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Introduction |
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A pressure pulse, created by the use of downhole gas generators, can be effective as a stimulation technique to improve the transmissibility of oil and gas-bearing formations. The principle is relatively simple: increase the pressure in the wellbore until it reaches a level sufficient to "split" or fracture the formation rock. The fractures become highly conductive flow channels, which allow free movement of hydrocarbons to the wellbore. The pressure used for fracturing is provided by the rapid combustion (burning) of propellant in the wellbore, which generates large quantities of CO2 gas. When this pressure exceeds the in-situ stresses, the rock fails in tension, and the high velocity gasses erode the face of the subsequent fractures. The operation may be conducted in both open and cased wellbores. The technique is known by several generic names, such as Dynamic Gas Pulse Loading® (Servo-Dynamics), High Energy Gas Frac (Sandia National Laboratories), and Controlled Pulse Fracturing (Mobil Research and Development Corporation). To the casual observer, the technique may seem to resemble other methods of creating fractures in rock, such as hydraulic fracturing or explosive stimulation. However, closer examination of the techniques indicates that Dynamic Gas Pulse Loading® represents an entirely new regime. |
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In Situ Stress |
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Every part of an earth formation is subject to stresses that are caused by the cumulative weight of material above (overburden) and tectonic forces. Although it is considered impossible to ascertain the direction and magnitude of all the individual stress components, it has been shown mathematically that any system of stresses for a given point, regardless of number of direction, can always be described by three "normal" stresses which are called the "principal" stresses. These principal stresses are mutually perpendicular and can be approximated graphically by envisioning the geometric form of a cube, with the stresses directed toward opposing faces. Over the extended period of geologic time, the movement of the earth's various components has resulted in repeated deformation of the rocks, as evidenced by faulting and folding. This indicates that the general condition beneath the surface is one in which the three mutually perpendicular principal stresses are unequal. The porous sedimentary rocks associated with oil and gas production are usually saturated with fluid under pressure, which leads to a combined fluid/solid stress field. The components of this stress field are (1) the hydrostatic pressure, which acts upon both the fluids and solids in the formation, and (2) tectonic stress which only affects the solid constituent. The main components, then, are pore pressure and matrix stress. |
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Hydraulic Fracturing |
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In hydraulic fracturing, fluid pressure is applied to the formation and progressively increased. As the pore pressure increases, the matrix stress is reduced equally in all three principal stress directions. When the least principal stress reaches a value of zero, the rock will be placed in tension toward that direction. Pressure increase above this level will exceed the tensile strength of the rock, and it will split along a plane approximately perpendicular to least principal stress. The fluids used to exert sufficient pressure for hydraulic fracturing of a formation are also used to transport a solid agent, such as sand, which remains within the fracture to prop it open after pressure is relieved. This provides a highly conductive flow channel from the formation to the wellbore. |
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Explosive Stimulation (Well Shooting) |
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Stimulation of hydrocarbon-bearing formations with explosives, such as liquid or solidified nitroglycerin, has been conducted since the 1800s. The intended objective of well shooting is to fracture or "rubbilize" the rock to alter permeability near the wellbore, thus increasing production. When the explosive is detonated, a brief, extremely high and compressive pressure pulse, or shock wave is generated, which far exceeds the tensile strength of the formation rock. The high pressures of the detonation cause the rock to yield and compact. After the stress wave passes, the rock unloads
elastically, leaving an enlarged, deformed wellbore, a zone of compacted rock and a region of greater compressive stress. This
crushed region contains a substantial quantity of fines, which reduce permeability and productivity around the wellbore. The resultant shattering causes sloughing and enlargement of the wellbore, and usually requires subsequent clean-up operations. Naturally, this stimulation treatment is restricted to the open hole, because the shock wave could damage well casing and production hardware. |
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Dynamic Gas Pulse Loading® |
