Application Issue on Air Hammers Drilling Technique

Hu Gui, Meng Qing-kun, Wang Xiang-dong, Tao Ye Research Institute of petroleum Exploration & Development, Beijing,10083

China

ABSTRACT Air hammers, also called Down-To-Hole hammers, are previously applied in mining industry, civil engineering, hydrology well drilling and have long been considered an effective approach to improve the rate of penetration (ROP) especially in hard formations. The development of air hammers suitable for oil & gas deep well drilling industry in 2000s substantially increase the ROP in hard to mediate hard formations and effectively prevent hole deviation in tilt formations. However, its expanding application in oil & gas field shows that some problems still exist in following operations: drilling in ultra-hard formations, drilling in wet or water formations, and drilling in sloughing wellbore. This paper discusses these issues, presents the possible resolutions and aim to extend the air hammers’ application range and level. KEYWORDS: air hammer, air hammer drilling Technique, air drilling, wet formation, sloughing wellbore, oil & gas field

INTRODUCTION Compared to conventional drilling techniques, percussion tools are capable of increasing

penetration rates in hard rock (Meng et al., 2007). In civil engineering, hydrologic geology and mining industry, air hammers (a kind of percussion tools used in air drilling, previously called Down-To-Hole hammers) has long been used to drilling shallow wells in hard rock from 1950s (Du, 1988; Geng, 1995; Joesph et al., 2010).

In 1980s, air drilling technology developed rapidly in oil & gas drilling industry and could increase the rate of penetration (ROP) more than 5 times using roller bit with high weight on bit(WOB) (usually more than 3 tons, average 10 tons) owing to the reducing of chip-hold-down effect on rocks(Yu et al., 2009; Chen et al., 2009; Zhang et al., 2010). Nevertheless, high WOB is benefit for ROP but easily induce well deviation. A new kind of air drilling tool with low WOB is needed. Therefore, a new kind of air hammers suitable for deep wells drilling characterized with compatibility of high temperature, high air consumption (AC) and high longevity is developed from 2003 (Meng et al., 2007).

This kind of air hammers is suitable for drilling well depth of 0~5000 m. Its AC is more than 80 m3/min and favors the cuttings transportation a lot. The down-hole working longevity is more than 120 hr. with a long drilling section of 2000 m. Application in oil & gas field showed this tool

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However, air hammers are not always all round. In ultra-hard formations, borehole sloughing formations, wet or water formations, directional wells, air hammer drilling technique is still not suitable or has no obvious ROP improvement. This paper discusses these issues.

AIR HAMMERS’ SPECIFICAITONS Table 1 shows the specifications and drilling parameters of KQC air hammers which are used

in oil & gas field in China. Up till now, this kind of air hammers has been drilling more than 250,000 m in Sichuan a province in southwest of China. The highest ROP was 32.56 m/h compared with 2~3m/h of mud drilling. The deepest operation was completed in more than 5000 m (measure depth) with a high temperature of more than 140 ℃. The longest section the bit drilled in one trip was 1,946 m. Such indicators show that the air hammers provide a new way of ROP enhancing measure in lower pressure and loss strata and new approach of controlling the deviation in tilt formation for the oil & gas drilling industry.

Table 1: Specifications of KQC series air hammers

Hammer Type hammer OD (mm)

Bit OD (mm)

Hole Size (inch)

Mass Weight

(kg)

WOB (kN)

Rotary Speed (r/min)

Stand Pipe Pressure (MPa)

AC (m3/min)

KQC275-445 275 444.5 17 1/2 618 30-50

30-50

1.8-2.5 90-150 KQC275-312 275 311.15 12 1/4 618 KQC180-242 180 241.3 9 1/2 277

20-30 2.0-3.0 75-120 KQC180-216 180 215.9 8 1/2 277

KQC135 135 152.4 6 106 20-30 2.0-3.0 48-80 KQC90 90 113.4 4 1/2 86 15-25 2.0-3.0 30-80

ISSUES ON AIR HAMMERS DRILLING TECHNIQUE

Operations of long-term reaming Operation of long-term reaming using air hammers is disallowed. Reaming operation is the

solution of well under-gauge. The bit of air hammers has no cutting inserts, but cylindrical and composite ball teeth. When used to reaming the hole, the air hammers would start to work and the impulse energy would not be transmitted directly to the bit teeth due to the incompletable contact of bit with rocks. This would cause the bit’s vibration and the rapid worn of bit inserts (shows in figure 1), finally, influences the rock fragmentation efficiency.

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Figure 1: Worn Bits of Air Hammer

Drilling in ultra-hard formations Most air hammers used in oil & gas field is currently not suitable for ultra-hard formations,

and also not suitable for drilling in cement containing casing accessories. When forced into such operations, the inserts may be damaged drastically and the matrix may be worn speedily. All the results affect the bit’s longevity and penetration rate. In one case of application, we employed the air hammers to drill the casing accessories to save one pass of tripping in/out. After drilling the casing accessories, we found the ROP decreased rapidly, only 4~5m/h.

