A stimulation method used in Russia has been successfully implemented in three US sedimentary basins.

Caseless, charge-powdered pressure generators (in Russian - poroxovie generatorie davlenea beskorpusnie, or PGDBK) have been effectively, safely and economically applied in more than 30,000 wells in the Eastern Hemisphere to increase productivity. The pulsating pressure generated upon combustion of caseless, cylindrical powdered charges overcomes formation rock tensile strength to create multiple fractures, with horizontal and vertical orientation, that extend more than 100 ft (31 m) from the wellbore.
Through a combined effect of mechanical, thermal and chemical actions, the use of PGDBK increases pressure communication between the extended reservoir and the wellbore. The pressure pulsating action of generated noncombustible gases erodes fracture walls, keeping the flow channels open for a long period of time without propping agents. Heat generated through the combustion process cleans the wellbore region of heavy hydrocarbons, reduces oil viscosity and enhances relative permeability of oil to water. Additional oil recovery is achieved by the effect of combustion products lowering the surface tension. Skin is significantly reduced, and permeability in vicinity of the wellbore is increased.
The in situ process ends with a negative pressure gradient toward the wellbore, washing impediments from flow channels. The well is immediately swabbed or circulated, equipment is reinstalled, and the well is brought back on line.
A better way to fracture
The decline in a well's productivity may be attributed largely to obstruction of flow channels by formation fines, heavy-end hydrocarbons and contaminants from drilling and production practices. If no remedial action is taken to clear the flow channels, larger pressure drawdown will be required to sustain production, causing movement of more formation fines and exacerbating the wellbore damage.
Conventionally, fractures are created by hydraulic fracturing or acid fracturing techniques, which are usually expensive and create only single fractures along least stress planes. The PGDBK powdered gas pressure generators create desired multiple fractures around the wellbore. Other advantages of this technology are the simplicity of its application, high mobility, reduced downtime and minimal cost.
Background
PGDBK well stimulation technology was developed for the USSR Ministry of Geology by the former Soviet Union Military Research and Production Facility, Federal Research & Production Center (ALTAI). The science was developed over 14 years with documented results from 6,500 wells, concluding in 1986. The gas generators are used in Russia, Ukraine, Uzbekistan, Kazakhstan, Turkmenistan, Vietnam, India and China.
Results have been remarkable. In a single oil field, the use of the PGDBK gas generators resulted in an incremental production of 200 million bbl of oil. The PGDBK treatment of wells in Russia's Tyumen oil basin resulted in an incremental production of 2.4 billion bbl during a 5-year period.
Previous work
There have been attempts to develop other technologies based on stimulating wells by burning charges in wells to generate pressures. Such products typically are solid-propellant gas generators conveyed by wireline into wells. These vary in their design and the effect on formation rock. These devices typically are encased in metallic or polyvinylchloride tubes. Duration of the pressure created is brief - on the order of a few milliseconds. By combining different types of powdered charges and amounts of charges, the generated pressure pulse may be varied. However, the opportunities to adjust the pressure duration are limited. The number of fractures created typically does not exceed 10, and length ranges from 16 ft to 23 ft (5 m to 7 m). Increasing the amount of propellant charges could lead to a sharp increase in the peak explosion pressure and could damage the well or have an undesirable, irreversible effect.
The phenomenon
The combustion process of PGDBK powdered generators creates high temperature and pressure. The pressure exhibits an attenuating sinusoidal action, and the duration of this phenomenon is significantly longer than the time required to burn the charges (Figure 1).
The rate of pressure increase during the combustion process is an order of magnitude slower than that of an explosive reaction, and the maximum pressure attained upon combustion is significantly less. Pressure increase in the zone of combustion is accompanied by the propagation of compression waves through the surrounding fluid at close to the speed of sound. The combustion process is thus not accompanied by shock waves, and the generated impulse does not crush the surrounding region. The pressure generated by the burning charges also can be regulated in a wide range by changing the forms and types of charges. Fractures are held open partly due to rock deformation exceeding the elastic limit (irreversible deformation) and partly due to the erosion of fracture walls by the pulsating action of the combustion gases.
In order to create stable vertical fractures, the following condition is necessary but may not be adequate:
where E1 and E2 are Young's modulus of formation under compression and tension, respectively.
The high-pressure impulse quickly overloads any existing fractures around the wellbore and continuously creates new fractures until the generated pressure declines. Typically, the pressure generated will exceed the fracture initiation pressure. Note that the fractures are not filled with large volumes of well fluid during the fracturing process.
The heat from combustion melts paraffin, asphaltenes and resins, and reduces viscosity of oil in the near-wellbore region. Temperatures of the gaseous combustion products can reach 1,400°F (760°C) (Figure 2). The physico-chemical action of the combustion products (CO, CO2, N2 and HCl) reduces viscosity and surface tension at the oil-water interface and also partially dissolves carbonate rock and some cementation in the wellbore region. At the end of the combustion process, the combustion products are washed from the reservoir, cleaning the fractures and perforations of solid sediments, heavy-end precipitates of hydrocarbons and products of chemical reaction. The use of PGDBK technology is not recommended for poorly consolidated or unconsolidated sandstone formations.
The number of perforations and the shot density are important parameters for an effective well treatment. The pressure acting on the formation rock is less than pressure created in wellbore by the combustion products. The differential pressure is the pressure loss due to liquid movement through perforations. Figure 3 shows a relationship between fluid penetration coefficient ao and the number of perforations. Several perforations are desirable for effective treatments (typically 6 to 8 shots/ft). For a treated interval the fluid penetration coefficient increases sharply with the number of perforations (up to about 40 to 60 shots in Figure 3). Beyond 60 shots, the gradient of this relationship declines. Therefore 40 to 60 perforations may be considered optimal.
