OIL WELL STIMULATION BY ENERGY FRACTURING METHOD (ENERGYFRAC)

Prof. Vladislav G. Slutsky, Prof. Sergei A. Tsyganov, Ph.D. Eugeny S. Severin
Semenov Institute of Chemical Physics, Russian Academy of Sciences
4 Kosygin Street, 119991 Moscow, Russia
Tel.: +7(495)939-7257, Fax: +7(495)938-5906
E-mail: slutsky@list.ru

Energyfrac is a new method of High Energy Gas Fracturing (HEGF) for well stimulation.

Version 1 (Single-Stage Energyfrac). This version is fully developed and protected by Russian Patents nos. 2154733 and 2178073.

This version is designed for stimulating wells with high formation pressures (naturally flowing and gas-lift wells). In this version, the formation is fractured by the high-pressure gas produced by burning a liquid fuel–oxidizer solution (FOS) pumped into the well. FOS is an aqueous solution of an organic fuel and a mineral oxidizer. FOS does not react at atmospheric pressure, which makes it fire/explosion safe. The combustion of the FOS pumped into the well is initiated by a promoter designed to make a self-igniting binary system with the FOS. The promoter is enclosed in containers and lowered down the hole on a logging cable. The FOS combustion begins when the promoter containers are broken by means of a detonating cord. The high efficiency of this version (increase in oil production by 100% and higher) has been demonstrated for wells with high formation pressures (naturally flowing and gas-lift wells). The fractures created by treating wells of this kind remain open.

Version 2 (Two-Stage Energyfrac with acid treatment). This version is under development.

This version is designed for stimulating wells with low formation pressures (pumping wells) in carbonate reservoirs. It has been found that fractures in wells with low formation pressure (pumping wells) close unless subjected to additional treatment. Carbonate reservoirs can be effectively treated by acidizing. The acid dissolves carbonates and serves to enlarge fractures so that they remain open after the formation has reached a new steady state. The stimulation treatment is performed in two stages. At the first stage, the formation is fractured as in Version 1. At the second stage, the created fractures are treated with an acid.

Version 3 (Two-Stage Energyfrac with proppant).This version is under development.

This version is designed for stimulating wells with low formation pressures (pumping wells) in sandstone reservoirs. Such wells are stimulated in two stages. At the first stage, the formation is fractured as in Version 1. At the second stage, a proppant gel is pumped into the created fractures to hold them open.

Energyfrac is a new method of High Energy Gas Fracturing (HEGF) for well stimulation.

The most effective method for enhancing oil production, Hydrofrac, makes use of hydraulic fracturing of the oil-bearing formation under the high pressure of fluids pumped into the reservoir to be treated. The fractures created in the formation provide passageways for additional oil influx into the well. To hold the fractures open, the treatment fluid is mixed with proppant, such as ceramic grains of certain size. To keep the proppant suspended, the fluid is gelled. Treatment costs amount to several hundred thousand US dollars because hydraulic fracturing equipment is very expensive. In less expensive technologies, such as High Energy Gas Fracturing (HEGF), the formation is fractured under the high pressure of the gas produced by propellant combustion in the pay zone. The companies using HEGF systems include Servo-Dynamics, Inc.; J Integral Engineering, Inc.; Owen Oil Tools, Inc.; and Geotec Thermal Generators, Inc. The available HEGF technologies have the following common disadvantages: (1) the propellant charges are fire and explosion hazardous; (2) fracture closure can occur, which may lead to a substantially lower efficiency of HEGF treatment as compared to hydraulic fracturing.

In what follows, a description is given of a new HEGF-type method that is free of the disadvantages of the available HEGF technologies. The efficiency of treatment by this method is comparable to that of hydraulic treatment while treatment costs are substantially lower. The method is called Energy Fracturing (Energyfrac).

Version 1 (Single-Stage Energyfrac) is designed for stimulating wells with high formation pressures (naturally flowing and gas-lift wells). This version is fully developed and protected by Russian Patents nos. 2154733 and 2178073.