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Dynamic Gas Pulse Loading® (DGPL®) is a technique for inducing multiple, radial fractures in a wellbore, using rapid gas-pressure loading of the rock during the deflagration (burning) of downhole gas generators. By controlling the energy release during this process, the in situ stresses can be exceeded by a substantial amount while pressures remain significantly below the level which deforms and crushes the rock. Explosives and propellants have similar energy contents, but different ingredients. Both release energy as they "burn", and the type of energy released is directly related to the rate at which they chemically convert into gas. By necessity, the burn rate characteristics of Servo-Dynamics' DGPL®/STRESSFRAC® are significantly more tolerant to extreme well temperatures, hydrostatic pressure and exposure to hostile well fluids than solid rocket fuel. Test observation at Lawrence Livermore Laboratories, and mathematical stimulators for stimulation technology used by Sandia National Laboratories indicate the following requirements: |
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For inducing multiple, radial fractures, the rate at which the borehole is pressurized is the controlling factor. Peak pressure is important, but secondary. The pressure-loading rate is an inverse function of borehole diameter. Continued or repeated gas-pressurization of the induced fractures is necessary for increasing fracture length. an increase in wellbore peak gas pressure will not increase the length of the longest induced fractures, unless an adequate gas volume is available in the wellbore. For a given wellbore diameter, there is a range or loading rates that will produce the same number of induced fractures. |
| The time, tmax during which the gas pressure rises from ambient to its peak, pmax, is called the "rise time", is used to derive the loading rate of the borehole (pmax/tmax). The loading rate is a function of formation characteristics, wellbore configuration, chemical properties of the propellant, degree of confinement (hydrostatic pressure), geometrical distribution of perforations and cross sectional area open in casing. Pressure-loading rates from 1 to 150 psi/usec have been observed to generate multiple fractures, but a prcise definition of the effective range for pulse duration has not yet been derived. |
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Controlling the Number and Orientation of Induced Fractures |
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In many of the producing horizons of the world, in situ natural fracture netwroks control well productivity. Published technical literature indicates that these pre-existing fractures are ordinarily vertical and widely spaced relative to the well diameter. They also tend to be oriented in one or only a few preferential directions, and are only occasionally interconnected. The natural fractures oriented to the direction of the least principal tectonic stress will probably have the largest apertures, thus providing that major conduits for transmission of oil and gas. Optimal productivity in these formations is dependent upon establishing effective flow channels between the most conductive existing fractures and the wellbore. Hydraulically induced fractures usually propagate in a direction perpendicular to the least principal tectonic stress. This is not often favourable for intersection of a major, conductive natural fracture system. Dynamic Gas Pulse Loading®, on the other hand, can be used to induce multiple cross cutting fractures, increasing the likelihood of intersection. Published literature has described the following method of evaluation for naturally fractured reservoirs: |
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"Pre-notching" the wellbore can control the gas generator-induced fracture direction. This could be used in an open hole to direct the induced fracture
so that it could optimize the number of natural fractures that are intercepted. However, since multiple fractures are created by gas generation in open holes, substantial flow channels can be created in heavily fractured intervals without notching. In cased wellbores, the circumferential distribution of perforations determines radial coverage of the interval. The perforations are used as "chokes" to transmit the pressurized gas to the formation. Selection of the "optimal" angular phasing will be limited by other treatment design considerations, such as wellbore configuration, casing diameter, weight and grade, formation characteristics, downhole hardware and the amount of containment (fluid head) that will be available during the subsequent treatment. |
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Summary |
| The chief differences among the various methods of inducing fractures in a producing formation are related to the rate of wellbore pressurization during the event. Of the three methods discussed, the most universally useful is Dynamic Gas Pulse Loading®, due to its ability to induce fractures in specific horizons without the need for mechanical zone isolation, and at peak pressures compatible with the well casing and other completion components. |
For technical assistance please contact:
| Gary Loman | (805) 967-3578 | Santa Barbara, California |
| Doug Hartle | (403) 270-0787 | Calgary, Alberta |
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