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Introduction |
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The Dynamic Gas Pulse Loading® (DGPL®) system is an advanced well stimulation technique based on high-pressure gas generation by propellant combustion designed to enhance existing completion methods, and in many applications provide a more cost-efficient alternative. This process permits simultaneous initiation of numerous multi-oriented fractures at most well depths and conditions. Activated in-zone under a liquid column, the system acts as its own high capacity pump, and generates its own stimulation in the form of gas. DGPL® is conveyed to zone on conductor cable or on tubing. Today's DGPL® system, which was 25 years in development, has been successfully employed in thousands of well completions covering a variety of applications. The term DGPL® is broadly applicable, and may include uses such as sand control applications, limited perforation breakdown and similar uses. STRESSFRAC®, on of the DGPL® services, is applicable when the prime objective is to create fractures with large gas generators. The primary application of the DGPL®/STRESSFRAC® gas generator is for the improvement of transmissibility in oil and gas wells. Sometimes, the assembly is used as a single service. More often, it will be combined with other services as part of a technical completions package. No single product, service or technique can ensure a high efficiency completion by configurations, operational constraints, reservoir characteristics, skin damage and flow potential, production mechanics can only be enhanced by viewing the completion as a whole. Drilling, logging, formation evaluation, casing, cementing, perforating, stimulation, and subsequent production considerations are only a few components in a complex series of decisions that petroleum engineers must make as each well is drilled. Increased expenditure and attention to detail in one area often reduces costs and/or increases efficiency in more of the other areas. DGPL®/STRESSFRAC® gas generators have a variety of specific applications as part of a synergistic approach to solution of problems commonly encountered in production operations. A general summary of these applications includes, but is not limited to: - |
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Candidates for primary stimulation include wells in highly porous and permeable conventional reservoirs, which are damaged beyond the distance that can normally be reached by perforating. Commonly employed methods to overcome this problem include acid and/or hydraulic fracturing treatments, sometimes with ball sealers or treating packers. DGPL®/STRESSFRAC® services will produce fractures several metres beyond the perforations and in every direction, particularly when a high volume tool is used (i.e., through casing, rather than through tubing). DGPL®/STRESSFRAC® may be used in place of acid or hydraulic fracturing to initiate and keep fractures in-zone, or to preclude the use of a damaging fluid in many fluid sensitive formations. In some carbonates, the use of DGPL®/STRESSFRAC® and acid has provided results superior to either method. In all cases, use of controlled multi-fracture initiation and propagation should be employed as a component of a carefully engineered technical completions program. Another primary stimulation application is in secondary porous reservoirs. In reservoirs which are jointed, sheeted, or otherwise naturally fractured, the use of DGPL®/STRESSFRAC® can improve performance and recovery by improving communication between the joint system and the well bore. Simply stated, DGPL®/STRESSFRAC® with its many radial fractures connects more natural fractures to the well than does hydraulic fracturing. This has been frequently demonstrated in the Devonian Shale play of central West Virginia, where many high performance oil wells were stimulated in the early 1980s through high density perforations by means of one or two sequential DGPL®/STRESSFRAC® treatments, followed by a nitrogen clean-up. Since matrix porosity and permeability were very low, and a proppant was not employed, the success of the treatment was dependent on DGPL®/STRESSFRAC® intercepting plural natural fractures in all quadrants. Wells stimulated without DGPL®/STRESSFRAC® had flow rates up to 100 barrels of oil a day. Wells treated with DGPL®/STRESSFRAC® followed by nitrogen produced 200-500 bbl/day. The Clinton sandstone in Ohio and the tight Prairie du Chien formation in Michigan respond well to DGPL®/STRESSFRAC® in long gas zones, and stringers as small as 4-ft. (1.2m) Of these wells, 65% produce 400 MCF - 1,000 MCF of gas per day with DGPL®/STRESSFRAC® alone, while 35% require additional treatment after STRESSFRAC®. Shallow oil and gas sands in the Mid-Continent are routinely STRESSFRAC'd with good results. DGPL®/STRESSFRAC® in the Gardner sandstone raised production from 5-bbl/day pumping to 70-bbl/day flowing on a 60-day test. Comparable results are obtained in the Strawn sand with production rising from less than 5bbl/day to 40-60 bbl/day. Less frequent, but more spectacular have been stimulations with large DGPL®/STRESSFRAC® tools in the open hole Ellenburger formation. Acid and frac treatments had been applied unsuccessfully. Typically, two DGPL®/STRESSFRAC® tools were energized below 13,000-ft, resulting in sustained production of 200 bbl/day. |