In this version, propellant charges are replaced by a fuel–oxidizer solution (FOS, aqueous solution of an organic fuel and a mineral oxidizer), which presents no fire/explosion hazard since it burns only at pressures above 5 MPa after it is pumped into a well. The combustion of a FOS is initiated by bringing it in contact with a promoter designed to make a self-igniting binary system. The promoter is enclosed in containers and lowered down the hole on a logging cable. The FOS combustion begins when the promoter containers are broken by means of a detonating cord.

The high efficiency of this version of Energyfrac (increase in oil production by 100% and higher) has been demonstrated for wells with high formation pressures (naturally flowing and gas-lift wells).

As applied to pumping wells (with low formation pressure), the currently available Energyfrac technology is much less efficient: the post-treatment increase in oil production does not exceed 15%. The low efficiency of Energyfrac treatment of such wells is explained by fracture closure during well operation.

Version 2 (Two-Stage Energyfrac with acid treatment) is designed for stimulating wells with low formation pressures (pumping wells) in carbonate reservoirs. This version is under development.

When pumps are used, the fluid level in the well is substantially lowered. This leads to a large difference in fluid pressure (differential pressure) between the formation and the wellbore, which causes the newly created fractures to close as the formation approaches a new steady state.

To improve the efficiency of Energyfrac as applied to pumping wells, an additional treatment that prevents fracture closure is required. Carbonate reservoirs can be effectively treated by acidizing. The acid dissolves carbonates and serves to enlarge fractures so that they remain open after the formation has reached a new steady state. The stimulation treatment is performed in two stages. At the first stage, the formation is fractured as in Version 1. At the second stage, the created fractures are treated with an acid.

Version 3 (Two-Stage Energyfrac with proppant) is designed for stimulating wells with low formation pressures (pumping wells) in sandstone reservoirs. This version is under development.

To improve the efficiency of Energyfrac as applied to pumping wells in sandstone reservoirs, fractures must be held open by means of a proppant. Such wells are stimulated in two stages. At the first stage, the formation is fractured as in Version 1. At the second stage, a proppant gel is pumped into the created fractures. Since the gel is pumped into already created fractures under a relatively low pressure (several MPa), this can be done by means of standard equipment available in most oil-producing or oil service companies. Thus, the cost of a proppant fracture treatment is significantly reduced as compared to the cost of a typical gel-proppant hydraulic fracturing treatment, because the latter makes use of special expensive high-pressure pumping equipment (tens of MPa) required to create fractures.

OIL WELL STIMULATION BY ENERGY FRACTURING METHOD (ENERGYFRAC)

Well treatment by Version 1 results in formation fracturing. To ensure an enhanced oil influx, the created fractures must be held open during subsequent well operation. The fractures can be closed by forces associated with the flow of oil to fractures. These forces are proportional to the differential pressure ΔP (difference between the formation pressure and the bottomhole fluid pressure). The closing of the fractures is impeded by friction forces proportional to the rock pressure, which increases with the vertical well depth H. The ratio between the fracture-closing and friction forces is proportional to the normalized differential pressure ΔP/H. The newly created fractures persist when ΔP/H is low and tend to close as ΔP/H exceeds a certain threshold value (ΔP/H)thr.

The figure shows the ratio of post-treatment to pre-treatment oil production rates, Q/Qo, for several wells. It is demonstrated here that Version 1 increases production rate by a factor of 2 to 2.5 when the normalized differential pressure is relatively low. The post-treatment production rate rapidly decreases as ΔP/H exceeds a threshold value of 2.5–3 MPa/km and almost drops to the pre-treatment rate at ΔP/H above 4.5–5.0 MPa/km. This means that newly created fractures persist at ΔP/H < (ΔP/H)thr = 2.5–3 MPa/km and close at ΔP/H > 2.5–3 MPa/km.

Thus, Version 1 (Single-Stage Energyfrac) is effective as applied to wells operating at normalized differential pressures below 2.5–3 MPa/km. Most wells of this kind are naturally flowing or gas-lift ones. In these wells, the fluid level is close to the surface, which is possible only when the formation pressure is high. Naturally flowing or gas-lift wells are best suited for effective stimulation by Version 1. Stimulation results for wells treated by Version 1 are listed in the table.

