FIELD TEST OF SWTORPEDO^TM (SHOCK WAVE TORPEDO) AS AN
EXPLOSIVE METHOD BASED ON EFFECT OF ROCK DILATANCY
TO INCREASE PRODUCTIVITY/INJECTIVITY FOR OIL AND GAS
WELLS
 

Date Report Completed: August 13, 2004

By Igor Skakovsky

Work performed under Rocky Mountain Oilfield Testing Center (RMOTC) CRADA 2002-02
 

Abstract

Dilatancy is a permanent deformation registered in rocks that are subjected to non-uniform dynamic stress. As the rock-volume changes, porosity can increase up to 60% and permeability increases 200% or more, as a result of the microfracturing or cracking that have been measured in laboratory experiments using core samples and in the field tests at Momontov in West Siberia by implementing SWTorpedo^TM services.

During the life time of the producing well accumulation of fines from the formation can eventually plug the formation in the near wellbore area. In some cases, a fairly recent drilling or completion could cause damage to the formation and result in lower and sometimes not acceptable productivity of the well.

To solve the problem of low production caused by near wellbore damage, application of the SWTorpedo^TM can be inexpensive and effective method to clean perforations and near wellbore area. The expected effective radius can be adjusted and undesirable increase of water production can be avoided.

The SWTorpedo^TM stimulation allows using small amounts of explosives resulting in no damage to the casing string. When it is necessary, reinforcement to casing is provided. Appropriate application of the exact amount of explosives is the key to optimizing the results of this new rock deconsolidation technique.

Introduction

Dilatancy is achieved by use of high explosives that are placed strategically in the tool and detonated in rapid succession to generate multiple shock waves, or dynamic stress. As shown in Fig. 1, at this point fast growing increase in volume of rock can be observed, even though active forces are still working in compressive regime.

All of the SWTorpedo^TM services allow using small amounts of explosives resulting in no damage to the casing string. When it is necessary, casing protection is provided. The secondary effect of shock waves is cleaning and flushing out near borehole area by forcing the fluid in to formation through the perforated casing. The application of SWTorpedo stimulation services is not restricted by the size or density of the perforations because shock wave will fracture the rock before fluid become mobile. Nevertheless, to create more effective flushing effect it is preferred to have larger size of perforation's entry holes with the density at 4 holes per foot or higher. The change in cumulative production is measured in short period of time after stimulation, where positive or favorable oil water ratio is consistent, and has tendency to change over time and the duration of the change can not be predicted with high accuracy.

Figure 1.  Distinctive characteristics of the dilatant stress state created by SWTorpedo during its detonation. ζ - is Dilatancy and where ζ = σ_φ/σ_r , msec-is microseconds.

In Figure 1 displayed registered tensile stress and rock fracturing in shear when 2 stage SWTorpedo with cumulative TNT amount of 8lb was used, where distance ( ) from the center of the charge to the point where maximum principal stress was measured is 3ft. Dash line shows the how θ (volumetric deformation) is vary with change in ζ  value. During detonation of SWTorpedo compressive volumetric deformation appears only in the initial stage of wave’s propagation when maximum principal stress as low as 17.5MPa or 23.3% of its amplitude value. In the short period of time differential stress begin to grow rapidly and at the amplitude value of wave’s propagation reach value of 0 (ζ → 0) which is approximates uniaxial stress and continued decline of amplitude value of wave’s propagation reaches value of negative 1 it is close to "ideal" shearing (ζ → -1) (area Ι, and ΙΙ).

Testing Goals

·      Stimulate stripper well with SWTorpedo^TM to solve the problem of low production caused by near wellbore damage resulting in increase of injected gas production at RMOTC.

·      Protect casing and create temporary bridge to localize the explosion, as a response to a safety concerns and future applications of high explosives in shallow wells. Sigor designed a sandbag system wrapped around the wireline just above the device. The system included four sand bags, each 5 ft. long, and spaced at 5 ft. intervals. The sandbags were designed to bridge-off in the wellbore, thereby constraining the blast energy to the perforated interval.

·      Evaluate a technology design. If proven effective the technology could have wide application at NPR3 and similar fields in the Rocky Mountain region. Several NPR3 wells have declined in production recently and DOE would welcome new stimulation technology to enhance field economics.

History of the well

Well 85-AX-10 was drilled in 1977 to a TD of 3264 and completed in January 1978. Initial production was 148 BBL/M of oil. Eleven months later production was: for oil 0 BBL/M, for gas 0 MCF/M, for water 92 BBL/M. In April 1979 well was acidized and put back in production for 3 month than well being shut-in until 1991. In February 1991 well was re-perforated from 2,990' to 3,010', 80 shots, 3,014' to 3,052', 152 shots,4 spf (.75" diaholes), fracture treated and stimulated with 2400 gallons Terra Frac 111 pad and 1500 gallons Terra Frac 111 8 ppg 12/20 sand (12,000 #). Initial production was: for oil 30 BBL/M, for gas 3,319 MCF/M, for water 175 BBL/M. Core was cut over the Second Wall Creek (2WC) formation gross sand interval from 3014 to 3054 MD with 2SP3F, 21 shots/31 feet. Casing of 5.5 in. diameter was hung at 3138 MD, and perforations shot over the intervals 2990-3010, and 3014-3052. Production histories since 1997 are in Figures 2, and 3. The gas spike in September 2001 is due to a gas storage project.

