Prudhoe Bay Field, Alaska, Waterflood Injection Water Quality and Remedial Treatment Study

Abstract

SPE Members Abstract A comprehensive laboratory and field study was conducted at Prudhoe Bay Field, Alaska to identify the cause for rapid injectivity decline and injection profile degradation in produced water injection wells and to seek remedial treatment methods to improve water injectivity. A specially designed portable core flow station was installed in the produced water treatment facility at Prudhoe Bay Field to evaluate permeability damage characteristics of produced water on reservoir core plugs and to evaluate performance of different surfactant solutions for restoring water injectivity. Test results show that Prudhoe Bay produced water caused a slow but significant permeability reduction to reservoir cores. Commercially available surfactant solutions were effective initially in restoring water injectivity when applied as slug treatments at 1 to 2% concentration levels. No surfactant solution was effective in preventing damage when injected continuously in the produced water stream at less than 50 to 100 ppm levels. Remedial acid stimulation treatments and solvent washes can restore part of the water injectivity. Study results identify the need to further improve the present produced water treatment facilities. Introduction Filtered seawater and gravity separated produced water are being injected in the down structure area of the Prudhoe Bay Field in Alaska to maintain reservoir pressure and improve oil recovery. Rapid injectivity decline and injection profile degradation have been observed particularly in some produced water injection wells. A comprehensive laboratory and field study was conducted to identify the cause for this rapid permeability reduction and to seek potential remedial treatment methods to regain water injectivity. When conducting a waterflood project, it is important that the injection water is sufficiently clean. Solid particles and oil droplets entrained in waterflood water can block reservoir pore throats and cause rapid injectivity decline. Total removal of these particles may not be economically feasible, so a balance must be found between the number and size of particles removed and the acceptable level of blocking by the particulate materials remaining. Membrane filter filtration tests are frequently employed to evaluate injection water qualities. A 0.45 um membrane filter is most commonly used in these tests. However, membrane filters do not closely simulate real formation permeabilities and pore geometries. Sintered glass disks have been suggested as an alternative to better simulate real formation cores. Particle damage core flow tests have also been used in the past to evaluate injection seawater qualities. However, previous studies were conducted under ambient temperature and pressure conditions using 100% brine saturated cores. This may not be directly applicable in the produced water case. If sufficient oil droplets are present in the produced water, they can be filtered out at the core face to increase oil saturation. P. 717^