air drilling

Introduction to Air Drilling

Tuesday, April 11, 2023

1 Why Air Drilling

2 Technical Considerations

© 2007 Weatherford. All rights reserved.2

• Identify the key advantages of drilling sub-hydrostatically,

• Recognize why drilling sub hydrostatically increases ROP,

• Identify the basic parameters involved in selecting fluids for air drilling applications,

• Recognize the main considerations to take into account when establishing air drill procedures,

• Describe the operation and advantages of hammer drilling,

Learning Objectives


Why Air Drilling

1 Air Drilling Advantage

2 ROP

3 Bit Life

4 Differential Sticking


The Air Drilling Advantage

• Increase ROP

• Increase Drilling Bit Life (ft/bit)

• Minimize/Avoid Differential Sticking

• Formation Stability (Reactive Shale)

• Lower Bit weight is required

• Deviation Control

• Hammers Can Improve ROP and Deviation Control

• Decreasing Lost Circulation

Hard Rock / Depleted Zones / Shale /Fractured Zones / Deviation Tendencies


Increased Penetration Rate

• Overbalanced reduces ROP,

– Increased rock shear strength,

– Increased chip hold down effect,

• Confining stress = differential pressure,

• Increasing confining stress (overbalance) increases rock shear strength,

• Chip hold down effect,

– Rock may break pseudo plastically under positive confining stress,

– Chip released after pressure equalizes,

• Hydraulic horsepower not key in underbalanced

From: Burgoyne Applied Drilling Engineering.


Air Drilling Increasing ROP


WOB x RPM

ROP Pb

WOB x RPM ROP Po

Penetration Rate Prediction

Normalized Drilling Strength Index =

A value of 5 means ROP at an specific BHP is 1/5 of the ROP at atmospheric pressure.

Norm

alize

d R

ock D

rillin

g S

tren

gth

, D

SN

orm

alize

d R

ock D

rillin

g S

tren

gth

, D

Snn

Bottomhole Pressure (psi)Bottomhole Pressure (psi)

10

9

8

7

6

5

4

3

2

1

00 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

DTM/Jurassic-2 Shale SDM/Welsh Shale Results from Cheatham et al. SDM/Jurassic-2 Shale

DSn =


Increased Bit Life

• Air drilling uses Less energy / Per foot drilled,

• Less energy Less Tooth wear / foot More feet / Bit,

• Limiting Factor: Increased vibration Lower bearing life,

• Use appropriate bearing / Air Bit


Differential Sticking

• Differential sticking occurs when the pressure in the hole is larger than the pore pressure forcing the pipe against a permeable formation,

• Fs = Force required to free pipe (lbf) is calculated as

Fs = Ac X P X s X 144 sq.in./sq.ft.

Ac = Contact area (sq. ft)

P = Pressure differential across the mud cake (psi)

s = Coefficient of friction between string and mud cake.

• Example: 4.5” 16.6 #/ft S-135 premium pipe is differentially stuck after drilling 30 ft of 8 ¾” hole with 9.0 ppg mud at 8,000 ft into a depleted 5.0 ppg permeable formation. The coefficient of friction is 0.3 and the area of contact is 1/12 of the hole. Can you free the pipe by pulling?


Technical Considerations

1

Hammer Drilling2

Fluid Selection

Procedures / Operations3

Rig Up4


Fluid Selection

• The lower the ECD the higher the ROP,

• Application Summary:

Dry Gas

Mist FoamSmall Liquid Inflow

Large Liquid InflowPoor CleaningHydrocarbon

• It may be required to mud up when the hole becomes wet and there is a water sensitive formation in open hole,

Foam Chemical / disposal cost, mechanical Instability

Two Phase


Fluid Selection

• Other factors to take into account are:

– There may be a minimum ECD to maintain formation stability,

– It is recommended to use a gas with low or no oxygen (natural gas, nitrogen) when drilling with hydrocarbon influx,

– Drilling with returns to a pressurized separator requires a gas with low or no oxygen (natural gas, nitrogen),

– Drilling with returns to a pressurized separator requires a gas with low or no oxygen (natural gas, nitrogen),

– The optimal cost/benefit between higher ROP and operational cost may not be at the lower ECD,

• Fluid surface handling and disposal along with chemical cost,

• ROP can reach a practical maximum,


Hammer - How it Works?

