Abstract:
Methods including the pumping of fracturing fluid carrying proppant. Simultaneously particulate is pumped made of substance that under subterranean temperature releases hydrochloric or etching acid precursor, wherein the said acid precursor reacts with the formation water and produces acid. The methods stimulate the inflow of formation fluid towards the well due to cleaning of the surface of hydrofracture and due to growth of its area.

Description:
[0001]    This application claims foreign priority benefits to Russian Patent Application No. 2007105188, filed on Feb. 13, 2007. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    This invention relates to the oil and gas industry, and in particular, to methods of oil and gas production, and can be applied to improve hydrocarbon recovery from a fractured subterranean reservoir due to fracture clean up. 
         [0004]    The method of hydraulic fracturing of oil-bearing formation is an efficient method for stimulation of oil/gas production from a well. The goal of hydraulic fracturing is to pump a fluid under the pressure and rate sufficient for cracking the formation of the reservoir; this creates two fractures on opposite sides of wellbore traveling in opposite directions. These large-scale fractures are required as conduits for draining of hydrocarbon fluids into the borehole; these conduits have a higher fluid conductivity than the formation itself. To prevent the fracture closing when the fluid pumping has ended, propping agents are delivered with the fluid into the fractures. This proppant particulate is carried into the formation by fracturing fluid with a required certain density and viscosity. The preferable variant of fracturing fluid is viscous solution of viscoelastic polymers (guar or hydroxypropylcellulose). 
         [0005]    The disadvantage of traditional fracturing methods is damaging the fracture with polymers and products of their decomposition. The residue of undamaged polymer stays in pores and considerably reduces fracture permeability. Research data shows that 45 to 75% of polymer remains in the fracture after an initial flowback period. To counteract this damage breakers are used to reduce gel viscosity and help remove concentrated polymer residues. 
         [0006]    Some methods are known where low-molecular oxidizers (persulfates/peroxides of metals or ammonium) or organic peroxides are applied as gel breaker. These oxidizers and peroxides are effective up to 120° C. Sometimes these breakers are poorly compatible with the fracturing fluid or with resin coated proppant. 
         [0007]    Some methods describe treatment of near-wellbore zone with hydrochloric acid pumped. This method is not adapted for removal of gel and filter cake damage from propped fractures. 
         [0008]    Other methods a method for acid treatment of the near-wellbore zone. A mixture of polyvinylchloride and ammonium bifluoride is thermally decomposed (due to in-situ ignition or impact of formation temperature) and produces acid. The resulting mixture of acids is used to break apart the colloidal sediments of ferrous oxide. This method has not been adapted for removal of gel and filter cake damage from propped fractures because is only used to treat the near-wellbore zone. 
         [0009]    Also known is a method of using acid to dissolve filter cake via a proppant with coating made of organic acid precursor (e.g., polylactic acid). This method may also be applied to aid with gel cleanup in gravel packs. The solid acid precursor can make up to 10% by weight of the total proppant mass. This invention is the most similar to the disclosed invention. 
         [0010]    There are many known methods for production improvement after stimulation, by removal of concentrated gel and filter cake from a propped fracture. Removal of gel damage is achieved by using of polymers that are capable to produce organic or nonorganic acids under subterranean conditions. 
       DESCRIPTION OF THE INVENTION 
       [0011]    At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0012]    The description and examples are presented solely for the purpose of illustrating the preferred embodiments of the invention and should not be construed as a limitation to the scope and applicability of the invention. While the compositions of the present invention are described herein as comprising certain materials, it should be understood that the composition could optionally comprise two or more chemically different materials. In addition, the composition can also comprise some components other than the ones already cited. In the summary of the invention and this detailed description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary of the invention and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration within the range, including the end points, is to be considered as having been stated. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors possession of the entire range and all points within the range. 
         [0013]    The current invention describes a method for cleaning up a propped fracture by aiding the breakdown and removal of gel and gel residue from the fracture. 
