Earth boring – well treating – and oil field chemistry – Well treating – Contains organic component
Reexamination Certificate
1999-11-01
2001-02-13
Tucker, Philip (Department: 1712)
Earth boring, well treating, and oil field chemistry
Well treating
Contains organic component
C507S269000, C507S902000, C507S921000, C507S922000, C166S308400
Reexamination Certificate
active
06187720
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to the treatment of subterranean formations using gelled hydrocarbons. In one aspect, it relates to the use of breaker chemicals in gelled oil-base fracturing fluids. In another aspect, the invention relates to the use of granules containing breaker chemicals in fracturing operations.
DESCRIPTION OF THE PRIOR ART
Hydraulic fracturing has been widely used as a means for improving the rates at which fluids can be injected into or withdrawn from subterranean formations surrounding oil wells and similar boreholes. The methods employed normally involve the injection of a viscous fracturing fluid having a low fluid loss value into the well at a rate sufficient to generate a fracture in the exposed formation, the introduction of fluid containing suspended propping agent particles into the resultant fracture, and the subsequent shutting in of the well until the formation has closed on the injected particles. This results in the formation of high conductivity channels through which fluids can be injected or produced. The conductivity in the propped fracture is a function of the fracture dimensions and the permeability of the bed of propping agent particles within the fracture.
In order to generate the fracture of sufficient length, height, and width, and also to carry the propping agent particles into the fracture, it is necessary for the fluid to have relatively high viscosity. This requires the use of gelling agents in the fracturing fluid.
Fracturing fluids are either water-based liquids (gelled with water-soluble polymers) or oil-based liquids (gelled with in-situ formed reversible networks). The water-based liquids cannot be used in the fracturing of many water sensitive formations. Fracturing in these formations requires the oil-based liquids.
As with all fracturing fluids, the gel or viscosity of the fracturing fluid must be retained for sufficient time to generate the fracture and place the proppant therein, but must eventually be “broken” to permit the well to cleanup. The fracturing fluid accordingly will generally incorporate a “delayed breaker” which after a predetermined time or event, activates the breaker to degrade the polymer or gelling a gent. Water-based “breakers” are described in SPE paper 18862. Breakers for oil-based systems are more varied because of the differences in the hydrocarbon viscosifiers.
The breaker used in accordance with the present invention is for pH sensitive oil-based gelling agents. By far the most common gelling agents, which exhibit pH sensitivity, are the phosphate gelling agents. These well-known and commercially available gelling agents for oil-based systems are alkyl phosphate esters.
Breaker chemicals useful in oil-based systems include sodium bicarbonate (U.S. Pat. No. 4,877,894), calcium hydroxide, and magnesium hydroxide, and similar bases.
The breaker chemicals may be encapsulated and activated by a variety of mechanisms including crushing by formation closure (U.S. Pat No. 4,506,734), and dissolution by formation fluids (U.S. Pat. No. 4,741,401 and U.S. Pat. No. 5,110,486, U.S. Pat. No. 4,919,209 and U.S. Pat No. 5,649,596).
U.S. Pat. No. 3,163,219 discloses water insoluble agents such as rosin asphalts and waxes used as binding agents for sulfur and calcium sulfate gel breakers for water-based systems.
SUMMARY OF THE INVENTION
The method of the present invention involves the use of wax granules containing a particulate gel breaker which are introduced into an oil-based fracturing fluid and function as delayed breakers in the fracturing operation. The granules exhibit a delayed release of the active chemical (gel breaker) so the degradation or the breaking down of the gelled structure occurs well after the fracturing fluid has been pumped into the formation. Moreover, the breakers display the desired activity within reasonable time periods so that long shut-in times are not required.
The preferred gelling agents for oil-based liquids are the metal salts of alkyl or orthophosphate esters. These gelling agents are effective viscosifiers in a wide range of oil types and are most effective when neutralized (i.e. no excess presence of base or acid). In the presence of excess base or acid, the gelled structure is destroyed and the viscosity of the fracturing fluid is reduced. Acids and bases thus can be used to break the phosphate ester gelling agents. The present invention relies on this mechanism for breaking gels, and employs granules containing the base or acid.
The oil gelled with phosphate ester gelling agents are effective in the absence of excess base or acid. This requires the apparent pH of the liquid to be in the neutral range. In adding the gelling agent to the oil, the pH need not be measured because sufficient gelling of the oil inherently requires the absence of excess base or acid.
The addition of the base or acid provides the apparent pH change necessary to break the gelled structure.
REFERENCES:
patent: 4770796 (1988-09-01), Jacobs
patent: 4919209 (1990-04-01), King
patent: 5110486 (1992-05-01), Manalastas et al.
patent: 5164099 (1992-11-01), Gupta et al.
patent: 5649596 (1997-07-01), Jones et al.
patent: 5948735 (1999-09-01), Newlove et al.
Acker David B.
Malekahmadi Fati
Graham R. L.
Tucker Philip
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