• Description from diagram

• Description on Hammer

• Hammer on Rig

Check Valve

Back Head

Check Valve Spring

Feed Tube

Piston

Guide Sleeve

Bit

Driver Sub

Retainer

• General recommendations

– Drilling w Hammers.pdf

– Hammer Program


Hammer Bits

Bit Retainer Thread

Strike Face (piston

impacts)

Lower Bearing

area

Spline - transfers rotation

Load Shoulder

Blow Tube “foot valve”

• RPM: move bit to on fresh rock,

• 20 - 30 Common Range,

• 10 – 20 hard formations,

• WOB 200 - 500 lb/inch of hole size

• WOB ~ ½ of conventional WOB when rotary Air Drilling


Hammer Drilling

Advantages

• Very low required bit weights,

• High compressive force at the bit,

• Minimal deviation tendencies,

• Excellent bit life,

• Excellent on-bottom time

– Less bits and fewer trips,

• Higher Penetration rates,

Limitations

• Same as air/mist drilling,

• Liquid inflows affect hammer performance,

• Designed to work with minimal liquids injection,

• Not as effective in soft formations,

• Lack of tools to predict behavior on bottom,


Hammer Drilling

Candidates

• Hard rock areas with associated low drillability,

• Areas where hole deviation is a concern,

• Horizons with minor water influxes,

• Horizons with water sensitive claystones,


Exercise - Hammers

• An operator plans to drill a 17.5 in hole to 3500 ft in a 20 ksi compressive strength formation with hammers. Drill string is 5.5”. The air requirements for hole cleaning are 5000 scfm. Select a hammer from the ones included in the Hammer spreadsheet provided.

– What is the expected pressure differential?

– What are the expected beats per minute?

– If a choke is required, what is the required size?

– What is the expected ROP?


Planning and Operations

Procedures

• Condition the hole,

– Drill shoe,

– Circulate CPD Fluid - Unload

– Dry Hole

• Drilling,

• Connections,

• Tripping,

• Secure the well and Complete,

• Extensive Pre-Job Planning,

– Key in new areas or lacking UB experience,

• Flow Modeling,

• Drill pipe burst & casing collapse in empty hole,

• UB Equipment Increases Safety,

• BOP Stack & Choke for secondary well control,

– Kill Fluid,


Procedures

Drilling out of Casing

• If possible, use a roller cone bit,

• Use a PDC-drillable shoe when drilling the shoe with a hammer.

Unload

• Can unload in stages or from bottom,

• Minimize pumping with motors or hammers inside casing,

• From bottom when feasible - Establish schedule with Unload program.

• Move pipe often when circulating inside casing with a motor .


Drying the Hole

Drying the hole

• Clean up the hole with soap,

• Blow the hole until the water is just a drip,

• Drill 5 feet of hole,

• Work the pipe up to the top and back to the bottom twice,

• Drill 5 feet of hole, watch for dust and pressure changes,

• Repeat the two above steps until you have drilled thirty feet or have an indication that the well is not drying.

• If not dusting after thirty feet start mist and re-clean the hole.

– The thirty feet can be increased if there is no torque or drag.

– Start over if attempting to dry the hole after re-clean.


Procedures

Drilling

• Keep the hole clean,

• Know what size and quantity cuttings are returning,

• Be vigilant for the changes in pressure,

• When making connections be sure the hole is clean,

• Reestablish circulating pressure prior to commencing drilling,

• Keep the pipe moving,

• When drilling with dry gas keep a constant watch for wet cuttings,

• When circulating prior to connection work the pipe up to the top and back to the bottom,


Connections

• Minimize the time not pumping

• Pump down to the float with gas

• Always run string floats (minimum 500’ to 700’)

• Make safe connections as fast as practical

• Consider shutting in on foam connections


Tripping

• Rig up a blooie line jet when is not suitable to kill the well,

– Use two inch connection at 400 angle with screw in nozzle ¾ to ½ inch,

– It can suck ~ 6 million/day (120 m3/min) out of the RCD,

– Plan how to get the drill string out by pulling through the RCD rubber if necessary,

– Know your pipe light conditions,


Air/ Mist / Foam Flow Diagram


Exercise

• 9 5/8” 42 #/ft casing will be set and cemented at 9000 ft. Cement is to be displaced with water. Indicate the main steps you would plan to take to hammer drill a 8 3/4” hole from 9000’ to 12000’. Water inflows sometimes appear in this section and the formations are stable. It is planned to start drilling with 4.5” 16.6 #/ft NC46 Drill Pipe, 6.75” X 2.5” Drill Collars, 1900 scfm of air and WB-12 booster .

– Take into account recommendations in “Drilling w Hammers.pdf”,

– If unloading is required use Unload program to prepare unloading schedule.

• The float is at 400 ft and the injection pressure is 900 psi. You want to circulate pass the float. How long do you have to inject after stop pumping water before starting to bleed of the pressure?


Homework

• Unload the hole the fastest,

• Read document “air drilling with motors”.

– Identify at least 5 “rules of thumb” that make sense to you and prepare to explain to the group,

– Identify at least two “rules of thumb” that do not make sense to you or that you would like to know more about,

– Why do you think it is recommended not to use the full motor rate when circulating to bottom,

– Do you think we in CPD should use the methodology they propose to calculate flowrate at the motor or should we use something else?

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