The first topic will "theory".  I am planning to expand the section over the next couple of weeks.

Theory - Propellant Stimulation                                                       

To understand how propellants work - and where to use them, you need to understand some fundamental principles of  combustion, confinement, and burn rate. This is just the start of this section.  

Propellants - by definition - burn, they do not explode.  The total burn time might be just 1 millisecond - but it is still an oxidation reaction, ie. combustion.  It does get a little confusing because the Department of Transportation (DOT) classification of most propellants is as an Explosive.  StimGun is classified as an Oxidizer; while other propellants are classified as a Flammable Solid.  But anyways...

The burn of a propellant is a rapid oxidation reaction resulting in the release of both heat and gaseous energy. This high pressure combustion gas mixes with liquid in the well and is forced into the perforations causing breakdown and fracture propagation.  

To enable this process to be effective - the burn rate be must adequate and there must be adequate confinement.

"Adequate" sort of means there is a large window of effective parameters. 

Burn rate - the ignition of the propellant must result in a rapid increase of pressure in a minimal time span - rise time - dP/dT.  The formation is typically broken down in a few milliseconds with remainder of the propellant burn (up to 500 milliseconds) is used for fracture propagation / extension.  There is also an optimum peak pressure - if the propellant generates too high a peak pressure - stimulation efficiency is decreased (resultant total burn time will be reduced) and there is an increased risk of casing damage.  If the peak pressure is too low, the formation will not breakdown, regardless of the length of burn time. 

Using down hole pressure data and computer modelling, the burn rate of StimGun has been optimized to increase effectiveness and minimize risk of casing damage.

Confinement - the least understood concept.  It is somewhat difficult to understand why this combustion gas goes into perforations - and not just up the wellbore.  It all has to do with "path of least resistance".  If you have 1000' of non gasified water above your propellant tool - when the tool is ignited, the generation of combustion gases has to result in an increase in the system volume.  Something has to break or move - it is easier to breakdown the rock and extend fractures (increasing the system volume) than it is to lift the fluid column to increase system volume - in a few milliseconds.  If the fluid in the well is gasified, it is easier to compress this fluid column than breakdown the rock - the stimulation will be ineffective.  

As the amount of fluid is reduced, the stimulation efficiency is also reduced - more and more of the combustion gases go up the wellbore.   Also, StimGun products require a minimum amount of fluid to ignite and burn at a optimum rate.  We typically recommend a 1000 feet minimum - but type of rock can affect this number.  Tight zones requires less fluid tamp than extremely permeable zones. (generally, minimum fluid range is 500 - 1500').

The last concept - the easiest to understand, and the one least discussed, and probably responsible for most of ineffective stimulations - is flow area into the casing.    To get the combustion gas into the formation, it must go through the perforations.  The stimulation efficiency is directly controlled by the perforation density and perforation hole size.  Using a charge with a 0.3" entry hole compared to one with a 0.4" can reduce fracture length by up to 40%.  What we recommend at least 6 spf for guns up to 3 3/8" OD, and more for the larger guns. We need a good hole size and a good depth of penetration.

Propellants have been successfully used to stimulate formations for over forty years.  However, the stimulation success rate has generally been low.   Why?  Poor pre-job evaluation / screening; and poor job design.  This is the reason the StimGun Group came together - to use a little science in the stimulation pre - job screening and stimulation design. A good application with adequate well fluid, adequate perforations, and adequate volume of propellant (with adequate burn parameters) - will be successful 100% of the time.

Experience is always the key parameter in job design