According to the structure of the bit face, there are three types of bit face (Geng, 1988): flat face, concave and convex cone face. Typically, the relationship of drillable strata hardness among them is following: flat face bits < concave cone face bits < convex cone face bits. Bits with flat face are suitable for drilling in soft to medium hard formations; and bits with concave cone face are able to drill most medium hard to hard strata; while bits with convex cone face are mainly used in ultra-hard formations. Presently, most air hammers’ bits need to drill a long section and most of the formations are medium hard to hard. Therefore, the bits always adopt structure of concave cone face.

In oil & gas field, we should use roller bits to drill the casing accessories; when drilling in ultra-hard layers we should choose structure of convex cone face bits if necessary. This kind of bit can generate large impacting power in a smaller area and obtain high drilling rate. However, the ability of drilling well straightly may not be so well than flat and concave cone face bit.

Drilling in sloughing formations Borehole collapse is a big challenge for drilling operators. If the roller cone bits were chosen

for the air drilling, borehole collapse can be relieved by back-reaming operations, rotating of drilling string with high torque and lifting the drilling string up and down with high gas injection rate. Since the air hammers have no function of back-reaming, once encounters borehole

Vol. 19 [2014], Bund. Q 4128 sloughing, the only measure operators may adopt is increasing gas injection rate to blowout the sloughing debris. And this is unsafe if the highest gas injection rate (always no more than 250 m3/min owing to the air compressors’ capacity) is not enough to blowout the all the sloughing debris. Consequently, a serious drill pipe frozen may occur. Usually, under such circumstances, operators give up the air hammers drilling technique and select the air drilling with roller bits or traditional mud drilling technologies.

Figure 2: Schematic Diagram of Air Hammer Bit with back-reaming Ability

Air drilling is a kind of underbalanced drilling. It is inevitable to induce borehole collapse

due to the high under-pressure situation. No or low capacity of processing the hole-caving in air drilling limits the wide application of air hammers. Therefore, a new type of air hammers with ability of handling the trouble of well collapse should be developed.

Meng et al. (2012) proposes a bit structure of air hammers with the function of back-reaming (shows in figure 2). Certain amounts of cutting teeth are inserted on the back of air hammers’ bit. Normally, the cutting teeth do not work. When there are shrinkage hole, first lift the drilling strings to the shrink point and simultaneously the air hammers change from the work state into the circulation state automatically, then rotate the drilling string and start the operation of back-reaming. This kind of air hammers’ bit structure provides a solution to the operation in sloughing formations, however, much work is still needed to do to be applied in field.

Drilling in wet and water formations When Air drilling technology are applied in wet or water formations, the powdery cuttings

are extremely easy to absorb the water and paste together on the drilling tools, especially the bits; form a pie of "mud collar" and seal off the annulus between drilling tools/strings and wellbore; at last cause the stand pipe pressure (SPP) to increase and the AC to reduce. In serious situations, the moisten cuttings would block the flowing channel of gas, causing the failure of air drilling operation. Then the operators would change into misted drilling, foamed drilling or mud drilling according the amount of water produced. If the bit were roller cone bit, the change of medium would be convenient since there was no necessity of bit-change. If the drilling tools are air hammers, the change of medium would cause the failure of air hammers and necessity of bit-change. Study (Hu et al., 2009) reveals that the well bottom pressure influences the internal normal work of air hammers. When the equivalent density of drilling medium in annulus (causing the increase of well bottom pressure) increases, the energy output of air hammers decreases, even

Vol. 19 [2014], Bund. Q 4129 no energy output. In misted or foamed drilling, the equivalent density of drilling medium is higher than that of gas. And in water formations, the gas need to blowout the water. The added water in annulus also increases the well bottom pressure. These two listed above would lead to the low energy output of air hammers, and lower the ROP for drilling operations. Except for the low energy output of air hammers in wet formations, reservoir fluid backflow into the air chamber of air hammers is another problem. When stopping gas circulation during the pipe connection time, the water combined with the cruds from the formation gathers into the hole and backflow into the air chambers through the bit nozzles. The cruds are easily to contaminate air chambers and cause the failure of air hammers.

Therefore, operators are carefully to select the air hammers drilling technology if the target section may imbed with wet or water layer. Such application situations limit the wide use of air hammers.

Three schemes may solve this problem: misted hammers drilling technology, foamed hammers drilling technology, and reverse cycle air hammers drilling technology. We have tested the air hammers drilling in misted & foam drilling medium and found that the air hammers only drilled a short section (no more than 100 m) with a low ROP (smaller than 5 m/h) before its failure. Through the research on the working mechanism of air hammers, we came to conclusions that the key point of misted or foamed hammers is how to guarantee the air hammer impact power output when the bottom well pressure increases. The hammers must self-adapt the shift of well bottom pressure and keep a high impulse energy output.

