Sand consolidation is a well known term applying to procedures routinely practiced in the commercial production of petroleum, whereby wells are treated in order to reduce a problem generally referred to as unconsolidated sand production. When wells are completed in petroleum-containing formations which also contain unconsolidated granular mineral material such as sand or gravel, production of fluids from the formation causes the flow of the particulate matter into the wellbore, which often leads to any of several difficult and expensive problems. Sometimes a well is said to "sand up", meaning the lower portion of the production well becomes filled with sand, after which further production of fluid from the formation becomes difficult or impossible. In other instances, sand production along with the fluid results in passage of granular mineral material into the pump and associated hardware of the producing well, which causes accelerated wear of the mechanical components of the producing oil well. Sustained production of sand sometimes forms a cavity in the formation which collapses and destroys the well. All of these problems are known to exist and many methods have been disclosed in the prior art and applied in oil fields in order to reduce or eliminate production of unconsolidated sand from a petroleum formation during the course of oil production.
The above-described problem and potential solutions to the problem have been the subject of extensive research by the petroleum industry in the hope of developing techniques which minimize or eliminate the movement of sand particles into the producing well and associated equipment during the course of producing fluids from the formation. One such general approach suggested in the prior art involves treating the porous, unconsolidated sand mass around the wellbore in order to cement the loose sand grains together, thereby forming a permeable consolidated sand mass which will allow production of fluids but which will restrain the movement of sand particles into the wellbore. The objective of such procedures is to create a permeable barrier or sieve adjacent to the perforations or other openings in the well casing which establish communication between the production formation and the production tubing, which restrains the flow of loose particulate mineral matter such as sand. Another approach involves removing a portion of the formation around the well and packing specially prepared resin-coated granular material into the formation around the wellbore which is subsequently caused to become cemented together.
It is a primary objective of any operable sand consolidation method to form a barrier around the wellbore which restrains the movement of sand particles into the well while offering little or no restriction to the flow of fluids, particularly oil, from the formation into the wellbore where it can be pumped to the surface of the earth.
Another very important quality of a satisfactory sand consolidation method is durability of the permeable barrier formed around the wellbore. Once the barrier is formed and the well is placed on production, there will be a substantial continuing flow of fluids through the flow channels within the permeable barrier, and it is important that the barrier last for a significant period of time, e.g. several months and preferably years, without excessive abrasive wear or other deterioration of the consolidation matrix which would allow the particulate matter to once again flow into the wellbore. This is a particularly difficult objective to accomplish in the instance of sand consolidation procedures applied to wells completed in formations subjected to steam flooding or other thermal recovery methods. The production of fluids in steam flooding operations involve higher temperatures and higher pH fluids than are normally encountered in ordinary primary production, and this greatly aggravates the stability problem of sand consolidation procedures.
It is also important that the material injected into the formation should be essentially unreactive during the period it is inside the wellbore, i.e. while it is being pumped down the well and positioned where it is desired adjacent to the perforations of the production casing. It is this desire to delay the consolidation reaction that has led to multi-step procedures in which first a catalyst is injected into the formation, after which the polymerizable resin-containing fluid is injected separately. While this reduces the propensity for the fluid to polymerize in the injection string, it does give rise to several problems which constitute inherent weaknesses in many prior art methods for accomplishing sand consolidation. First, each separate injection step increases the time and cost of the well treatment by which sand consolidation is accomplished. Second, when the only catalyst employed is injected into the formation in advance of the polymerizable fluid, uniform mixing of catalyst with all of the subsequently-injected polymerizable fluid is not achieved to the degree necessary to ensure optimum polymerization of the resin, and thus often fails to achieve maximum, uniform strength and durability of the consolidated mass.
Many materials have been utilized for consolidating sand in the formation adjacent to production of wellbores. One of the more successful agents utilized for this purpose are fluids comprising monomers or oligomers of furfuryl alcohol which can be polymerized in situ to form a solid matrix which binds the sand grains together, while at the same time offering superior resistance to high temperatures and to caustic substances which may be encountered in steam flood operations. One of the problems in utilizing furfuryl alcohol oligomers to polymerize in the formation for the purpose of consolidating sand grains is failing to achieve uniform catalysis of the polymerization. Many catalysts that are effective for polymerizing furfuryl alcohol resins cannot be admixed with the furfuryl alcohol to permit a single fluid containing both the resin and the catalyst to be injected into the formation, because the time of polymerization is so short or unpredictable that there is excessive danger that the resin will polymerize in the injection wellbore.
In U.S. Pat. No. 4,427,069 there is disclosed a procedure for consolidating sand in a formation adjacent to a wellbore using an oligomer of furfuryl alcohol, in which the catalyst used is a water soluble acidic salt, preferably zirconyl chloride, which is injected in an aqueous solution into the formation prior to injection of the resin-containing fluid. The salt absorbs on the sand grains, and sufficient acidic salt remains adsorbed on the sand grain during the subsequent resin fluid injection stage that adequate polymerization occurs. Although this has been very effective in difficult situations where sand consolidation procedures are utilized, particularly in connection with thermal flooding such as steam injection procedures, the procedure nevertheless requires a multi-fluid injection procedure which requires more time and is more expensive than is desired. Usually a preliminary sand cleaning step is required before injecting the aqueous-catalyst solution in order to remove the naturally-occurring oil film from the sand grains to ensure good catalyst adsorption on the sand. Also, although catalyst mixes with the subsequently injected polymer to a limited degree, usually sufficient to cause polymerization, it is believed that superior performance would result if the catalyst resin mixture can be made more homogenous prior to polymerization, in order to achieve a dense, strong, durable consolidation mass.
In U.S. Pat. No. 4,669,543 which issued June 2, 1987, there is described a method for consolidating sand using an acid curable resin and utilizing as a catalyst, the reaction product of an acid and an alkyl metal or ammonium molybdate. In that instance, the catalyst is incorporated in an aqueous carrier fluid which comprises the continuous phase of an emulsion in which the polymerizable resin is the dispersed or discontinuous phase. Thus this process requires that the emulsion be resolved or broken after it is located in the portion of the formation where the permeable consolidating mass is desired, which is difficult to achieve the degree of completion and accuracy of timing necessary to accomplish the desired strong durable consolidating matrix necessary for a long lasting sand consolidation process.
In our U.S. Pat. No. 4,482,072 for "SAND CONSOLIDATION METHODS" we disclosed a particularly effective method for consolidating sand utilizing a mixture of a polymerizable resin such as an oligomer of furfuryl alcohol and a diluent such as butyl acetate and an oil soluble, slightly water soluble acid catalyst such as orthonitrobenzoic acid is injected followed by injection of salt water to reestablish permeability.
In U.S. Pat. No. 4,903,770 for "SAND CONSOLIDATION" we disclosed a preferred process which is more easily removed after a period of use and which is quite inexpensive. The process employs a fluid comprising a polymerizable monomer such as furfuryl alcohol and as a diluent, a polar organic solvent such as methanol and a strong, non-volatile acid catalyst such as sulfuric acid, mixed with steam to form a multiphase or aerosol treating fluid, and injected into the formation to be consolidated. An ester such as ethyl or butyl acetate is incorporated in the fluid when the steam quality is less than 80 percent.
Both of the above methods have produced excellent results in many field applications including several viscous oil-containing formations being stimulated by steam flooding. In some applications, however, results are adversely affected by a slight shrinkage of the porous, polymerized mass which can cause cracks in the polymerized mass which causes leaks. There is a need for an inexpensive modification to the sand control treatment using monomers or oligomers of furfuryl alcohol to reduce or eliminate shrinkage of the polymerized polymer-sand mass.