Abstract:
A method and a composition for stimulating oil production from wells which have ceased production by pumping in a mixture of gluconic, oxalic and citric acids, isopropanol, ethylene glycol and sodium citrate between pre-flush and over-flush intervals, which composition and method does not include the use of hydrochloric acid or hydrofluoric acid.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to a method and a composition to effectively stimulate oil production from a subterranean hydrocarbon-bearing rock formation penetrated by a wellbore and, more particularly, to a composition and method to acidize a hydrocarbon-bearing sandstone formation to stimulate production without the use of either hydrochloric or hydrofluoric acids. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    It is well known in petroleum production that the use of acid treatments to stimulate oil flow from a subterranean sandstone reservoir is a comparatively difficult and complex task when compared to the relative ease of stimulating oil flow from a carbonate reservoir. While the treatment of a carbonate reservoir with hydrochloric acid and/or hydrofluoric acid readily forms water-soluble products, such as calcium chloride in water, while sandstone acidizing with “mud acid” (HCl/HF) generates several reaction products which may precipitate and cause formation damage. 
         [0003]    Formation damage occurring in oil wells in sandstone-based fields results from a combination of naturally occurring factors due to reservoir fluids, as well as factors due to operations, such as drilling, workovers, previous acid stimulation treatments or during well production. The resulting damage can include fines migration, scale formation, deposition of paraffins, asphaltenes, or a combination of both organic and inorganic deposition. In addition, the drilling fluid can also be a source of formation damage when it invades a sandstone reservoir. Still other causes of formation damage may result from plugging due to particles within the injected fluid, wetability alteration, and the dispersion or the swelling of clays, emulsions, sludges, bacterial products and water-block. 
         [0004]    An acid wash or matrix acidizing is typically used to remove the “skin effect” near the well bore and is introduced at a pressure below the formation&#39;s fracture pressure. Furthermore, the use of conventional acid systems, such as HCl and HF acids, causes the production of sand since it dissolves the cementitious portion beneath the quartz and silica particles. 
         [0005]    It is an object of the present invention to provide a composition and method to enhance the permeability of a hydrocarbon-bearing sandstone formation to increase oil production therefrom. 
         [0006]    It is another object of the present invention to remove formation damage in off-shore sandstone reservoirs by removing calcium carbonate particles, iron precipitates and dispersed or swollen clays from the vicinity of the wellbore. 
         [0007]    It is still another object of the present invention to provide a method for introducing a stimulation fluid into a sandstone formation effectively and at a low cost. 
       SUMMARY OF THE INVENTION 
       [0008]    A stimulation fluid has been found which avoids the need for using hydrochloric or hydrofluoric acids, and which has been used successfully to stimulate oil production in sandstone formations, from what had been considered to be dead wells or low production wells. The newly discovered stimulation fluid and its use both serves to improve permeability by removing formation damage due to calcium carbonate chips that are used as a “killing fluid” in drilling and work-over operations and to improve the oil production rate from a sandstone reservoir. 
         [0009]    The composition of the present invention also has the capability of retaining iron and calcium in solution, when compared with prior art stimulating compositions that contain conventional strong acids, such as hydrochloric and hydrofluoric acids, which cause the iron and calcium to precipitate. 
         [0010]    The composition of the present invention, which includes both organic and inorganic chemicals, is a mixture of gluconic acid, oxalic acid, citric acid, isopropanol, ethylene glycol and sodium citrate. 
         [0011]    The stimulative mixture of the present invention has been successfully used to enhance permeability and increase oil production when delivered to the wellbore of a sandstone formation in accordance with the method of the present invention. The method is a single stage process involving three intervals or steps. 
         [0012]    In the first interval or step, a solvent pre-flush consisting of a mutual solvent and water is pumped into the wellbore. 
         [0013]    In the second interval or step, the composition of the present invention is pumped into the wellbore and acts to dissolve and remove precipitates in the wellbore region and thus reestablish permeability and improve production. 