Reverse cycle air hammers drilling technology (Du, 1988; Geng, 1995; Han et al., 2008) is another method to solve the drilling problem in water formations. It refers to an engineering that carry the drilling cuttings from the center of drilling string instead of annulus between wellbore and drilling string to ground. This drilling method has been employed in shallow well drilling such as civil engineering, hydrologic geology and mining industries. Whether can it server in oil & gas drilling effectively is still in the period of theoretical and experimental study (Han et al., 2008). The advantages of reverse cycle air hammer drilling technology are following: a. good cuttings carrying ability and high ROP. Because the drill pipes’ inner bore are smaller than the annulus between borehole and drilling strings, the gas velocity is higher and can effectively carry the drilling debris, so avoid the rock regrinding and increase the penetration rate. b. adaptability to the wet or water formation. With high gas flow velocity, all the water can carry from the inner hole of drilling string effective and have no impact on the wellbore. c. lower trouble frequency. There is no washing on the wellbore with gas or water from the wet formation, thus the wellbore stability is good, so reduce the troubles frequency. However, the reverse circulation drilling technology is totally different from the traditional and normal circulation cycle, and many traditional drilling tools need to redesign, such as wellhead equipment, BOPs, RAMs, swivels, double wall drill pipes. The most challengeable tools may be the double wall drill pipes suitable for deep wells. Much work has been done on these pipes but with no effective progress.

Drilling in directional wells Air hammers are not only able to increase the ROP and control the well deviation in vertical

wells, also are capable of applying in directional wells. Presently, more and more wells are designed to be directional wells and need more ROP enhancing means. Air hammers are previously thought to be suitable for drilling in vertical wells. Study (Hu et al., 2012) shows that: because of the low WOB, the influences of rock bit’s side force on drilling direction are small. The drilling direction mainly depends on the direction of normal line of bit bottom face. If the air

Vol. 19 [2014], Bund. Q 4130 hammers were well assembled with other directional tools, by adjusting the normal line of bit bottom surface, the air hammers are able to be employed in directional drilling.

There are two means of directional drilling with air hammers. Previous efforts to drill directional wells with an air hammer have included a directional air hammer (Bui et al., 1997) in which the piston was designed to rotate and impact on the hammer bit as it oscillates inside the hammer. Surface rotation is not required, since the hammer itself was self-rotating. A surface adjustable bent sub was run above the hammer to enable kickoff and steering. This type of technology has been used primarily to deviate up to 30 degrees in “S” shaped directional wellbores.

Another approach that has been used to drill directionally with air hammers involved running a packed-hole BHA to maintain inclination, after the curve was built with a motor BHA. A number of high-angle directional wells were successfully drilled using air hammers in the Granite Wash formation in the Texas Panhandle in 2005-2006.

However, Chinese directional technology with air hammers has not yet been applied as it was restricted by development of efficient gas PDM and reliability of self-rotary air hammers. In fact, the world’s directional technology with air hammers is not so widely applied. Much work is still needed to do.

a. Measurement While Drilling (MWD) technology in gas drilling medium. To control the hole direction, we need to know the trajectory parameters when drilling. Thus survey is important. However, Traditional MWD only works in mud medium. In gas medium, presently, the only mean for survey is single shot survey which is too simple, complex and inefficient. Another possible approach is the EM-MWD, which refers to Electromagnetic measurement while drilling. Lots of efforts has been put on the research of EM-MWD and currently it still need to be improved (Wang et al., 2013; Su et al., 2013). With the improvement of this technique, the air hammer drilling in directional drilling would prospective.

b. The low-speed and high longevity gas motors technology. Motors can drive the bit rotate without the drive of drilling string. Most of the gas PDMs rotary speed are high (Shang et al., 2009) and cannot meet the air hammers’ requirement of 30-50 r/min rotary speed. Thus the low-speed and high longevity gas motors is still need to be developed.

c. Air hammers with ability of directional drilling. Under the condition of deviated and horizontal wells, the kinematic characteristics of the piston of air hammer change (Hu et al., 2009) a lot. The piston lies in a slanting or horizontal way (so center of piston’s gravity has no or little changes), so reduce the performance of piston’s energy storage and intensify the friction between piston and cylinder. These all influence the working performance of air hammer.

d. Self-rotary air hammers. Self-rotary air hammers (He et al., 2011; He et al., 2012) are sensitive to the drilling parameter of WOB. The WOB easily influences the energy output and even the reliability of self-rotary air hammers. Little literature shows their relationships, much work still needs to do.

CONCLUSIONS Air hammers drilling technology cannot be applied in long-term reaming operation. The long-

term reaming operation would wear the bit rapidly.

The air hammer with bit of concave cone face do not adapt to ultra-hard strata and the casing accessories. When meets such formations, we should select the bit with convex cone bit face.

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Bit with ability of back-reaming should be further developed to meet the drilling requirements in sloughing formations.

Misted and foamed hammers adaptive to the increases of well bottom pressure need to be further studied and developed.

Air hammers drilling in directional wells has been applied in some blocks but not widely used. EM-MWD, gas motors, air hammer with ability of directional drilling, self-rotary air hammer still need to be improved and advanced.

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