Well no. Oilfield Rock	Vertical depth H m	Differential pressure ΔP MPa	ΔP/H MPa /km	Daily oil production barrels/day		Increase in daily oil production %	Water cut vol %
Well no. Oilfield Rock	Vertical depth H m	Differential pressure ΔP MPa	ΔP/H MPa /km	before	*after)**	Increase in daily oil production %	before	*after)**
1321 Enoruskinskoe carbonate	1185	3.1	2.6	5.7	29	410	8	8
343 Serginskoe sandstone	2221	6.6	3.0	6.3	25	300	68	24
122 Gubkinskoe sandstone	2732	2.0	0.7	50	110	120	--	--

2. Wells best suited for stimulation by Version 2 (Two-Stage Energyfrac with acid treatment) and Version 3 (Two-Stage Energyfrac with proppant)

Oil production in wells with low formation pressure requires the use of pumps. When pumps are used, the fluid level in the well is substantially lowered and the normalized differential pressure is generally higher than 3 MPa/km. In wells of this kind, fractures will close unless subjected to an additional treatment that prevents fracture closure. Carbonate reservoirs are effectively treated by acidizing. Pumping wells in carbonate reservoirs are best suited for stimulation by Version 2. In sandstone reservoirs, fractures are effectively held open by propping. Pumping wells in sandstone reservoirs should be stimulated by Version 3.

Version 1. Single-Stage Energyfrac
Best suited for naturally flowing and gas-lift wells

Version 2. Two-Stage Energyfrac with acid treatment
Best suited for pumping wells in carbonate reservoirs

Stage 1. Fracturing is performed as in Version 1.
Stage 2. Acid is pumped into the created fractures.

Version 3. Two-Stage Energyfrac with proppant
Best suited for pumping wells in sandstone reservoirs

Stage 1. Fracturing is performed as in Version 1.
Stage 2. Proppant gel is pumped into the created fractures.

OIL WELL STIMULATION BY ENERGY FRACTURING METHOD (ENERGYFRAC)

This version is designed for stimulating wells with high formation pressures (naturally flowing and gas-lift wells). The version is fully developed. It has demonstrated a high efficiency when tested on wells in Russia. The patent rights to this version are owned by V.G. Slutsky, S.A. Tsyganov, and E.S. Severin. The Patent Owners are willing to negotiate the transfer of their rights to this technology version with an interested party (Organization).

Possible scenario for transfer of rights
The Patent Owners perform demonstration treatments of several wells under a contract between the Patent Owners and the Organization. The wells are chosen by the Organization in coordination with the Patent Owners. The Patent Owners cover both chemical procurement costs and travel expenses. The services provided by workover and geophysical crews are paid for by the Organization. If the ensuing increase in average production rate exceeds a stipulated percentage of the pre-treatment production rate, then the rights to the technology are transferred under the terms and conditions of the contract.

Version 2 (Two-Stage Energyfrac with acid treatment)

This version is designed for stimulating wells with low formation pressures (pumping wells) in carbonate reservoirs. This version is currently being developed. The Developers are the Patent Owners for Version 1.

The stimulation technology consists of fracturing performed as in Version 1 and subsequent acid treatment of the created fractures. The Developers are willing to cooperate on completing the development of the version.

Possible scenario for cooperation
The Developers perform a series of tests of Version 2 under a contract between the Developers and an interested party (Organization). The Organization covers chemical procurement costs and travel expenses of the Developers, as well as services provided by workover and geophysical crews. If the ensuing increase in average production rate exceeds a stipulated percentage of the pre-treatment production rate, then the rights to the technology are transferred under the terms and conditions of the contract.

Version 3 (Two-Stage Energyfrac with proppant)

This version is designed for stimulating wells with low formation pressures (pumping wells) in sandstone reservoirs. This version is currently being developed. The Developers are the Patent Owners for Version 1.

The stimulation technology consists of fracturing performed as in Version 1 and subsequent pumping of a proppant gel into fractures. The Developers will be willing to cooperate on completing the development of Version 3 after the rights to Version 1 or 2 of the technology are transferred to an interested party.