Nitroglycerin and other

explosives came into general

use as stimulants in 1867 and

prevailed until the late 1940s

when explosive-based

stimulation was replaced by

hydraulic fracturing.

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15S t i m G u n T e c h n o l o g y

B a c k g r o u n d

Rapid energy-release stimulation of oil and gaswells commenced nearly 150 years ago. In

1860, a black powder “torpedo,” a 3 ft (0.9 m)length of 2 in. (50.8 mm) copper tubing filled withrifle powder, was first used successfully to stimulatean oil well.

Nitroglycerin and other explosives came intogeneral use as stimulants in 1867 and prevaileduntil the late 1940s when explosive-based stimula-tion was replaced by hydraulic fracturing.

Solid propellants were introduced in the 1970sand are the basis of modern propellant technolo-gy for oil field use. The performance and successrate of the initially slow burning cylindrical toolwith a top-to-bottom ignition system was animprovement but still marginal by today’s stan-dards and requirements. In successive years, con-siderable research, development, improved engi-neering design, and testing have been applied tosolid propellant technology in order to enhanceits stimulation effectiveness.

The latest steps forward, enabled by the com-mercialization of high-speed downhole datarecorders, more sophisticated computer modeling,and newer tool designs, have been made by thePropellant Technology Development Group. Thisrecent work has resulted in three distinct tooldesigns now available and in use:

✳ Well Stimulation Tool

✳ StimGun™ assembly

✳ StimTube™ tool

Well Stimulation Tool (WST)

The WST is used primarily to stimulate: perforat-ed cased intervals, open hole and long intervals inhorizontal wells. It is a cast cylindrical rod of pro-pellant with a full length central ignition system.The ignition system has been improved to increasethe tool’s reliability, burn rate, and reproducibility.Although these steps have enhanced WST per-formance and reliability, its use is somewhat limit-ed by its design.

In cased wells, the interval must be perforatedprior to running the WST. This two-run require-ment is less economical and not always physicallypossible. Another consideration is that the toolmust be depth-correlated with the existing perfo-rations. This is rarely a problem, but warrantsmentioning.

The WST requires an external steel, supportingcarrier similar in appearance to a perforating gun.The carrier’s outside diameter sometimes preventsWST use in wells with inside diameter restrictions.

Nevertheless, the WST remains an attractivechoice for many previously perforated wells andopen hole intervals.

StimGun™ assembly

The WST two-run requirement, mentionedabove, pointed out the need for a combined perfo-rating gun/propellant stimulation tool and prompt-ed the formation of the Propellant TechnologyDevelopment Group. In 1996, this group proposeda new design comprised of a propellant sleeveplaced over the outside of the perforating gun. Thepropellant sleeve is ignited by the perforatingcharge; as the propellant sleeve burns energeticgases are released.

The group conducted extensive field tests of dif-ferent sleeve designs to optimize the ignition andburn rate of this new StimGun™ assembly. The col-lection and interpretation of the down-hole, high-speed pressure data were essential in the optimiza-tion. Computer modeling demonstrated that com-bining propellant stimulation with the perforatingin one run resulted in a more efficient stimulation

Historical and technical perspectivesJoe Haney, HTH Technical Services, Inc.John Schatz, John F. Schatz Research & Consulting, Inc.

The successes of modern oil-field propellant technology developed by this group are due to:✳ An integrated science and engineering

package

✳ New propellant tool designs

✳ High-speed data acquisition

✳ Computer modeling

✳ Data analysis and job optimization

✳ Extensive field experience

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16S t i m G u n T e c h n o l o g y

as compared to the two-run perforating gun/WSTcombination. This new design was subsequentlypatented, trademarked, and is now in worldwide com-mercial use.

StimTube™ tool

When run without its required protective steel carrier,the WST failure rate is unacceptably high because of itsmechanical weakness. Work to strengthen the WST ledto the concept and testing of a tool with an endoskele-ton, or central steel mini-perforated strengthening tubewhich also contains the igniter. This design togetherwith a modified propellant type increases the maxi-mum temperature rating and enhances the burn rate.In addition to through-tube applications, theStimTube™ tool, because of reduced equipment require-ments, is an excellent replacement for the WST for fullinner diameter (ID), cased well stimulations.

The StimTube™ tool, both patented and trademarked,is the Propellant Technology Development Group’snewest product, and has shown good success since itsintroduction.

Propellant stimulation technology anddynamic fracturing

The burn of a propellant in a well is a rapid oxida-tion reaction causing the release of gaseous energy asshown in Figure 1. This gaseous energy mixes with the

liquid in the well to penetrate the perforations (perfs)and cause breakdown with fracture propagation intothe formation. It also causes the liquid to compressand move upward and downward in the well.