The process
Different types of PGDBK powdered generators are used for well treatments. Depending on geological and well conditions, a combination of several generators may be used to achieve the desired effect. The set of generators used for treatment is made up of a bottom primary charge and a number of other charges braided with a wireline and a support tube. The cylindrical generator typically has a length of 3 ft (1 m) and may be run through tubing or inside casing.
To control pressure during the combustion process, a well is filled with tamp fluid, such as lease oil, formation water, KCl or CaCl2 solution, depending on the formation to be treated. The tamp fluid provides hydrostatic head to ensure that pressure generated is directed into the formation to overcome the overburden pressure and create the desired fractures.
The generator assembly is lifted using a wireline truck and manually directed through the wellhead. The generator then is lowered to allow connection of the electrical conductors from the generator assembly to the wireline truck. A pair of mechanical pressure gauges is tied between the generator assembly and the point of electrical connection. The generators then are run to a predetermined interval in the well.
In the summer of 2000, Geotec Thermal Generators and ALTAI introduced the PGDBK treatment in 20 oil and gas wells owned and operated by four companies in the Powder River Basin, Anadarko Basin and the Austin Chalk Trend.
The treated interval depths ranged from 2,450 ft to 11,000 ft (747 m to 3,355 m). The formation types included sandstone, carbonate, dolomite and chert.
Case study: Powder River Basin
Five wells in the Springen Ranch field and three wells in the East Sandbar field were treated. Emerald Restoration and Production Co. operates these fields. The wells had been shut in for 2 to 15 years prior to the PGDBK treatment, primarily because they had reached their economic limit.
A conventional fracture design for the Lower Cretaceous Muddy formation in the Powder River Basin includes pumping 120,000 lb of 20/40 mesh sand in a 40-lb cross-linked borate gel fluid at a cost of US $100,000 to $130,000 per well. Estimated payout time typically is 4 to 10 months.
The PGDBK treatment has an instant payback, producing additional revenues at a minimal cost of about $2,000 to $4,000.
Discovered in 1969, the Springen Ranch field had an initial reservoir pressure of 3,600 psi to 4,000 psi. Estimated reservoir pressure prior to PGDBK treatment was 600 psi to 1,000 psi. Solution gas drive (primary production) and supplementary water flooding had depleted the field.
The Lower Cretaceous Muddy members are sandstones to shaly sandstones with porosity of 12% to 25% and permeability of 1 to 100 mD. Almost all the wells were treated twice in the 5½-in. production casing over a 6-hour period after cleaning out the well with a bailer and tamping with lease oil or KCl solution.
Five of the eight wells treated in the Powder River Basin are on production. The wells tested between 200 b/d to 960 b/d of fluid. The other three wells have not been tested or brought on line because of unavailability of equipment and services. The wells have sustained production rates of 12 b/d to 30 b/d of oil since they were treated with the PGDBK stimulation technology. This is an increase of 300% to 1,000%, compared to each well's production rates prior to last shut-in. More than 5,000 bbl of incremental oil had been produced from these five wells in less than 3 months. The wells are still on production as a result of the effective treatment.
Recommendations
If wells have been shut in for prolonged periods of time, it is recommended that the wells be circulated or otherwise thoroughly cleaned to remove silt, scales and other sediments in the wellbore prior to PGDBK treatment.
The PGDBK pressure surge "cleans" a lot of junk into the wellbore that needs to be removed by circulating the well, using a bailer or swabbing the well after the treatment.
Russian case studies
PGDBK technology is widely used for stimulating wells in Russia. In 1988, more than 1,500 wells were treated, and in 1990, more than 2,000 wells were treated. Some of the exploratory wells treated have porosity in the range of 2% to 20% and permeability ranging from 0.1 to 100 mD. Western Siberian oil fields typically have porosity of 18% to 28% and permeability of 1.3 D, and treatments there boosted production by two to 16 times with improved production lasting beyond 6 months.
Recent PGDBK treatments have achieved incremental production for Glavtumenneftegas of 2,750 bbl, for Nishnevartorsckneftegas 7,075 bbl, and for Streshevoineft 1,965 bbl. For water-injection wells, Glavtumenneftegas increased injection by about 188,700 bbl, and Streshevoineft increased injection by 125,800 bbl.
The Samotlorskoye oil field is the world's second largest after the Ghawar field in Saudi Arabia. Discovered in 1965, Samotlorskoye has remaining reserves of 9 billion bbl. Initial estimated oil-in-place and initial reserves were 49 billion bbl and 25 billion bbl, respectively.
The results on this field show an average sevenfold increase in production after PGDBK treatment. The average incremental oil production for successfully treated wells was 18,900 bbl, and one well had a production increase of 162,000 bbl.
Well stimulation with the PGDBK gas generators has been confirmed as an effective and inexpensive technology to enhance oil and gas well productivity. Production rates in treated wells have increased by 300% to 2,500% over rates prior to treatment. Tight formations with high reservoir pressures are some of the best candidates for treatment.
Acknowledgment
This paper was prepared for presentation at the Petroleum Technology Transfer Council Symposium on "Developments in Well Stimulation and Slim-Hole Technology" in Lafayette, La., Dec. 5, 2000.