Secondary stimulation is a relatively limited, but a very important application of DGPL®. Candidates are wells that have been completed by hydraulic fracturing employing limited entry techniques. In such completions, treatment hydraulics are of primary importance, but often result in excessive pressure drop across the completion during production, which progressively worsens with compaction of migrating fines, scale growth, and in low temperature wells, precipitating waxes and asphaltenes. By re-perforating and STRESSFRACing after production decline, the pressure drop is reduced, and radial flow is improved. Many operators recognize these problems and attempt to overcome them simply by re-perforating. Where 120° or 90° phasing of perforations is possible, the addition of STRESSFRAC® with re-perforating greatly reduces skin effect and improves radial flow. In some fields it has become the economical practise to omit re-perforating, and to restore production by STRESSFRAC® alone. |
| Preconditioning for Subsequent Operations |
| Preconditioning of well bores for subsequent treatments is a very broad application of DGPL® services. DGPL® in some cases, provides a more desirable and less costly alternative to more conventional treatments such as acid washes, acid breakdowns, mini-hydraulic fracs, ball sealers, treating packers, etc. As previously mentioned, even in formations as tight as Ohio's Clinton sandstone, STRESSFRAC® so effectively overcomes skin damage that wells initially produce as if they have been hydraulically fractured. Such a treatment allows subsequent treatments to be designed on the basis of production tests. Benefits of preconditioing by DGPL® in under-balanced conditions include assured in-zone fractures, and low wellhead pressure during subsequent treatment where breakdown pressures are prohibitively high for hydraulic breakdowns. |
Acidizing has proven successful in reduction or removal of many types of wellbore skin associated with perforating, drilling, cementing, gravel packing, excessive drawdown during production, precipitated scale, migration of fines, "dirty" workover fluids, incompatible chemical treatments, and incompatible injection fluids. Acid has also been effective for increasing the permeability of natural or induced fractures in some formations, by etching the fracture face and increasing its aperture. However, the degree of success when placing acid through perforated casing has been limited by the proximity of the damage or fracture network to the perforation tunnel. The closer the damage or fracture, the more easily acid can be placed. Additionally, if perforations are plugged, substantial injection pressures and/or ball sealers may be required to ensure effective placement. Logging methods can be used to locate fracture networks, and to verify the presence of hydrocarbons. Perforating techniques can maximize vertical and circumferential coverage of the interval, and DGPL®/STRESSFRAC® gas generators assemblies to optimize placement of the acid. The improved transmissibility through perforations reduces pressure requirements and allows thorough treatment of the interval without the need for diverting agents. |
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Case History: Preconditioning for Acid Artificial Lift, Sour Oil Production |
| Depth: | 14,500-ft. (4,421m) |
| Interval: | 20-ft. (6m), Perforated 6 SPF, 0° Phasing |
| Formation: | Fractured Carbonate |
| Prior Production: | 90 BOPD for 1 day, Recharge Fracture Network for 9 days (Net Production: 9 BOPD) |
| After Stimulation: | 85 BOPD |
| After 3 Months: | 50 BOPD, Stabilized |
Successful hydraulic fracturing operations are dependent upon application of sufficient pressure to overcome the principal tectonic stresses, as well as exceeding the tensile strength of the rock. Even within a single reservoir formation, the rock properties can vary substantially in successive strata. The use of STRESSFRAC® gas generators provides a simple mechanism for overcoming high fracture initiation pressures, whether across an entire formation interval, or within selected sections of a complex, stratified formation. The STRESSFRAC® System insures that fractures are initiated in the zone of interest and that all perforations are open. Furthermore, STRESSFRAC® gas generators may have application for preconditioing multiple intervals that are scheduled for limited-entry treatment, particularly when anticipated breakdown pressures are substantially different among the individual formations. |
During production of hydrocarbons, undesirable fluids, such as salt or brackish water, may also be produced. The law requires that these fluids be disposed in a manner that will prevent pollution of the environment or contamination of the water supply. Current practices involve perforating a relatively long, porous, non-productive interval at a low shot-density (1-4 SPF), or installing an open hole gravel pack with a screen or liner, and pumping the undesirable fluids into the interval. Eventually, the pressure requirements will exceed the capacity of surface equipment, and the interval must be re-perforated, or the screen/liner must be pulled. |
