Oilfield Applications of Colloidal Silica Gel

Abstract

Summary A practical reservoir fluid-flow control system based on colloidal silicagel was developed. Colloidal silica gel is an environmentally benign systemthat provides easy surface handling, reliable gel-time control at temperaturesup to 250 degrees F, and high in-situ performance. Extensive laboratory testingwas completed and is discussed elsewhere. Colloidal silica gel has been used in11 well work-overs for water-injection-profile modification (four wells), water-production control (three wells), and remedial casing repair (fourwells). Only one of the four injection-well treatments was a clear-cuttechnical success, with failure typically caused by pressure parting the gelplug after the well treatment. Two of the pressure parting the gel plug afterthe well treatment. Two of the three water-production-control jobs weretechnical and economic successes. Temporary success was achieved in three ofthe four casing-repair treatments. Introduction A wide variety of gel systems have been proposed and investigated for use inreservoir fluid-flow control. None has proved to be a universally effectivesolution; rather, each lends itself to particular applications. Work wasinitiated in 1982 to identify a gem system particular applications. Work wasinitiated in 1982 to identify a gem system that had low permeability (lessthan 1 md in typical applications); was stable in typical operatingenvironments; had controllable gel time; was easily injected; and posed minimalsafety, handling, and environmental problems. After a wide range of gelchemistries was screened, the search focused on silica-based systems. Colloidalsilica gels were selected for development instead of conventional sodiumsilicate gels because of their more robust gel-time control. Followingextensive laboratory testing, field testing of colloidal silica gel wasinitiated in production operations in 1985. Colloidal Silica Sols and Gels " Colloidal silica" refers to stable aqueous dispersions of discretenonporous particles of amorphous silicon dioxide (SiO2), Commercial solscontain 15 to 40 wt% SiO2 as spherical particles with diameters ranging from 4to 200 nm. Concentrated commercial sols are stable at moderate pH (9.5 to 10.5)and at high silicon dioxide/alkali ratios (e.g., SiO2/Na2O>50) because ofsilica particle repulsion resulting from surface ionization in alkalinesolution. Particle collision, bonding, and aggregation into long-chain networks arebelieved to cause colloidal silica to gel. Particle collision is promoted byreducing the pH of a stable alkaline sol, by adding cations to the solution, byincreasing particle concentration, or by increasing temperature. Particlebonding probably results from formation of siloxane (Si-O-Si) bonds at pointsof interparticle contact. Bonding is catalyzed by hydroxide ion. Gelationoccurs when particle aggregation ultimately forms a uniform 3D network of long, particle aggregation ultimately forms a uniform 3D network of long, bead-likestrings of silica particles. Fig. 1 illustrates the effects of pH and salinity on colloidal silicagelation. A 2 less than pH less than 5, the solution is deficient in hydroxidecatalyst. Gel time decreases with increasing pH and is insensitive to salinity. A broad minimum in gel time is exhibited in the range 5 less than pH less than7. As pH increases above 7, the silica particle surfaces progressively ionize, promoting charge repulsion and particle surfaces progressively ionize, promoting charge repulsion and increasing the gel time. Ultimately, at 9.5 lessthan pH less than 10. 5, a stable sol is obtained. The addition of salt to anakaline solution, however, results in charge screening and decreases gel time. Divalent cations (Ca2+ and Mg2+) screen silica particles more effectively thanmonovalent cations (Na + and K +) do and consequently have a greater effect ongelation kinetics. Increasing the number of silica particles in solution or increasing thetemperature decreases gel time. Particle concentration can be increased eitherby increasing total silica concentration at fixed particle size or bydecreasing particle size at fixed total silica particle size or by decreasingparticle size at fixed total silica concentration. At fixed pH and salinity, the gel times of colloidal sila solutions follow a simple first-order Arrheniustemperature dependence. Upon initial gelation, the interparticle bonds are relatively weak, but theyare strengthened by deposition at the contact points of silica dissolved fromthe particle surfaces. This curing reaction diminishes asymptotically withtime, and the gel reaches an ultimate strength. Sodium and potassium silicate solutions differ from colloidal silica sols inthe form and distribution of silica in solution. The gel-time behavior ofsilicate solutions is qualitatively similar to that shown in Fig. 1 forcolloidal silica, but for a given set of reaction conditions, silicate gelsform faster (often by several orders of magnitude) than colloidal silica eels. At a given silica concentration, the ultimate gel strength of a silicate gel isgreater than that of a colloidal silica gel but is more dependent on initialreaction conditions. Colloidal silica was selected for extensive investigation because itsgelation is less sensitive to salinity and pH variations than the gelation ofsilicates is, providing more reliable gel-time control. The higher silicaconcentrations required for colloidal silica gels than for silicate gels wasaccepted as a necessary consequence. Du Pont Ludox colloidal silica (7-nm particle size) was used in all thelaboratory work and field testing reported here, but the trends discussed holdfor colloidal silica in general. The properties of Ludox colloidal silica solare given in Table 1, along with analogous properties for a 3.3:1 sodiumsilicate. Laboratory Testing In liquid-saturated consolidated core samples with permeabilities rangingfrom 10 to 500 md, 0.01-md permeabilities were routinely obtained for colloidalsilica gel plugs. In unconsolidated sand-packs with initial permeabilitiesranging from 1 to 3 darcies, gel-plug permeabilities between 0.5 and 1.0 mdwere achieved. Residual oil had little effect on gel-plug permeability in thewater-wet systems tested. Effective permeability reduction to CO2 injection wasalso demonstrated. Fully cured colloidal silica gels in consolidated core plugs were found towithstand applied pressure gradients of more than 2,500 psi/ft beforeexhibiting any change in permeability. Silica gel psi/ft before exhibiting anychange in permeability. Silica gel strength was found to be a function ofelapsed time after initial gelation, with at least three times the initial geltime required for the gel plug to reach 50% of its ultimate pressure-gradientstability. The ultimate gel strength was reached after curing about 20 timesthe initial gel time. Silica gel plugs were demonstrated to be stable to 100-PVthroughput of neutral brine, were resistant to HCI, and were tolerant oftemperatures exceeding 350 degrees F. However, colloidal silica gels were foundto be susceptible to attack by fluids with pH >9.5. Also, thepressure-gradient stability of silica gel in pH >9.5. Also, thepressure-gradient stability of silica gel in arti-ficially fractured core plugswas found to be much lower than is competent core plugs. In-situ gel times were controlled reliably from several hours to severaldays by initial adjustment of silica-solution pH and salinity at applicationtemperatures below 180 degrees F. The highly buffered silica solutionsexhibited minimal pH variation when injected through core plugs. Ion-exchangepreflushes provided adequate gel-time control in reservoir core plugs with highion-exchange capacity by es-tablishing the proper salinity environment. Attemperatures abov180 degrees F, a chemical triggering agent was used to achievegel-time delays greater than a couple hours. SPEPE P. 406