         [0014]    The technical benefit of this method is the improved fluid mobility into and inside the propped fracture and therefore enhancement of hydrocarbon production from the fracture. 
         [0015]    The said technical result is achieved by the following means: the fracturing fluid carries proppant and particulate into the propped fracture; the said particulate under formation temperature releases a substance that reacts with the formation fluid and produces hydrochloric or etching acid. The preferable embodiment of this patent uses the particles of polyvinyl chloride or polyvinylden chloride or their copolymers comprising monomers of vinyl chloride or vinylden chloride, as well as their chlorinate analogs. Another variant is the use of material granulated and encapsulated into oil-dissolving coating; the said material is ammonium chloride or fluoride, ammonium difluoride, pyridine fluoride, and fluoride-bearing polymers, e.g., fluoride of polyvinylpyridine. The usual size of polymer particulate varies from 0.1 microns to 10 mm. The preferable amount of vinyl chloride is from 0.1% to 99.9% wt., and the content of chloride in the polymer is from 0.01% to 85% wt. The proportion of polymeric particulate as a percentage of proppant mass varies from 0.1% to 99.9%. 
         [0016]    The method is based on using the substances which release hydrochloride under conditions of the formation temperature and in a water-oil medium; the produced hydrogen chloride destroys the polymer gel and dissolves the gel residue, typically filter cake. The disclosed method is based on using a new material for this technique (preferably, polyvinylchloride or co-polymers). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0017]      FIG. 1  shows the diagram that illustrate a loss of polymer mass over the time of the thermal treatment. 
       
    
    
       [0018]    These new substances enable efficient cleanup of polymers typically used in hydraulic fracturing fluid by removal of concentrated gel and gel residues, such as filter cake. The advantages of using a polymer based on vinylchloride monomer in contrast to other methods of destruction the polymer gel in propped fractures are the following: 
         [0019]    1. It is possible to reduce the concentration of costly gel breakers or abandon their use completely. 
         [0020]    2. The yield of hydrogen chloride from the said polymeric material is a long-term process (tens of days). A long time interval facilitates more uniform distribution of produced hydrochloric acid over the fracture volume and ensures more complete breaking of polymer gel. 
         [0021]    3. The particulate of polymer based on vinylchloride monomer releases hydrogen chloride at elevated temperatures (130 . . . 200° C.), where most commercially available peroxide breakers lose effectiveness (e.g., peroxide and persulfate of metal or ammonium). 
         [0022]    4. Unlike common peroxide breakers, the polymer particles with vinylchloride monomers do not react with resin coated proppant (RCP) or reduce the strength of the proppant packing that can lead to a reduction of fracture width. 
         [0023]    5. Unlike peroxide-type (persulfate-type) gel breakers, the polymer particles with vinylchloride monomers are non-reactive with the fracturing fluid during the fracturing process and fracture closure; the particulate does not affect the rheology of fracturing fluids or solids transport properties. 
         [0024]    6. Hydrogen chloride released from particulates with vinylchloride monomers can dissolve a carbonate rock and create microchannels in the formation. This creates a divaricated system of drainage and improves the hydrocarbon flow towards the wellbore. 
         [0025]    7. Hydrogen chloride released from polymer particulates can dissolve the filter cake formed during filtration on gel by the rock matrix. 
         [0026]    If other types of substances meet the conditions formulated in the independent claim of the invention formula, the advantages must be the same. 
         [0027]    A new method of conducting of hydraulic fracturing is disclosed in the following claims; according to this method, proppant is delivered to the fracture with a mixture polymer particles (polyvinylchloride or copolymers of vinylchloride), wherein the main proppant and polymeric particulate can be mixed before the job or on-the-fly and then delivered to the subterranean formation. Under the conditions of high subterranean temperature the polymeric material produces hydrogen chloride that destroys the network of intermolecular cross-links in the polymer gel; the said cross-linked network is formed due to intermolecular bonds between hydroxyl groups of polymer gel and ions of multivalent metals (the cross-linking agents). The produced hydrogen chloride breaks the gel and improves the water solubility of polymer components suspended in the fracturing fluid; this reduces the viscosity of fracturing solution. In general, the said factors facilitate a more complete cleanup of polymer gel from the fracture and improve the fracture permeability. In addition, the method ensures dissolving of the filer cake and formation of micro channels in the formation; the later creates a divaricated system of drainage and facilitates the flow of hydrocarbons towards the wellbore. 