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Case History: Preconditioning for Injection or Disposal Waterflood Project, Mature Oilfield |
| Depth: | 3,116-ft. (950m) |
| Interval: | 35-ft. (10.5m), 6 SPF Existing, 120° phasing added |
| Formation: | Friable Sandstone |
| Injected Fluid: | Produced Formation Water |
| Initial Injection: | 12 BPD (20m³) @ 1,200 psi |
| Rate: | Surface Pressure (8,200 kPa) |
| After Stimulation: | 4,830 BPD (700m³) @ 900 psi |
| Rate: | Surface Pressure (6,200 kPa) |
In order to meet seasonal demand for natural gas, a significant percentage of annual production is stored in otherwise non-productive reservoirs of the Northeastern United States. In other parts of the continent, salt domes are used for storage of oil. There are two major engineering concerns related to these wells; maximum efficiency during injection operations, and adequate transmissibility if there is a need for withdrawal of the hydrocarbons. The advantage of the stimulation package above is readily apparent. DGPL®/STRESSFRAC® can be applied with relative convenience, especially if casing inspection or routine nuclear logging operations are already scheduled. |
Placement of chemicals within the matrix or cement to restore zone isolation through perforations will be subject to the same limitations discussed in "Acidizing" above. The importance of zone isolation improvement is obvious. One example would be for reduction of a high Gas/Oil Ratio (GOR) in a solution-drive reservoir. Since the dissolved gas provides the energy needed to lift the oil to the surface, any reduction in the GOR can extend the productive life of the well before artificial lift hardware must be installed. |
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Case History: Reduction of High GOR Solution-Drive Reservoir |
| Depth: | 7,280-ft. (2,220m) |
| Interval: | 13-ft. (4m), Perforated 4 SPF (13 spm) 120° phasing |
| Formation: | Carbonate |
| Prior Production: | 150 BOPD (24m³) GOR of 3,360 scf/bbl (600 scm/m³) |
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After Stimulation and Squeeze: | 125 BOPD (20m³) GOR of 850 scf/bbl (150 scm/m³) |
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Reduction of Wellbore Skin |
| The effects of wellbore skin, or impairment of low have been discussed thoroughly in the literature. Total skin is comprised of a number of components, including drilling damage, cementing damage, insufficient perforation performance or density, perforating damage, convergent flow and turbulence, to name a few. Technical completions packages which include downhole gas generators can reduce the effects of skin dramatically, hence improving inflow performance of the well. |
Selection of a particular perforator is often limited, due to other production operations considerations. For example, a deep, high-pressure reservoir might require the use of through-tubing perforators. The design considerations for small shaped charges make it difficult to provide performance equivalence to the larger casing guns. Additionally, as casing weight and grade increase, the perforation entry hole diameter in casing will be reduced. This can cause problems if subsequent injection operations, such as hydraulic fracturing are required. Also, in the case of high-rate gas wells, smaller entry holes and lower shot densities can cause turbulence, which severely impairs inflow performance. Ordinarily, a choice must be made between sufficient penetration to perforate as far beyond the wellbore as possible, and an entry hole diameter adequate for subsequent production. By using downhole gas generators to increase the length of the perforation tunnel, it becomes possible to solve both production problems. |
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Case History: Extension of Inadequate Perforations |
| Depth: | 3,136-ft. (956m) |
| Interval: | 7-ft. (2.2m), Perforated 4 SPF (13 spm) 120° phasing |
| Formation: | Medium-grained Sandstone |
| Initial Production: | Nil after perforating, offset production was adequate |
| After Stimulation: | 32 BOPD (5m³) |
Over the productive life of a well, perforations eventually become plugged. This causes a decline in production, until it reaches an uneconomic level. Previous remedies often involved re-perforating. Additional perforating, followed by downhole gas-generation is an improvement upon this procedure. |
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Case History: Opening Plugged Perforations |
| Depth: | 3,995-ft. (1,218m) |
| Interval: | 7-ft. (2.2m), Perforated 4 SPF (13 spm) 120° phasing |
| Formation: | Carbonaceous Sandstone |
| Initial Production: | 94 BOPD (15m³), declined to 3 BOPD (0.5m³) |
| After Stimulation: | 63 BOPD (10m³) |
| Dispersal of Scale, Migrated Fines and Precipitates |
