Abstract:
A sand consolidating method is provided for use in a borehole within an unconsolidated or loosely consolidated oil or gas reservoir which is likely to introduce substantial amounts of sand into the borehole and cause caving. After perforating the borehole&#39;s casing at an interval of the formation where sand will be produced, an aqueous silicate solution is injected into said interval. Next, a spacer volume of a water-immiscible hydrocarbonaceous liquid is introduced into the interval. Thereafter, a water-miscible organic solvent containing an alkylpolysilicate and inorganic salt or chelated calcium is injected into the interval. A permeability retentive silicate cement is formed in the interval. Injection of the aqueous silicate and organic solvent is continued until the interval has been consolidated by the silicate cement to an extent sufficient to prevent sand migration and thereby prevent caving.

Description:
This application is a continuation-in-part of Ser. No. 07/622,586, now U.S. Pat. No. 5,088,555, which was filed on Dec. 3, 1990. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the consolidation of subterranean formations and, more particularly, to a method of introducing two consolidating fluids into a zone of an incompetent formation so as to form a silicate cement adjacent to a well penetrating the formation. The method of this invention is especially useful in promoting more uniform fluid injection patterns in a consolidated interval of the formation so as to tolerate high pH&#39;s and high temperatures when conducting a steam-flooding or fire-flooding enhanced oil recovery operation. 
     BACKGROUND OF THE INVENTION 
     It is well known in the art that wells in sandy, oil-bearing formations are frequently difficult to operate because the sand in the formation is poorly consolidated and tends to flow into the well with the oil. This &#34;sand production&#34; is a serious problem because the sand causes erosion and premature wearing out of the pumping equipment, and is a nuisance to remove from the oil at a later point in the production operation. 
     In some wells, particularly in the Saskatchewan area of Canada, oil with sand suspended therein must be pumped into large tanks for storage so that sand can settle out. Frequently, the oil can then only be removed from the upper half of the tank because the lower half of the tank is full of sand. This, too, must be removed at some time and pumped out. Moreover, fine sand is not always removed by this method and this causes substantial problems later in production operations which can lead to rejection of sand-bearing oil by the pipeline operator. 
     Also, removal of oil from tar sand formations is particularly challenging because high temperature steam with high pH is used. A suitable consolidating agent must withstand a similar harsh environment. In order to prevent caving around a wellbore and damage thereto, during the production of oil from a tar sand formation, it is often necessary to consolidate the formation. 
     Steam or fire stimulation recovery techniques ar used to increase production from viscous oil-bearing formations. In steam stimulation techniques, steam is used to heat a section of the formation adjacent to a wellbore so that production rates are increased through lowered oil viscosities. 
     In a typical conventional steam stimulation injection cycle, steam is injected into a desired section of a reservoir or formation. A shut-in or soak phase may follow, in which thermal energy diffuses through the formation. A production phase follows in which oil is produced until oil production rates decrease to an uneconomical amount. Subsequently, injection cycles are often used to increase recovery. During the production phase, sand flowing from a subsurface formation may leave therein a cavity which may result in caving of the formation and collapse of the casing. 
     Caving of the formation and collapsing of the casing is not peculiar to the production of oil from a reservoir by steam stimulation. It may also occur during a water-flooding, fire-flooding, or carbon dioxide stimulation oil recovery operation. 
     Therefore, what is needed is a method to consolidate a formation so as to prevent caving of an interval near the wellbore which interval requires stability to withstand high pH and high temperatures during a steam stimulation or thermal oil recovery process. Similarly, prevention of caving is also required during a water-flooding or carbon dioxide stimulation oil recovery operation. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a method for consolidating sand in an unconsolidated or loosely consolidated oil or hydrocarbonaceous fluid containing formation or reservoir. In the practice of this invention, an aqueous organoammonium silicate, alkali metal or ammonium silicate solution is injected into an interval of the formation where sand consolidation is desired. The aqueous silicate solution enters the interval through perforations made in a cased well penetrating the formation. By use of a mechanical packer, penetration of the fluid into the interval can be controlled. As the aqueous silicate enters the interval, it saturates said interval. 
     Thereafter, a spacer volume of a water-immiscible hydrocarbonaceous liquid is directed into the interval. Hydrocarbonaceous liquids for use herein comprise parafinnic and aromatic liquids. Paraffinic liquids are preferred. Preferred parafinnic liquids are selected from a member of the group consisting of mineral oils, naphthas, C 5  -C 40  alkanes and mixtures thereof. 
     After a desired spacer volume of hydrocarbonaceous liquid has been placed into the interval requiring sand consolidation, a water-miscible organic solvent containing an alkylpolysilicate and hydrated calcium chloride is next injected into the interval. Upon coming into contact with the organoammonium silicate, alkali metal or ammonium silicate solution which remains on the sand grains and between the sand grain contact points, alkylpolysilicate and hydrated calcium chloride react with the organoammonium silicate, alkali metal or ammonium silicate to form calcium silicate cement in the interval being treated. The calcium silicate cement which is formed is stable at high pH&#39;s and temperatures in excess of about 400° F. These steps can be repeated until the interval has been consolidated to the extent desired. 