Perforation breakdown and tortuosity reduction

Perforating guns release energy in microseconds(1/1,000,000 of a second), the gas energy from pro-pellant burn releases in tens to hundreds of a millisec-onds (1/1000 of a second), and hydraulic fracturingstimulations expend energy over many minutes. A pro-pellant is therefore “intermediate” to the other meth-ods in its energy release rate.

Figure 2 (a, b, c) shows laboratory test results andtypical pressure-time records for (a) explosive events,(b) propellant events, (c) hydraulic fracture events.

✳ Explosive events, such as perforating, create veryhigh pressures, necessary to penetrate the wellcasing, that also crush and damage the rock.

✳ Propellant events rapidly exceed the fracturepressure of the rock and maintain the pressure butdo not crush the rock.

✳ Hydraulic fracture events balance the drivingpressure and the fracture pressure.

As a result, propellants pressurize and break downmany perforations along a significant proportion oftheir length; while a hydraulic fracture, entering onlythe path energetically allowed, breaks down only asmall subset of perforations. These breakdowns maybe very near to the sand face or in a micro annulus.

Fracture propagation and orientation

Propellant-driven fractures originating at the perfo-ration tunnels will then move (propagate) into theformation from a few feet to a few tens of feet, asshown schematically in Figure 1 and in the scaled lab-oratory simulation of Figure 3. The high pressuregas/fluid pulse generated by the burning of the pro-pellant temporarily creates local stress concentrationsthat are two to three times the normal fracture gradi-ent. These stresses are oriented on a plane throughthe axis of the wellbore/perforation tunnels.Propellant-driven fractures will always tend to initiatein the plane of the axis of the wellbore. This is truewhether the well is vertical or deviated, no matterwhat the depth of the well. Contrarily, hydraulic frac-tures only will initiate in the in-situ stress preferredplane in deeper vertical wells (Figure 5a). For wells inthe depth range of 500 to 2000 ft (152 to 609.6 m)

Tampingliquid

Expandinggasifiedbubble

Propellantenergy source

High-speedrecorder

Wellboreinterior

Fluid motionin well

Flow throughperforations andinto fracture

(Two fracturesshown)

Figure 1 – Representation of the release of gaseous propel-lant energy and its exit from the wellbore into the formation.

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17S t i m G u n T e c h n o l o g y

B a c k g r o u n d

Explosive event

Propellant event

Pressure vs. time for typical hydraulic fracturing.(c) Laboratory hydraulic fracture test.

Pressure vs. time for typical propellant event.(b) Laboratory propellant test.

Pressure vs. time for typical explosive event.(a) Laboratory explosive test

P

Time – Microseconds

Rock Crushing

~>105 psi

Fracturing

P

Time – Milliseconds

Fracturing

~>104 psi

P

P

Time – Seconds

Fracturing

~>103 psi

Figure 2 – Laboratory test results and typical pressure-time records for explosive, propellant, and hydraulic fractureevents.

Gas/liquid event

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18S t i m G u n T e c h n o l o g y

Figure 4 – Perforation erosion caused by propellant gas energy (Large Block Surface Test)

Figure 3 – Propellant stimulation with 90° perforation phasing (Laboratory Scale).

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19S t i m G u n T e c h n o l o g y

(depending on local conditions) and in highly devi-ated wells, hydraulic fractures will tend to be outof the plane of the wellbore axis (Figure 5b).Propellant-driven fractures will tend to curve backinto the in-situ stress preferred direction, but gen-erally propagation is finished before much of thecurvature occurs. Because late-time propellant-driven fracture propagation is ultimately controlledby in-situ stress, the longest propellant fracturestend to be bi-wings and in the plane nearest thein-situ stress preferred plane, although shorterfractures will occur in the other planes.

Near-wellbore stimulation

Propellant-driven fractures will not containproppant in the formal sense, but will retainsome aperture due to erosion, ablation, debris,

and closure misalignment caused by shear asshown in Figure 4 from a surface field test.Although these fractures cannot compete withlong propped hydraulic fractures in absolute con-ductivity values, they can penetrate near-well-bore damage, reducing skin and mildly stimulat-ing wells. They can also act as effective pre-hydraulic frac treatments, reducing breakdownpressure and improving proppant placement.

Propellants can be used economically toimprove well productivity or injectivity. They arenot meant to be replacements for other processessuch as hydraulic fracturing, but they can beexcellent solutions or solution enhancements inmany situations to perforating limitations, nearwellbore damage, or reservoir problems thatrestrict well potential.

B a c k g r o u n d

Figure 5 – (a) Initial orientation of propellant-driven fractures in all wells and hydraulic fractures in deeper vertical

wells. (b) Orientation of hydraulic fractures in shallower wells and some deviated wells.

a b

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