Migration of fines and the deposition of scale or precipitates can cause severe impairment to flow in the immediate wellbore region, and this damage can extend far beyond the maximum penetration depth of the most advanced-performance perforators. Re-perforating the interval will only be partially successful, and the use of acids and other solvents can cause re-precipitation or even precipitation of entirely new compounds if the formation components are not thoroughly understood. A typical problem encountered in carbonates is the solution of precipitation of anhydrite. During an acid treatment it might be dissolved near the wellbore, then re-precipitated deeper in the formation as acid is spent on highly soluble dolomite and calcite. Other carbonates are high in siderite or ankerite (iron carbonates) or pyrites (iron sulfide). These minerals are dissolved easily in HC 1, but will precipitate as the acid reaches equilibrium with the formation. Diagenetic mineral reaction can be even more of an impairment to flow in sandstones, where precipitates formed during an acid treatment can bridge on the undissolved grain framework. Clay particles, such as kaolinite, illite, smectite and chlorite, can also present problems. Kaolinite is a hexagonal crystal, sometimes resembling "booklets", which usually adheres to the pore walls. High injection pressure during acidizing, or high drawdown during production can cause kaolinite to disperse and migrate, bridging in pore throats. Illite is fibrous clay, and can be broken off by high flow velocities or surges adjacent to the wellbore. Chlorite, commonly associated with deep, abnormally pressured gas reservoirs, contains significant quantities of iron, and is partially soluble in HC 1. However, when the iron is extracted, a silica residue is left behind. STRESSFRAC® gas generators, can disperse scales, precipitates and migrated fines near the wellbore. Furthermore, the treatment can also induce fractures that will penetrate beyond the damage zone, up to 20-ft. or 25-ft. Circumferential and vertical distribution of perforations are critical factors of this type of treatment. Another example of impairment by migrated fines is illustrated by the following example. Inadequate quality control of sand proppant resulted in "bridging" of fines due to poor sorting of grain sizes in a hydraulic fracture. Rather than a skin factor of -3, as expected, subsequent testing indicates a factor of +12: |
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Case History: Dispersal of Migrated Fines Depth: 7,545-ft. (2,300m) |
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| Flow Capacity: | 83.8 md-ft. | 131.6 md-ft. |
| Skin Factor: | (25.6 md-m) | (40.1 md-m) |
| Flow Efficiency: | 41% | 55% |
| Damage Ratio: | 2.43 | 1.82 |
| Stabilized Flow Rate: | 2.8 MMCFD | 4.3 MMCFD |
| (79,511m³/day) | (121,880m³ /day) |
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Remedial Treatments |
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The DGPL®/STRESSFRAC® systems can be successfully applied in certain types of remedial problems in producing oil, gas and condensate wells; gas storage injection, withdrawal and observer wells; and enhanced oil recovery projects in both injection and producing wells. In EOR injection wells, including limited entry steam injectors with critical profile, there may be no better solution that DGPL®/STRESSFRAC® for steam control, improvement of injection rates, and reduction of injection pressure. In less critical applications, the typical solutions include repeated perforations, matrix acid treatments, nitrogen or carbon dioxide flushes, or acid washes in the cased and open hole. DGPL®/STRESSFRAC® has the advantage of overcoming performance deterioration without affecting conformance, such as is experienced with hydraulic fracturing or acid fracturing. It works in either the cased or open hole. The condition of close oil/water contact is a challenge to the capabilities of conventional acidizing and hydraulic perf breakdown and stimulation. These methods rely on a prolonged application (minutes) of escalating pressure from surface pumps, which may result in progressive degradation of the cement sheath by worm-holing or bond deterioration. The fracture stimulation intended for the target formation may be deflected into the water zone below with predictable consequences. The classic remedy consists of squeeze cementing to re-establish the permeability barrier. Re-perforating and renewing attempts at perf breakdown and stimulation usually follows this. A striking example of this, and a tribute to the perseverance and optimism of a completions engineer, was a well drilled in the San Andres formation, where the above described procedure was carried out three times in succession with identical negative results. A DGPL®/STRESSFRAC® treatment was substituted on the fourth attempt, followed by a small acid treatment. This combination resulted in commercial production, stabilized at 30 barrels of oil a day with 5 barrels of water. The absence of channeling may be explained by the DGPL®/STRESSFRAC® loading rate, which initiates fractures into the pay zone in milliseconds. The high-pressure phase of the pulse subsides immediately, relieving the major stress on the cement sheath and on any natural permeability barriers, such as shale stringers, before failure can occur. The fracture extension phase in the formation proceeds at diminished pressure, and as such presents no threat to the zone isolation barriers. |