     Once the treated interval has been consolidated to a desired strength, a water-flooding, carbon dioxide stimulation, steam-flooding, or fire-flooding enhanced oil recovery method can be used to product hydrocarbonaceous fluids to the surface. By controlling the concentration and rate of injection of the aqueous silicate and the organic solvent containing the alkylpolysilicate and calcium chloride which are injected into the interval being treated, the consolidation strength of the formation can be tailored as desired. 
     It is therefore an object of this invention to provide for an in-situ calcium silicate composition for consolidating an interval of a formation which composition is more natural to a formation&#39;s environment. 
     It is another object of this invention to provide for a composition which will ensure an even flow front and a homogeneous consolidation of an interval of a formation requiring treatment. 
     It is yet another object of this invention to consolidate an unconsolidated or loosely consolidated interval in a formation to prevent caving and damage to an adjacent wellbore. 
     It is still yet further object of this invention to provide for a a method to obtain a desired consolidation within an interval of a formation which can be reversed by treating the interval with a strong acid. 
     It is an even still yet further object of this invention to provide for a formation consolidation agent which is resistant to high temperatures and high pH&#39;s. 
     It is yet an even still further object of this invention to provide for a consolidation composition lacking a particulate matter therein which matter might prevent penetration of the composition in an area requiring consolidation, flow alteration, or pore size reduction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a schematic representation showing how the composition is injected into the formation so as to consolidate sand grains while maintaining the porosity of the formation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the practice of this invention, a shown in the drawing, an aqueous organoammonium silicate, alkali metal or ammonium silicate slug is injected into well 10 where it enters formation 12 via perforations 14. A method for perforating a wellbore is disclosed in U.S. Pat. No. 3,437,143 which issued to Cook on Apr. 8, 1969. This patent is hereby incorporated by reference herein. As the aqueous slug containing the organoammonium silicate, alkali metal or ammonium silicate proceeds through formation 12, it fills the pores in the formation. 
     As the aqueous organoammonium silicate, alkali metal or ammonium silicate solution proceeds through zone 12, it deposits a film of said aqueous silicate on sand grains therein. This aqueous silicate also fills intersitial spaces between the sand grains. A spacer volume of a water-immiscible hydrocarbonaceous liquid 16 is directed through zone 12 so as to remove excess aqueous silicate from the intersitial spaces while leaving sufficient aqueous silicate adhering filmwise to the sand grains. The hydrocarbonaceous liquid comprises paraffinic and aromatic hydrocarbons. 
     This spacer volume of water-immiscible hydrocarbonaceous liquid 16 is selected from a member of the group consisting of mineral oils naphthas, C 5  -C 40  alkanes and mixtures thereof. Hydrocarbonaceous liquid used as a spacer volume can be of an industrial grade. A spacer volume of hydrocarbonaceous liquid is used to remove excess aqueous silicate from between the sand grains while allowing a thin silicate film to remain on the surface to obtain a cementing reaction with a subsequently injected water-miscible organic solvent containing an alkylpolysilicate and hydrated calcium chloride. 
     Afterwards, a water-miscible organic solvent containing an alkylpolysilicate and hydrated calcium chloride mixture therein is injected into formation 12 where it forms in-situ a permeability retentive silicate cement which is stable to temperatures up to and in excess of about 500° F. Once the silicate cement has hardened and formation 12 has bee consolidated to the extent desired, by repeated applications if necessary, an EOR operation is initiated in formation 12. 
     The cementing reaction which takes place binds sand grains i the formation thereby forming a consolidated porous zone 22. Although the sand grains are consolidated, a cement is formed which results in a substantially high retention of the formation&#39;s permeability. 
     In order to increase the cement s consolidation strength, the concentration of the organoammonium silicate, alkali metal silicate or ammonium silicate contained in an aqueous slug or the alkylpolysilicate and hydrated calcium chloride contained in the organic solvent slug can be increased. Similarly, the flow rates of each of these slugs through the formation can be decreased to obtain better consolidation strength. A decreased flow rate is particularly beneficial for increasing the consolidation strength when the alkylpolysilicate and hydrated calcium chloride slug&#39;s flow rate is decreased. As will be understood by those skilled in the art, optimal concentrations and flow rates are formation dependent. Therefore, optimal concentrations and flow rates should be geared to field conditions and requirements. 
     Injection of aqueous organoammonium silicate, alkali metal or ammonium silicate slug and organic solvent slug 18 containing the alkylpolysilicate and hydrated calcium chloride can be continued until the formation has been consolidated to a strength sufficient to prevent caving and damage to the wellbore. As will be understood by those skilled in the art, the amount of components utilized is formation dependent and may vary from formation to formation. Core samples obtained from the interval to be treated can be tested to determine the required pore size and amount of cement needed. U.S. Pat. No. 4,549,608 which issued to Stowe et al. teaches a method of sand control where clay particles are stabilized along a face of a fracture. This patent is incorporated by reference herein. 
