Laboratory Development of Lignosulfonate Gels for Sweep Improvement

The development of a lignosulfonate gel system for improving sweep efficiency is discussed. The gel mixture is injected as a low-viscosity fluid into a loose streak. After gelation occurs, subsequently injected fluids are diverted into lower-permeability intervals. The developed system is composed of 95 percent or More water, ammonium lignosulfonate, and a mixed activator of sodium dichromate and salt. Laboratory studies show that lignosulfonate gels exhibit the following properties:long gel times can be designed, e.g., gel times up to 2 1/2 months were obtained at 190 degrees F;gel strength can be controlled to produce the level of flow reductions required for a particular application. Controlled flow reduction, rather than complete flow blockage, is needed where the loose streak may contain appreciable amounts of oil recoverable by further waterflooding or by a miscible flood;available injection waters, even highly saline ones, can be used for mixing the gel solutions; andlignosulfonates, with no activator, gel when exposed to temperatures in the range of 300 to 450 degrees F. A fluid with these properties should be useful in controlling sweep in high-temperature reservoirs or steamfloods. Introduction Reservoir heterogeneities, such as loose streaks or fractures, may limit oil recovery in waterflood operations. The injected water may break through prematurely and producing wells abandoned because prematurely and producing wells abandoned because of high water cuts, even though much of the oil in the reservoir is left behind. In spite of sweep problems, many waterfloods have been successful problems, many waterfloods have been successful because water is inexpensive and can be cycled to displace oil gradually from the matrix or lower permeability regions. permeability regions. Recent papers have pointed out that good volumetric sweep efficiency is particularly important in miscible recovery operations. An expensive slug of gas solvent or micellar fluid recovers only the oil that is contacted on a single pass through the reservoir. Thus, it is important that the miscible fluid contact a significant portion of the reservoir. One suggested method for improving sweep to micellar fluid involves prepolymer injection. Because of the growing awareness of the importance of good sweep, greater emphasis is being placed on the diagnosis of reservoir heterogeneities and development of materials for correcting diverse sweep problems. Polymers in various forms have been proposed for improving volumetric sweep caused by a poor mobility ratio or by permeability contrasts. Gels are used for the more severe channeling problems caused by loose streaks. Solid fines and gels are used for fracture plugging. plugging. A previous paper discussed the development and application of alkaline silica gels for selective plugging. Silica gels have been and will continue plugging. Silica gels have been and will continue to be used where the need is to eliminate flow through high-capacity loose streaks. However, with continued application, it has become apparent the silica gels do have some limitations. These includeshort gel times at high temperatures (e.g., about 10 hours at 200 degrees F) that limit the volume of gel mixture that can be injected, andsensitivity to salts that necessitates the use of fresh water for mixing and preflushing and that causes premature gelation in formations containing soluble compounds (e.g, gypsum or anhydrite). This paper describes the development of lignosulfonate gels for improvement of sweep. These gels overcome some of the above limitations of silica gels and also offer additional advantages. DESCRIPTION OF THE SYSTEM This selected system is composed of 95 percent or more water, a lignosulfonate, and a mixed activator of sodium dichromate and salt. This system has evolved from a systematic evaluation of the known reaction of a lignosulfonate with dichromate. SPEJ P. 391