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Fluid Sensitive Formations |
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In fluid sensitive formations, particularly in gas zones, DGPL®/STRESSFRAC® is used successfully in an environment of methanol or liquefied compressed gases, such as carbon dioxide, etc. This combination is very effective as a primary stimulation in that heat released by the DGPL®/STRESSFRAC® combustion vaporizes some liquid gas, increasing the efficiency of the stimulation by further extending the resulting fractures. Examples are the Colony sand in Alberta. Four-ft. zones at 1,600-ft. deep, STRESSFRAC®ed with 4-ft. (1.2m) tools produced 500 MCF a day at 20% drawdown. Long zone wells either have cemented casing, un-cemented perforated liners, or are open hole. In high capacity wells, no initial stimulation is required, but it may become necessary during the life of the well. In tight reservoirs, initial stimulation is imperative and is usually attempted by hydraulic fracturing, which involves very large amounts of liquid and prop sand. The limited entry method with ball sealers is used to provide some control over the injection. In other cases, the presence of perforated loose liners, with or without gravel packs, usually precludes the use of hydraulic stimulation, as no effective controls are available, except for minor treatments where wash cups can be used. DGPL®/STRESSFRAC® is effective in these various well configurations, either on initial completion or workover. Zones in excess of 200-ft. (61m) are routinely stimulated in one run with tubing conveyed DGPL®/STRESSFRAC®. The energy level of the treatment is matched to the well geometry and formation. In the long zones with non-producing intervals, DGPL®/STRESSFRAC® will stimulate the separate target zones almost simultaneously. The tubing work string can be provided with a packer, which is set prior to hydraulic activation of DGPL®/STRESSFRAC®. With this plan, flowing wells can be put on line minutes after DGPL®/STRESSFRAC®, producing through the workstring. |
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Geothermal Well Stimulation |
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As with any well, drilling operations represent a large percentage of total costs. In a geothermal well, however, there is very little that can be done to repair near-wellbore damage at such excessive temperatures, except to drill a new well. DGPL®/STRESSFRAC® can induce fractures that exceed 20-ft. (6m) in length, more than adequate distance to penetrate the damaged area. At the Geysers, production wells are known to be drilled as close as 10-ft. (3m) from non-productive wells. This is well within the range of STRESSFRAC® downhole fracture generation, which may allow an entirely new engineering program to be developed. |
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Horizontal Well Stimulation |
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The category of horizontal wells includes strongly deviated wells, such as offshore and wells penetrating massive homogeneous or fractured formations. The offshore candidates usually have complex tubing programs with several diameter restrictions, gas lift valves, etc. Since the cost of pulling the packer and tubing are high, STRESSFRAC® tools are run through the production tubing subject to its diameter limitations. Coiled tubing and electrical ignitors are used. The length of tools is limited by the height of the lubricator. Surveys are conducted to determine the contributions of every individual zone prior to treatment, and STRESSFRAC® stimulation scheduled accordingly. With certain exceptions, the wells are usually left flowing during STRESSFRAC® operations, and the production gains from each stimulation can be gauged immediately. Wells in Alaska and the North Sea have responded to STRESSFRAC® with increases of 50-200 bbl/day from 12-ft. (3.6m) zones. The on shore well configurations are usually less complex, with large casing cemented to the kick-off point, and a more or less abrupt radius to the horizontal section in the open hole. With the horizontal section of the well extending 2,000-ft. (610m) or more into the hydrocarbon reservoir, these wells are expected to replace 10 or more vertical wells and to produce accordingly. While open hole and slotted liner completions are numerous, it is generally held that cemented casing should be used if stimulation of the horizontal section is considered. Lacking this isolation method, conventional stimulations in open hole and slotted liners have not been successful. STRESSFRAC® / DGPL® has been used successfully in large numbers of conventional open hole and slotted liner wells, and recently in long horizontal well bores to 12,000-ft. (3,358m) measured depth. Tubing conveyed STRESSFRAC® of lengths in excess of 150-ft. (46m) per run have been used to stimulate horizontal open hole sections in the Ellenburger formation. A similar stimulation was applied to a slotted liner well in the Bakken shale. |
For technical assistance please contact:
| Gary Loman | (805) 967-3578 | Santa Barbara, California |
| Doug Hartle | (403) 270-0787 | Calgary, Alberta |
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