         [0028]    The molar concentration of vinylchloride monomer units in a copolymer varies from about 0.1% to about 99.9%. 
         [0029]    The copolymer may include plasticizers, thermostability agents, and organic and inorganic compounds. 
         [0030]    Other organic or inorganic compounds can be used for the same purpose if they produce hydrogen chloride or hydrogen fluoride under formation conditions; the released hydrogen chloride or hydrogen fluoride combine in water to form hydrochloric acid or etching acid. 
         [0031]    In the method disclosed, the particles of polymeric material can be employed during the entire operation of hydrofracturing or at the final stages. 
       EXAMPLES 
       [0032]    The application feasibility of the method disclosed was proven by example using a polymer with vinylchloride monomers placed under conditions imitating the conditions of a producing oil well. 
         [0033]    The release of hydrogen chloride by polyvinylchloride was proven by the following experiment. A sample of polyvinylchloride was stored for several days at a high temperature (110° C.) in crude oil. The polyvinylchloride sample initially had a glass transition temperature of 56° C. and crystalline degree equal to 12%. There was no plasticizer in the composition. 
         [0034]    The evolution of sample weight during 1-32 days (showed on the abscissa axis) is plotted in the  FIG. 1  (the sample mass is on the left ordinate axis). It is apparent in this diagram that over the time of the thermal treatment there is loss of polymer mass showed on the left ordinate axis. The data of elementary analysis (carbon, hydrogen, chlorine) for initial sample and current state of degraded sample demonstrated that the mass loss of the polyvinylchloride correlates with the production of hydrogen chloride, showed on the right ordinate axis in gram 
         [0035]    For example, a sample of polyvinylchloride with the initial mass of 0.100 kg was stored for the period of 19 days at temperature of 110° C. and produced 0.029 kg of hydrogen chloride, which is equivalent to 0.193 kg of hydrochloric acid with concentration 15%. This quantity of acid is enough to dissolve of 0.042 kg calcite rock, a typical component of carbonate formations. 
         [0036]    Advantages of the disclosed method in comparison to known at the art are the following: 
         [0037]    1. It is possible to reduce the concentration or abandon completely the costly gel breakers. 
         [0038]    2. The yield of hydrogen chloride from the said polymeric material is a long-run process (tens of days). A long time interval facilitates more uniform distribution of produced hydrochloric acid over the fracture volume and ensures more complete breaking of polymer gel. 
         [0039]    3. The particulate of polymer based on vinylchloride monomer effectively releases hydrogen chloride at elevated temperature of formation (130 . . . 200° C.), when most of commercially available peroxide gel breakers (e.g., peroxide and persulfate of metal or ammonium) fail. 
         [0040]    4. Unlike common peroxide-type gel breakers, the polymer particles with vinylchloride monomers does not react with components of resin coated proppant and therefore should not damage the proppant pack strength which can lead to a reduction of fracture width. 
         [0041]    5. Unlike peroxide-type (persulfate-type) gel breakers, the polymer particles with vinylchloride monomers do not react with fracturing fluids during any stage of the fracturing process or during fracture closure; the particulate will not affect the rheology of fracturing fluid or proppant transport properties. 
         [0042]    6. Hydrogen chloride or hydrogen fluoride released from particulates can dissolve a carbonate rock and create micro channels in the formation. This creates a divaricated system of drainage and facilitates the flow of hydrocarbons towards the wellbore. 
         [0043]    7. Hydrogen chloride or hydrogen fluoride released from polymer particulates is capable of filter cake removal because these particles will be trapped inside the fracture, near the fracture surface.