     After an interval of the formation has been consolidated, that interval or another adjacent to the wellbore can be perforated and an enhanced oil recovery method conducted therein. Steam-flooding processes which can be utilized when enhancing this sand consolidation process described herein are detailed in U.S. Pat. Nos. 4,489,783 and 3,918,521 which issued to Shu and Snavely, respectively. U.S. Pat. No. 4,479,894 that issued to Chen et al. describes a water-flooding process which may be used herein. Fire-flooding processes which can be utilized herein are disclosed in U.S. Pat. Nos. 4,440,227 and 4,669,542 which issued to Holmes and Venkatesan, respectively. These patents are hereby incorporated by reference herein. 
     A carbon dioxide EOR process which can be used after consolidating the higher permeability zone is disclosed in U.S. Pat. No. 4,513,821 which issued to W. R. Shu on Apr. 30, 1985. This patent is hereby incorporated by reference herein. 
     Organoammonium silicate, ammonium or alkali metal silicates having a SiO 2  /M 2  O molar ratio of about 0.5 to about 4 are suitable for forming a stable alkali silicate cement. The metal (M) which is utilized herein comprises sodium, potassium, or lithium. Preferably, the SiO 2  /M 2  O molar ratio is in the range of about greater than 2. The concentration of the silicate solution is about 10 to about 60 wt. percent, preferably 20 to about 50 wt. percent. As will be understood by those skilled in the art, the exact concentration should be determined for each application. In general, concentrated silicate solutions are more viscous and form a stronger consolidation due to a higher content of solids. 
     In those cases where it is not possible to control the viscosity of the silicate solution and preclude entry into a lower permeability zone, a mechanical packer may be used. The silicate cement which is formed can withstand pH&#39;s of 7 or more and temperatures up to and in excess of about 400° F. The preferred silicates are sodium, lithium and potassium. Potassium is preferred over sodium silicate because of its lower viscosity. Fumed silica, colloidal silica, or other alkali metal hydroxides can be added to modify the SiO 2  /M 2  O molar ratio of commercial silicate. Colloidal silicate can be used alone or suspended in alkali metal or ammonium silicate as a means of modifying silicate content, pH, and/or SiO 2  content. In a preferred embodiment, two parts of the aqueous silicate is mixed with one part colloidal silicate. 
     Organoammonium silicates which can be used in an aqueous solution include those that contain C 1  through C 8  alkyl or aryl groups and hetero atoms. Tetramethylammonium silicate is preferred. 
     Alkylpolysilicate (EPS) contained in the water-miscible organic solvent is the hydrolysis-condensation product of alkylorthosilicate according to the reaction equation below: ##STR1## where n≦2 
     R=C 1  -C 10   
     R should be ≦10 carbons for good solubility and high SiO 2  content. 
     Tetramethyl (TMS) or tetraethylorthosilicates (TEOS) are preferred. Mixed alkylorthosilicate can also be used. It is desirable to obtain an alkylpolysilicate with n&gt;0.5, preferably n greater than 1. As n increases, the SiO 2  content increases, resulting in stronger consolidation. It is desirable to use an alkylpolysilicate with a silica content of 30% or more, preferably about 50%. EPS which is used herein is placed into a water-miscible organic solvent. The preferred solvent is ethanol. Of course, other alcohols can be used. EPS, TMS, TEOS, or other alkylpolysilicates are contained in the solvent in an amount of from about 10 to about 90 weight percent sufficient to react with the silicates contained in the aqueous solution. Although alcohol is the solvent preferred because of its versatility and availability, other water-miscible organic solvents can be utilized. These solvents include methanol and higher alcohols, glycols, ketones, tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO). 
     Although ethanol is the preferred solvent, higher alcohols also can be utilized, as well as other solvents capable of dissolving alkylpolysilicates. The concentration of alkylpolysilicate should be in the range of about 10 to about 100 wt. percent, preferably 20 to about 80 wt. percent. Of course, enough alkylpolysilicate should be used to complete the reaction with the organoammonium silicate, alkali metal or ammonium silicate. 
     The calcium salt which can be used herein is one which is soluble in alcohol or the water-miscible organic solvent. Calcium chloride hydrate is preferred. However, chelated calcium forms can also be used. Higher alcohols also can be utilized, as well as other solvents capable of dissolving calcium salts and chelates. The concentration of calcium chloride hydrate should be in the range of about 10 to about 40 wt. percent, preferably 20 to about 30 wt. percent. Of course, enough EPS and calcium chloride solution should be used to complete the reaction with the aqueous silicate. 
     In another embodiment, calcium chloride can be used alone in the organic solvent to form a silicate cement in combination with EPS. Similarly, a spacer volume of hydrocarbonaceous liquid is used to separate the calcium chloride solution slug from the EPS organic solvent slug. 
     While hydrated calcium chloride is preferred, cations of other chlorides can be used. Other chlorides that can be used comprise titanium dichloride, zirconium chloride, aluminum chloride hydrate, ferrous chloride, and chromous chloride. 
     Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of this invention, as those skilled in the art readily understand. Such variations and modifications are considered to be within the purview and scope of the appended claims.