         [0014]    In the third interval or step, a solvent overflush comprising a mutual solvent and diesel fuel is pumped into the wellbore to displace the composition of the present invention and the resulting reaction products. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    As a result of experimentation it has been determined that to achieve optimum results, the quantities of the chemicals in the composition of the present invention are preferably employed within certain ranges and that the composition must be introduced in accordance with a method that includes the specific sequence of intervals or steps described above generally, and in more detail below. 
         [0016]    The chemicals and the quantities that are to be employed in accordance with a preferred embodiment of the present invention are as follows:
       Gluconic Acid: 10-50%, by weight;   Oxalic Acid: 0-0.5%, by weight;   Citric Acid: 10-50%, by weight;   Isopropanol: 100-600 ppm, by weight;   Ethylene Glycol: 10-100 ppm, by weight; and   Sodium Citrate: 100-600 ppm, by weight.       
 
         [0023]    The foregoing composition has been found to provide a highly effective stimulation fluid while simultaneously avoiding damage which would compromise the integrity of a subterranean sandstone formation. 
         [0024]    The method of the invention involves carrying out the stimulation in a single-stage using the “bull-heading” technique with three pumping intervals, namely, a pre-flush interval or step, a main chemical introduction step or interval, and an over-flush interval or step, while avoiding any lengthy soaking time between each of these three (3) pumping intervals. This is in contrast to the well-known three-stage treatment methods which are employed conventionally with soaking time after each of the stages when acidizing carbonate formations with HCL, HF, or both. The single-stage treatment in accordance with the present invention results in effectively stimulating the sandstone reservoir to increase its permeability and to revive so-called dead wells by lowering the treatment cost and significantly shortening the required treatment times. 
       EXAMPLE 1 
       [0025]    The single stage method of the present invention includes three separate intervals which are: 
         [0026]    Interval 1—Solvent Preflush 
         [0027]    The main objective of the solvent preflush interval (the “first interval”) is to clean the formation by dissolving/dispersing heavy hydrocarbons and breaking emulsions in the wellbore region. An ancillary purpose is to water-wet the formation using a mutual solvent surfactant. This step is employed to enhance the contact of the composition of the present invention, which is pumped in the second interval or step, with carbonate and iron precipitates that are present near the wellbore area. 
         [0028]    In the pre-flush interval, water and 1-5 vol % of a mutual solvent, such as ethylene glycol is pumped into the wellbore, preferably using the bull-heading technique, and allowed to soak for about 0.5 hrs. The bull-heading technique is a known and field-proven placement method during chemical treatment. It is also considered to be cost effective when compared to the coil tubing technique. The utilization of bull-heading often results in improving the chemical placement with no mechanical isolation required to treat zones. 
         [0029]    The mutual solvent serves to remove any organic residue and better enables the stimulating composition to react with the damaging minerals. Other mutual solvents which can be used to advantage are oxyalkylated alcohols and aromatic alcohols. 
         [0030]    Interval 2—Treatment Interval Using the Composition of the Present Invention 
         [0031]    The main objective of this treatment interval (the “second interval”) is the use of the composition of the present invention to dissolve and remove calcium and iron precipitates in the wellbore region. It also has the properties to dissolve filter cake, starch and clays and to remove downhole bacteria. 
         [0032]    The volume of the composition of the present invention which is pumped into the wellbore can range from 1-10 gal/ft (open-hole completion) and 10-20 gal/ft of perforations (if a perforated casing completion type is used). 
         [0033]    Interval 3—Over-Flush 
         [0034]    The objective of this interval (the “third interval”) is to provide an over-flush fluid to displace the chemical composition of the second interval from the vicinity of the wellbore. It consists of pumping in diesel fuel and 1-5% of a mutual solvent such as ethylene glycol. Other mutual solvents which can be utilized are, for example, oxyalkylated alcohols and aromatic alcohols. 
         [0035]    It thus can be see that the objects set forth are achieved and since certain changes may be made in carrying out the above process and in the composition without departing from the scope of the invention as defined by the claims that follow, it is intended that all matter in the foregoing description shall be interpreted as illustrative and not as limiting.