Figure 2.  Cumulative production for 85-AX-10 since 1977:

2002 maximum gas rates (MCFD) 85-AX-10 for 6 months before stimulation:

2002-03 max gas rates (MCFD) 85-AX-10 for 5 months after stimulation:

Second Wall Creek Member lithology

The Second Wall Creek Member of the Upper Cretaceous Frontier Formation is a marine sequence of shelf sands and shales. Figure 4 is a neutron/density porosity log of the completed interval. The well is completed over three sand intervals separated by tight calcareous concretions between the depths 3009 - 3014 MD, and 3025 - 3029 MD. The well was originally fracture-treated, so vertical communication across the concretion among the three sands is likely.

The sandstone neutron/density curves converge with decreasing shaliness upward, culminating in the best quality sand in the interval 3002 - 3009 MD. It is this sand that most likely is contributing most of the production. The neutron curve deviation to the right in this interval may be an indication of gas.

Hot water and chemical stimulations of this well have resulted in temporary increases in gas production. A typical treatment involves 110 BBLs of hot Tensleep water followed by pumping off the load to regain production. The post-treatment gas response suggests paraffin blockage at the sand face and perforations. Sigor requested that RMOTC treat the well prior to testing.
 

Figure 3.  Neutron/density log of 85-AX-10, showing lithologies, cored interval, and perforated intervals. Most production is probably coming from the upper gas sand interval 3002'-3009'. Neutron/density curve separation is related to shaliness. Note how sand shaliness increases with depth between 3002' - 3054'.

Analysis, materials and equipment provided by Sigor Corp

Sigor conducted advanced core analysis that included:

·      Hydraulic observation.

·      Determine the rock static properties (Young's Modulus and Poisson's Ratio) and compressive strength under uniaxial and triaxial conditions.

·      Determine sonic velocities, unstressed (uniaxial) condition and stressed (triaxial) condition with Young's modulus and Poisson's ratio calculations.

·      Determine the changes in rock permeability after dynamic loading for rock specimens in regime of laminar hydraulic flow according to Darcy's law using a liquid with low viscosity.

·      Determine the general regularities of rock behavior as dilatant media, in accordance with Sigor Corporation proprietary information.

·      Investigate rock's dilatant and elastic characteristics and its dependence on its natural properties such as permeability under triaxial dynamic compression with imitation of geological conditions of the formation at the productive zone under different non-uniformity of the dynamic stress state, in accordance with Sigor Corporation proprietary information.

·      Ascertainment of the relationship between rock properties (porosity, strength, permeability and so on) in the state of dilatant deconsolidation, in accordance with Sigor Corporation proprietary information.

Sample plugs taken every foot in the core interval 2993' - 3050'. The highest permeability measured in the cored interval 2998' - 3005', with an average of 135' MD. This interval corresponds with the log interval 3002' - 3009' MD (the difference in depth reflects depth measurement shifts). Average porosity over this interval is 20% and oil saturation 65%. The lower sand packages in the interval 3009' - 3049' core depth average 37' MD permeability. Core analysis suggests the upper interval is most productive.

Following well data provided by the RMOTC has being used by Sigor to design the service:

·      Initial production rate (intake/injection rate) and its change during exploitation (subdividing production rates for oil, gas, water, and indicating the amount of water-cut of the entire fluid).

·      Range radius of well and its external reservoir boundary.

·      Whether any other methods to increase production rate were used, their characteristics and results.

·      Geologic section of the well's producing zone.

·      Data on the casing string (presence of filters and their construction, depth of casing shoes).

·      General sizes of casing such as diameter and wall thickness, strength of steel.

Sigor provided materials and equipment

Sigor's device consisted of an 8 foot long aluminum cylinder, with 12 lbs of TNT explosive strategically placed within the tube.

Figure 4.  Top view with Tool Head attached (Tool Head was not used during the test).
 

Figure 5.  TWrapping sandbags around the wireline cable.These sandbags bridged off and cushioned the blast.
 

Explosive forces create pressure of 10⁸...10⁹ MPa per second. In such environment areas of initiated fractures and rock crushing are multiple. The area of Dilatancy or micro-fractures is on average 6 times larger than an area of radial fractures.

Sigor provided subcontracted wireline service truck. Wireline company's technicians wired the blasting caps, and triggered the detonation.

Testing sequence of events

(Quotation from RMOTC official report)

Sigor requested that RMOTC conduct a paraffin treatment on the well prior to their stimulation attempt. The well was treated with 110 BBLs of hot water on November 7, 2002, and returned to production.

