An Oil-Coating Process To Stabilize Clays in Fresh Waterflooding Operations(includes associated paper 6405 )

Abstract

A new approach to stabilizing clays has been developed that uses anoil-soluble surfactant to coat the clays with a tenacious film of oil.Two alternative formulations of the oil-coating treatment an emulsion of diesel oil and an aqueous system consisting of a dispersion of the surfactant in brine have been developed. Introduction In some waterflood operations it is necessary to injectfresh water when a suitable brine is not available or is toocostly. If the reservoir rock contains interstitial clay thatswells and disperses in fresh water, permeability may beimpaired and injection rates may be lowered. Many methods of stabilizing clays are based on ioniccomposition of the water or the use of hydrolyzed metalliccations. This paper describes a new approach to stabilizingclays that uses an oil-soluble surfactant to cause the clays to be coated with a tenacious film of oil.Following laboratory development, the oil-coatingtechnique was field tested. It is an effective andinexpensive method giving protection from fresh-waterimpairment. Two alternative formulations of the oil-coatingtreatment have been developed:an emulsion of diesel oil, containing the oil-wetting surfactant, in brine(referred to as the OC emulsion), andan aqueous systemconsisting of a dispersion of the surfactant in brine(referred to as the OC dispersion). The second method, which uses the residual oil in the formation to coat theclays, is less expensive and is generally preferable.However, it is not effective with certain heavy crudes, forwhich the first method must be used. Both methods areapplied as a batch treatment that affects only a small radiusaround the wellbore. Since the pressure drop decreasesunder conditions of radial flow with the log of the distancefrom the wellbore, a treated zone of about 10 ft isgenerally adequate. Usually, the treatment is preceded by an acid job so that the formation may be cleaned and broughtto maximum permeability before being stabilized. It is recommended that laboratory tests of the treatmentbe made before applying it in a new waterflood. This isbest done by measuring the relative permeability to freshwater of preserved cores from the reservoir at residual oilbefore and after treating with the oil-coating process. Theeffect and comparability of acid and acid additives alsoshould be checked. In principle, the optimum volume oftreatment may be determined by assuming various radii oftreated zones, substituting into the radial flow equationtreated and untreated permeabilities as determined from the cores, and comparing the resultant injection ratesand costs. Laboratory Evaluation of the Oil-CoatingSystems Oil-Coating Emulsion Testing The OC emulsion was formulated with two considerationsin mind:that it should be effective in stabilizingthe clays, andthat it should be fine enough to beinjected easily into the formation. The composition ofthe emulsion is given in Table 1 and the function of eachingredient is as follows. The agent that causes the sandgrains to become oil wet is Redicote 75 TXO, a cationicsurfactant. Another cationic agent, E(11), is added tostabilize the emulsion. A nonionic agent, E(12L), controlsthe fineness of the emulsion. The diesel oil or tolueneprovides a low-viscosity oil to coat the sand grains. JPT P. 1053^