluids into the wellbore, resulting in less than maximum production (Dareing and others, 1981). Fluid intrusion into the fractures at the perimeter of the wellbore is reduced or eliminated when the circulating fluid pressure falls below the formation pressure Page 142 Geothermal Resources Council EUUmN May 1990

7 (Daering and others, 1981). Air permits control of this diffemntial pressure between the downhole circulating fluid and the formation. The use of an aerated drilling fluid system can reduce the total well cost by decreasing the loss of costly drilling fluids, increasing penetration rates, improving bit performance, and increasing long term production from geothermal reservoirs. "STIFF FOAM" DRILLING The "stiff foam" technique is a stable air-in-mud emulsion. The circulating fluid is formulated with mud additives, foaming agents, and compressed air. This technique is used when "air dust," "mud mist," or "aerated fluid" drilling techniques would not be practical because of economic, mechanical, or other reasons. This technique offers lower air volume, fluid volume, and annular velocity requirements. However, hole problems will dictate the actual amount of the air and fluid injection rates. The lower fluid volume requirements will enhance the ability to drill in-gauge, large diameter holes. The decreased circulating pressures allow the drilling of severe loss zones, with minimum fluid loss. The mud products provide hole stability, carrying capacity, foaming characteristics, and combat corrosion. They also decrease the tendency of the air to "breakout" of the foam in the annulus. The air lightens the downhole circulating pressure. The "stiff foam" mixture may be circulated one time oniy. After the mixture reaches the surface it cannot be rccirculated. The foam must be transfemd to a pit where it can disperse into liquid form. There is a possibility of environmental concerns with the large amounts of foam that are present in the surface pits. "Stiff foam" drilling does not work well when formation flows are encountered. As the formation fluid enters the wellbore the foam strength is reduced, decreasing the ability of the circulating system to clean the hole of cuttings. This requires an increase in the mud material concentration to strengthen the foam, increasing drilling costs. This air drilling technique is rarely used in gcothermal drilling. This technique is not recommended until all other air drilling systems have been exhausted. AIR VOLUME MEASUREMENT To properly apply any air drilling technique, it is important to measure and record the injected air volume and circulating pressure. The most common method used in the geothermal industry is the orifice metering method. The orifice flow meter measures the differential pressure drop aa-oss an orifice plate, placed in the air line, and the static line pressure. These values are morded using a standard 24-hour, two-pen chart. The actual air volume and circulating pressure can be calculated by using the API Standard 2530, and the recorded pressures. These records are valuable to the geothermal operator because they may indicate areas of potential problems, such as loss circulation, fluid intrusion, drillpipe washouts, etc. An experienced air operator can interpret the air charts and inform the operator of any problems, which may reduce drilling costs. CONCLUSION There are four types of air drilling techniques. Each one has a specific purpose and application for drilling geothermal reservoirs. By choosing the right technique, the operator can successfully drill into and beyond low pressure formations. The appropriate selection of an air drilling company is important to the success of the drilling program. The company should display and maintain characteristics of quality, reliability, and experience. Proper equipment and skilled personnel are essential to gain the maximum benefits from thcse air drilling techniques. Air drilling methods offer many advantages when compared to a conventional mud system. If appropriate procedures are followed the operator can save money, time, and obtain improved production from geothermal wells. REFERENCES Angel, R. R., Volume Rep irenien tsfor A ir and Gas Drilling, Gulf Pub1 ishing Company, 1958, Bannerman, J.K., and Davis, N., and Wolke, R.M., "Geothermal Drilling Fluid Systems, " Geothennal Resources Council Transactions, Volume 2, July 1978, pp Carson, C.C., and Lin, Y.T., "The Impact of Common Problems In Geothermal Drilling And Completion," GeothemlResources Council Transactions, Volume 6, October 1982, pp Dareing, D.W., and Kelscy, J.R., 'Balanced Pressure Techniques Applied To Geothermal Drilling" Geothermal Resources Council Transactions, Volume 5, October 1981, pp Hallmark, F.O., Wygle, P.R., Stockton, A.D., Blowout Prevention in Glqornb, Division of Oil and Gas, 1977, pp. 62. Ikoku, C.U., Azar, J.J., Williams, C.R., "Practical Approach to Volume Requirements for Air and Gas Drilling,' paper SPE 9445 presented atthe55thannualspefall Meeting, Dallas,Texas, September21-24, Lin, Y.T., 'The Impact Of Bit Performance On Geothermal Well Cost," Geothermal Resources Council Transactions, Volume 5, October 1981, pp M-I Air Drilling Handbook, Revised Geothermal Resources Council BUUmN May 1990 Page 143