Wells – Processes – Placing fluid into the formation
Reexamination Certificate
2001-10-31
2004-07-20
Kreck, John (Department: 3673)
Wells
Processes
Placing fluid into the formation
C166S300000, C507S201000, C507S267000
Reexamination Certificate
active
06763888
ABSTRACT:
The method of the present invention is generally applicable to the production of oil, gas or water from wells drilled into underground reservoirs.
During drilling, workover and production operations there are numerous situations where the production rate of an oil, gas or water well following these operations is limited due to the presence of formation damage. Types of damage include, but are not limited to, the presence of polymer-containing filter cakes, fluids (including hydraulic fracturing fluids) filtrates or residues including polysaccharide-containing filter cakes, fluids, filtrates or residues, particulate materials such as fluid loss control agents and rock fines, biofilms, scales and asphaltenes.
Damage can be near wellbore, for example the presence of filter cake, or damage may be present deeper into the formation, for example in natural or induced fractures or in the rock matrix.
The effective removal of damage, especially near wellbore damage such as filter cake, can significantly increase the production rate of hydrocarbon or water producing wells penetrating underground formations. The effective removal of damage can also increase the injectivity of injection wells.
Conventional acids have been used for many years to treat damage in underground formations and stimulate the rate of oil or gas production. However, conventional acids have several drawbacks. They react rapidly with acid soluble materials which can result in poor zonal coverage and they are hazardous in use. To improve zonal coverage the use of high pressure, high rate injection is often attempted, which increases the hazards associated with their use. Due to the high corrosivity of conventional acids the use of corrosion inhibitors is also generally required. There is a greater need for corrosion inhibitors in higher temperature formations and corrosion inhibitors are generally toxic.
Damage is often caused by a combination of several components. For example, certain drilling muds or drill-in fluids contain calcium carbonate or dolomite in combination with a polymer or polymers which provides suitable rheological properties. Both the carbonate and the polymer contribute to the formation of a filter cake. Carbonate fines generated during drilling of carbonate rocks may also be present in the filter cakes.
Filter cakes generally contain high densities of polysaccharide. U.S. Pat. No. 5,247,995 cites SPE Paper 21497 which states that they can contain up to about 48% polysaccharide versus about 4% in fracturing fluids.
Filter cakes which do not contain carbonate may be susceptible to treatment with acid if they are present on wellbore surfaces in carbonate formations, as acid may attack the underlying carbonate and cause disruption of the filter cake.
It is an object of the present invention to provide a simple and effective method for treating formation damage within an underground reservoir.
It is a particular object of the present invention to provide simple and effective methods for the effective removal of filter cakes over long horizontal intervals.
Another object of the present invention is to provide single stage treatment methods which can remove damage and provide further stimulation of production or injection rate by increasing the matrix permeability of adjacent undamaged regions of the formation.
It is a further object of the present invention to provide methods which are environmentally acceptable by utilising components which are of low environmental impact.
Accordingly, the present invention provides a method for treating an underground reservoir, which method comprises introducing into the reservoir a treatment fluid comprising, dissolved or dispersed in water, an ester and a polymer breaker, such that the ester hydrolyses to produce an organic acid to dissolve acid soluble material present within the reservoir and the polymer breaker degrades polymeric material present within the reservoir.
The reservoir may be a hydrocarbon, such as gas or oil, reservoir. Alternatively the reservoir may be a water reservoir.
Sufficient ester or esters is present in the treatment fluid to produce sufficient acid, when the ester is hydrolysed, to have a substantive effect on filter cakes or other types of damage. By substantive effect it is meant that sufficient acid is produced on the hydrolysis of the ester to give sufficient dissolution of acid soluble material, present in or adjacent to the filter cake or other damage, to assist in the removal of damage. For example the dissolution of carbonate present in a filter cake, the dissolution of carbonate rock adjacent to a filter cake or the dissolution of carbonate rock adjacent to a biofilm. To obtain sufficient dissolution, it is necessary that a minimum of several percent w/v of acid is produced. The concentration of ester incorporated into the treatment fluid of the present invention will typically be at least 1% w/v but may be up to 20% w/v or higher. In general it has been found that 5% to 10% w/v ester when used in combination with a suitable polymer breaker or breakers is sufficient to give good removal of damage caused by filter cake. Preferably 5% to 20% ester will be used.
It is also intended that sufficient polymer breaker is present in the treatment fluid to have a substantive effect on the polymeric material present. The concentration of polymer breaker incorporated into the treatment fluid will vary according to the type of breaker employed but will be of the order of 0.005 to 60 kg/m
3
, preferably 0.2 to 10 kg/m
3
.
In general it is desirable to use a concentration of breaker which results in the breaking of the polymer over a period of several hours to allow the effective placement of the fluid. For example, too rapid a degradation of a filter cakes or biofilm may lead to localised fluid leak off, adversely affecting placement of the treatment fluid. This is analogous to the situation experienced in treatments using conventional acids, where the fast reaction rate can result in rapid breakthrough and wormholing and uneven fluid leak off. This can prevent the even placement of fluid over long horizontal intervals or into fractures or the rock matrix. It is a feature of the treatment fluids of the present invention that use of an ester rather than a reactive acid avoids wormholing and improves the placement of the fluid. In order to maintain this advantage in certain applications such as the treatment of long horizontal intervals, too rapid a degradation of polymers in for example filter cakes or biofilms should in general be avoided. Ideally, breakthrough of filter cakes or biofilms will be achieved after a period longer than that amount of time needed to place the treatment fluid. A delay in producing a substantive amount of acid and in breaking polymer allows even treatment of the target zone and excellent zonal coverage.
Use of an ester gives the advantages of effective placement of the fluid and avoidance of the use of corrosion inhibitors. Where suitable esters are selected, in particular where low toxicity, high flash point esters are used, there are also health and safety and environmental advantages. The initially neutral or slightly acidic pH of the fluid permits the incorporation of polymer breakers such as enzymes and oxidative breakers into the fluid without the compatibility problems encountered when such breakers are incorporated into highly acidic formulations based on mineral or organic acids.
It is preferable to use polymer breakers which are activated by, or whose activity is enhanced by, the change in conditions as the ester hydrolyses to produce acid. For example, peroxides which are activated by the development of acidic conditions or enzymes which have their activity enhanced by the development of acidic conditions.
An example of the former is the decomposition of calcium peroxide . This decomposes under acidic conditions to generate hydrogen peroxide.
Enzymes useful as polysaccharide breakers generally have a pH optimum in the range ph 3 to ph 7. Such enzymes will therefore have their activity enhanced by the gen
Harris Ralph Edmund
McKay Ian Donald
Cleansorb Limited
Kreck John
Nixon & Vanderhye P.C.
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