Wells – Processes – Placing fluid into the formation
Reexamination Certificate
2002-01-29
2004-03-09
Bagnell, David (Department: 3672)
Wells
Processes
Placing fluid into the formation
C166S300000, C166S312000, C507S267000, C507S260000
Reexamination Certificate
active
06702023
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 including drilling mud 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 may arise as a result of drilling, production, injection workover or other oilfield operations.
Damage can be near wellbore, for example the presence of drilling mud or fracturing fluid filter cake, or damage may be present deeper into the formation, for example mineral scale deposited 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 prevent effective placement of reactive acid deep into carbonate formations or throughout long horizontal wellbores resulting in poor zonal coverage. These acids are also 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 of conventional acids. 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.
If acids can be delivered sufficiently well into the formation, acidising may also be effective in stimulating undamaged formations by increasing the permeability of the rock matrix around the wellbore. For example, increasing the permeability of a zone of 3 to 5 m radius around a vertical wellbore by 3-4 fold is expected to increase the rate of fluids production (or injection rate) by about 20% in a situation where there is no near wellbore damage. The efficient delivery of acids into fractures such as induced fractures or natural fractures can also increase the conductivity of the fractures allowing higher rates of fluid production or injection.
Acids may also be used to break acid sensitive gels such as cross linked guar-borate gels used in hydraulic fracturing and other oilfield applications. Efficient breaking of gels is generally required to obtain maximum production after such treatments.
One approach which can improve zonal coverage has been the use of solutions of carboxylic acid esters which hydrolyse at high temperatures to produce a carboxylic acid (U.S. Pat. No. 3,630,285). Preferably, the formation temperature for this process is greater than about 150° C. Because the acid is produced predominantly after placement of the fluid excellent zonal coverage can be achieved. The preferred esters used in U.S. Pat. No. 3,630,285 were ethyl acetate and methyl formate. These compounds have the disadvantage of low flash points and have other health and safety drawbacks such as some degree of toxicity.
It is an object of the present invention to provide simple and effective methods for increasing the rate of hydrolysis of esters to allow ester based acidising of underground formations to be carried out within a reasonable time scale. It is a further object of the present invention to provide simple and effective methods for acidising underground formations in combination with one or more polymer breakers.
It is also an object of the present invention to provide a method by which increased rates of ester hydrolysis can be obtained in heavy brines.
The present invention further provides a method by which controlled acidising, with or without suitable polymer breakers, may be used for the treatment of filter cakes following gravel packing operations, by incorporation of suitable components into the gravel packing fluid.
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 non-enzyme catalyst capable of increasing the rate of hydrolysis of the ester, such that the ester hydrolyses to produce an organic acid to dissolve acid soluble material present within the reservoir.
The reservoir may be a hydrocarbon reservoir, for instance a gas or oil reservoir. Alternatively the reservoir may be a water reservoir. When it is a hydrocarbon reservoir the method of the invention may further include recovering a hydrocarbon from the treated reservoir. Likewise, when it is a water reservoir the method of the invention may further include recovering water from the treated reservoir. Typically the reservoir is, or includes, a carbonate rock structure.
Preferred esters for incorporation into treatment fluids to be used in the present invention are carboxylic acid esters, preferably those with low toxicity, high flash point and high environmental acceptability. Esters of ethanoic and methanoic acid (acetic and formic acid) are particularly suitable. The calcium and magnesium salts of these acids have good solubility in water.
The ester should be at least slightly water soluble. Preferably the ester should be soluble to at least 1% w/v in water and most preferably soluble to at least 5% in water. The solubility of some esters may be less in high salt concentration fluids, such as a heavy brine. In such cases an ester which is completely soluble in the base fluid at the desired concentration will normally be selected.
The acid generated in situ according to the method of the present invention is an organic acid, generally an aliphatic carboxylic acid. Preferably the acid is of formula RCO
2
H wherein R is hydrogen, an alkyl group having from 1 to 6 carbon atoms or—R′—CO
2
H where R′ is a bond or an alkylene group having from 1 to 6 carbon atoms, the said alkyl or alkylene group being unsubstituted or substituted by halogen or hydroxy. Where the acid has a hydroxy substituent the ester may, for instance, be a cyclic ester such as a lactone.
The alcohol portion of the ester may be monohydric or polyhydric as long as the esters are sufficiently water soluble at formation temperatures. Partial esters of the polyhydric alcohols can be used in which case the unesterified hydroxyl groups serve to increase the water solubility of the ester.
Suitable examples of the ester include 1,2,3-propanetriol triacetate, 1,2,3-propanetriol diacetate, ethylene glycol diacetate, diethylene glycol diacetate and triethylene glycol diacetate. Most preferably the esters are acetic esters of 1,2,3-propanetriol (glycerol) and 1,2-ethanediol (ethylene glycol). The ester and also the alcohol which is produced when the ester hydrolyses can both act as mutual solvents. The presence of a mutual solvent is generally considered to be beneficial in treatments of hydrocarbon bearing formations, particularly when treating with water based treatment fluids.
The concentration of ester incorporated into the formulations 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.
The catalyst is any n
Harris Ralph Edmund
McKay Ian Donald
Bagnell David
Bomar T. Shane
Cleansorb Limited
Nixon & Vanderhye P.C.
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