Well Stimulation/Completion Using High Explosives

The objective of using high explosives in well stimulation is to increase the effective drainage area at reduced cost. One form of explosive stimulation uses dilatant technology, in which rock-volume is permanently changed through the use of explosives. Porosity can be increased 200% or more as a result of controlled microfracturing/cracking.

        Fig. 1—Dilatancy is an increase in volume of a granular
        substance when its shape has been altered as a result
        of increasing the distance between its component particles.

Introduced by Sigor Corp. in 2004, the SWTorpedo, a dilatant explosive technology (Fig. 1), is based on rock behavior under overbalanced dynamic stress, when rock fails at 5% of its strength and dilatancy/shearing is triggered as far as 60 ft from the wellbore.

Field applications of the technique have shown that extensive microfractures increase yield as much as five-fold in oil wells and seven-fold in gas wells; water production is doubled in irrigation wells. Stimulation of potentially productive intervals in which rock properties are known and hydrocarbons in place was 96% effective.

The dilatant method requires a relatively small amount of high explosive compared to the amount required for common explosive techniques/propellant tools. It can supplement acidizing and/or be used ahead of an acid job to increase acid/rock contact, and it is competitive with the hydraulic-fracturing method (designed to extend a fracture up to 150 ft in low-permeability sands). The dilatant method also can be compared against the use of nitrogen in shale when the tool is detonated in formation water or in diesel.

Fig. 2—When dilatant stimulation technology was applied in a
Berea sand interval in the Appalachian basin, there was an increase
in production from 4 to 20 BOPD and from 20 to 53 Mcf/D of gas.

The technology was applied in the Appalachian basin with JP Oil and BJ Oil and Gas in cased gas wells and openhole wells, respectively. The result of the stimulation effort for JP Oil in Medina sands was an increase in gas production from a maximum of 7 to 50 Mcf/D. Also in the Appalachian basin, this technique was used by BJ Oil and Gas to increase production in an openhole well that had been producing for 40 years before treatment. Production increased from 4 to 20 BOPD and from 20 to 53 Mcf/D of gas. The increase was sustained for 6 months and then declined to 12 B/D (Fig. 2). Coalbed-methane wells in the Powder River basin were stimulated using dilatant technology, and Williams Production saw an increase in beneficial water production from 76 to 128 B/D.

Dilatant technology has been successfully applied in the following formation types:

Strong, medium, and weak sandstones.
Strong, weak, and dense limestones.
Dolomites.
Strong and medium shales.
Strong coal.
Granite.

Increasing Drainage Area

Extremely high pressure on an average of 1 million psi created by high explosives usually lasts for a few microseconds, often causing rock compaction, but extending time for such high pressure up to 3 milliseconds will initiate shearing (all active forces are compressive), and allow microfractures to propagate as far as 120 times the radius of the explosives being used. For instance, a 3½-in.-diameter tool containing explosives will shear the rock as far as 30 to 35 ft from the wellbore. The drainage area will equal the area of an ellipse horizontally 70 to 90 ft and vertically 50 to 65 ft in size. This drainage area often exceeds the effective drainage area from a 150-ft-long fracture created by a hydraulic- or sand-fracturing method.

Process and Tool Characteristics

        Fig. 3—Volumetric deformation of strong sandstone
        and change in its permeability under an uneven stress
        state created by the SWTorpedo tool.

Simple sequential explosions have been used for decades, and delays between pulses were randomly selected, often leading to a decrease in permeability and formation damage. SWTorpedo designs (Guarant, Optimum, Econom) differ only in the amount of rock analysis that previously has been completed. Usually, the tool will contain two (in some cases, three) stages in which the initiation time of each stage is calculated to trigger perfect shift and allow dilatancy to expand (Fig. 3). The detonation regime, or timing between pulses, is essential to creating microfractures without significant formation damage. The optimal depth for the tool is 1,000 to 6,000 ft. Minimum depth is 300 ft and maximum depth for oil wells is 13,200 ft and 14,800 ft for gas wells. Nearly any type of fluid can be used to convey the shock to the formation (e.g., oil, diesel, weak acid, formation water, KCl brine, or water).

Comparing Methodology

Applicable to cased wells and open hole, shock-wave stimulation offers an alternative to propellant tools, common high-explosive stimulation techniques, certain acid jobs, and hydraulically created fractures that would extend up to 150 ft. Unlike propellant tools, shock-wave completion/stimulation creates permanent changes in rock, preventing recompression and prolonging the life of effective fractures/microfractures.

Compared to acidizing treatments, dilatant stimulation has a lower risk of compromising well integrity and requires a lower initial investment, according to clients. This methodology also results in higher aggregated permeability and the same or greater drainage area compared to the common 150-ft hydraulically created fracture. Because the volume of fractured rock created with shock waves is limited to the predetermined radius of the ellipse, unintentional penetration of nearby water reservoirs can be avoided.