(Quotation from RMOTC official report)

Sigor planned the stimulation for November 20, 2002. The RMOTC workover rig and the LogTech wireline crew rigged up and ran a casing collar locator (CCL) and 5.5 in. gauge ring (Figure 7). Fluid was encountered at a depth of 2750' MD, 240 ft. above the top perforation (at 2990 MD). A bridge or fill was tagged at 3031' MD, 107 ft. above the casing shoe at 3138' MD. Depths were correlated to the Cement Bond Log (CBL). RMOTC suspected the well to be filled with frac sand or formation material to the 3031' MD level. A bailer run, but the RMOTC workover rig crew recovered minimal sand and debris despite repeated stabbing attempts. RMOTC offered to conduct a cleanout operation, requiring an additional two or more days. Sigor determined that further delays were not acceptable. Sigor decided to proceed with testing, despite the lower 21 ft. of perforations not being open to blast energy. Darkness became a factor, and further work was delayed until the following day, November 21, 2002.

(Quotation from RMOTC official report)

On November 21, 2002, the device was armed for detonation by LogTech technicians and picked up on wireline. Sigor installed four sandbags on the wireline cable. Sigor requested the device be lowered until tagging the bottom at 3031' MD, and then raised 5 ft. to a depth of 3026' MD. Because the well cannot maintain a fluid level of sufficient hydraulic head to contain blast pressure, moved two water trucks to location to provide load fluid for an extra safety margin. About 80 BBLS of produced Tensleep water were put down the well immediately prior to detonation. At 10:35 AM, the device was detonated with little or no indication on the surface that a blast occurred. The sand bags were effective in isolating blast energy. The wireline unit pulled 7500 lbs. over to recover the cable; 3 feet of cable were missing and later recovered.

(Quotation from RMOTC official report)

... RMOTC ran a lead impression block to identify bridge/fish on December 29, 2002. The impression block required over 12,000 lbs to push through a constriction at an estimated depth of 3017' MD. The block tagged junk or fill at about 3025' MD. Impression block photos are in Appendix 2. On January 6, 2003 and at the request of Sigor, ran in the hole with a notched collar to clean out any fill at the estimated bridge depth of 3025' MD (Figure 9). High winds prevented strapping the tubing stands, and depths were estimated from an older tubing tally. The tubing string encountered drag at an estimated depth of 3017' MD, and stacked up at an estimated 3025' MD. RMOTC placed 12,000 lbs of weight on the string as it was stacked at 3025' MD. Sigor requested that a casing scraper to clean the perforations. Due to the casing constrictions, however, decided the risk was too high.

Production Engineering staff requested well 85-AX-10 be put back on production immediately to restore gas cycling. Sigor requested the tubing string be strapped-in. Sigor CEO Igor Skakovsky was present on January 7, 2003 to witness the measurement operation, and he verified the depths (Appendix 3). Tagged depth was 3047.48' (Figure 10).

Well 85-AX-10 was returned to production on January 8, 2003. The portable test treater was not used due to low ambient temperatures. Liquids were not measured, but gas rates were between 20 and 100 MCFD. Fluid rates were probably proportionately lower, according to Field Engineer.

(Quotation from RMOTC official report)

By January 20, 2003 the well had ceased production. Production Engineering determined the pump had been set too low, and the pump inlet had packed off with debris. The pump was replaced on January 21, 2003, and the inlet raised 6 ft. to 3020' MD. By January 29, 2003, Production Manager reported gas production had regained its pre-stimulation range of 50 - 350 MCFD. Again, no test treater was used due to cold weather. Additional 24 hour tests on April 9, 2003 and May 7, 2003 yielded 0.5/2.5/310 O/W/G. Figure 11 summarizes recent production history.

Summary

(Quotation from RMOTC official report)

Sigor Corp contacted RMOTC in April 2002. RMOTC invited Sigor Corp to visit the site in May 2002, whereupon Sigor Corp CEO Mr. Igor Skakovsky selected core from Tensleep well 62-TPX-10. The core was subsequently shipped to its laboratory in Ukraine for physical rock property analysis. In July, Sigor Corp stated that based on the information provided by RMOTC for the well 62-TPX-10 oil/water contact can not be determent and with out knowing oil/water contact successful results can not be achieved. In addition present oil-water ratio where production of water is 98.5 percent and oil 1.5 percent will not explicitly represent good results of Shock Wave Stimulation even if test is successful. In late July, Sigor Corp requested that RMOTC list of prospective wells in the Second Wall Creek Member of the Frontier Formation. Again, the well selection criteria included core availability and a history of economic production up to the present. There are very few Second Wall Creek wells currently producing with core available. Sigor Corp selected well 85-AX-10 as a fair candidate for the Case Study. The core was stored in the USGS facility in Lakewood, Colorado. The USGS provided Sigor Corp with an adequate amount of core.

Project was designed and stimulation of the well 85-AX-10 at Teapot Dome took place on November 21, 2002. Production stabilized in February and it was monitored through April 2003.

Contact information

Igor Skakovsky

President/CEO of Sigor Corporation

9934 E. Carolina Circle, Suite 202

Denver, CO 80247

Tel. 720 480-4642

Fax 303 337-5251

sigor@swtorpedo.com