In many of today’s most challenging land drilling applications, the lower hydrostatic head and weight on bit achieved with air drilling methods are key to reducing time and costs, and a low hydrostatic drilling fluid can improve drilling efficiency.

These versatile systems improve penetration rates; minimize problems such as lost circulation and differential sticking; and cut fluid, bit, and other expenses. Air drilling also drills a straighter hole. In vertical sections, this technique reduces deviation and doglegs that speed drilling and enhances drilling accuracy for well curves and laterals.

Well construction optimization requires careful matching of the technology to the application. Energized systems, including dry air, gas, mist, foam, and gasified fluids, are leveraged with a spectrum of chemicals and tools. For instance, corrosion inhibitors and surfactants enable air drilling through water influxes, and advances in percussion tool designs and materials have resulted in greater efficiency and durability.

These engineered systems provide unique performance characteristics in a wide range of applications, including formations that are extremely hard or consolidated, produce water, have lost circulation problems, or are sensitive to hydrostatic pressure.

Three examples in Texas illustrate how advancements are enabling familiar air drilling technologies to take on new extremes.

Canyon sands ROP

Slow penetration rates and hole deviation when drilling low-pressure, high-volume Canyon sands gas wells in West Texas have resulted in a change to underbalanced drilling (UBD) using downhole hammer bits.

The shift to air drilling was tracked in more than 50 wells where its use cut time from spud to total depth (TD) by approximately 30%, yielding a savings of about US $25,000 per well. At the same time, hammer drilling minimized borehole deviation to less than 4% without compromising penetration rates.

A conventional well plan in the hard dry rock set 8/ 8 -in. casing in a 12 1/ 4 -in. surface hole, then drilled a 7/ 8 -in. hole to TD. However, drilling with conventional muds and roller cone or PDC bits in the production section resulted in low penetration rates, high weights on bit, and deviation problems.

To improve ROP, underbalanced hammer drilling was used to drill the 7 7/ 8 -in. hole.

Conventional bits and a straight gas or a mist-foam system (due to water influx) were used until deviation problems occurred, followed by hammer tools using air or membrane-generated nitrogen to finish the drilling.

The longest hammer-bit run was 2,133 m (6,999 ft) in 66 hours, drilling at 32 m (106 ft)/hr; the shortest run was 198 m (650 ft) in 9.5 hours, drilling at 21 m (68 ft)/hr.

During one period in which 29 wells were drilled, 20 required only one trip for the production section. Of the remaining nine wells, six were drilled at a lower cost per foot and in fewer days than those drilled with conventional drilling techniques.

The additional costs of the air compressors, high-pressure boosters, and diesel fuel, as well as the diamond-insert hammer bit, were offset by mud cost savings and the reduced number of conventional bits, stabilization, and mud motor costs.

Barnett shale deviation

Air-hammer drilling in the Central Texas Barnett shale eliminated 12 to 15 days of drilling time by minimizing doglegs and reducing wellbore deviation by 50%. The significant reduction in deviation allowed the 8 3/ 4 -in. hole section to be drilled with a single bit run. Total drilling cost for the well was cut in half compared to conventional drilling methods.

Deviation and doglegs are problematic to the drilling curves and laterals typically designed in the Barnett shale. Hammer drilling places a light weight on the bit – typically just enough to keep the face of the bit in contact with the formation. With less weight, the bit drills a much straighter vertical hole.

Barnett production is highly dependent on accurately placed wellbore curve and lateral sections. Drilling a high-quality vertical borehole with an air hammer system enhances drilling of the deeper well geometry.

A new stabilized air hammer tech nology used in the vertical section of the well was key to the drilling performance and limiting the wellbore deviation and dogleg severity. The bit minimized hole deviation while still achieving an ROP of 15 m (50 ft)/hr.

In three vertical wells, UBD using a combination of hammer and tricone bits succeeded in more than doubling ROP compared to using a freshwater drilling system.

Travis Peak strength

The Travis Peak formation in East Texas is one of the hardest and most abrasive in the US, with unconfined compressive strength often in excess of 50,000 psia.

In three vertical wells, UBD using a combination of hammer and tricone bits succeeded in more than doubling ROP compared to using a freshwater drilling system.

The HP/HT wells were in excess of 4,877 m (16,000 ft) total vertical depth. Surface and intermediate sections were drilled conventionally, while the 8 1/ 2 -in. hole below 2,987 m (9,800 ft) was drilled underbalanced with hammer and tricone bits.

To improve ROP, the operator initially changed to freshwater from a conventional mud system. Further enhancement was sought using low-density single-phase gas and mist-foam systems.

Penetration rates also were enhanced with downhole hammers. Flat-bottomed with no valves or blow tubes, the diamond-enhanced bits are cycled at 1,600 to 1,800 beats per minute to drill the rock rapidly. The percussion bits also have a much longer life than conventional bits, resulting in fewer trips and lower bit expenses.

In addition to the hard, abrasive formation, the air drilling system had to address issues with water influxes common in the Travis Peak as well as noncommercial associated gas. Careful engineering of the system design, contingency planning, and rig crew training helped mitigate the risks.

Drilling the three wells involved compressed air or membrane-generated nitrogen injection rates ranging from 2,800 scf/m to 3,400 scf/m, and the liquid rates during mist-foam drilling were 14 gal/m to 16 gal/m (53 lb/m to 61 lb/m). The nitrogen drilling system provided a safe and economical means of gas drilling in the hydrocarbon-producing section of the formation. The first well section was drilled with three hammer bits and one tricone bit and averaged 13 m/h (42 ft/h) including connection times. Instantaneous ROP in the range of 107 m/h (350 ft/h) was observed with the hammer bits. Penetration rates while drilling underbalanced were significantly greater than the offset records, with 725 m (2,378 ft) drilled in 55.5 hours. In the second well, 838 m (2,750 ft) were drilled in 58.75 hours using three hammer bits and one tricone bit. The third well used four hammer bits and three tricone bits to drill 917 m (3,010 ft) in 61.7 hours.

Air-hammer drilling using a hammer bit (shown) eliminated 12 to 15 days of drilling time in the Barnett shale.

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