A diverting material and its, applications are introduced for use in stimulation of high-volume water injection wells. The paper describes laboratory work for diverter selection, field testing to develop application techniques and several case histories that demonstrate use and results. Also discussed are possible future developments.
Many North Sea oil fields have active water injection programs. Several of these are started the beginning of production history and they are considered essential for ultimate recovery. Any method of improving water injection profiles and of increasing water volumes is consequently of high priority.
A typical injection well has long perforated intervals (300 ft net or more) and water injection of at least 25,000 BPD. Early wells may have involved dump flooding but most now use power injection of produced water and seawater. Most wells have zones if widely varying permeability. A typical well completed is shown in Fig. 1 along with its corresponding, permeability profile.
After a certain injection history, skin damage, tends to increase. This is often the result of undissolved solids carried in the injection water. Previous stimulation treatments to remove such damage indicated that both hydrochloric acid and hydrochloric-hydrofluoric acid mixtures would improve well injectivity. The problem remained to improve the injectivity profile over the entire net interval.
Proper distribution of treating fluids over the entire interval is one of the major factors controlled E; the success' of any matrix treatment. An effective diverting agent must be used to ensure complete coverage.
A review was made of various diverting materials present 1'1 used in matrix acidising. These materials were evaluated in terms of (1) their ability to equalize acid flow into zones of varying permeability, and (2) cleanup when following the treatment immediately with continued water injection.
Due to the long intervals, treatment volumes are usually large and treatment times often. This places an additional requirement on the diverting technique in that the early stages must effect last the duration of the treatment but then clean up fairly rapidly after the end of the treatment.
Bottom hole treating temperatures (BHIT) were not a problem. Due to the large volume of injected water, BHTT are rarely above 140° F.
Ball sealers were not considered, either because of expected poor effiency (high-density type) or because of the lack of a recovery mechanism (low density)
Foams, gels and other liquid diverting agents were also rejected. Although these fluids will reduce flow rate into the formation, they are not long lasting and there may not be any actual redistribution in the ratio of treating fluid entering various zones. It is this latter point that is really the key to successful matrix treatments over long intervals. The diverting agent must equalize fluid flow into the various permeability zones. Based on the above considerations of fluid redistribution, longevity and cleanup, a particulate type of diverter seemed the best choice.