Temporary Zonal Isolation Minimizes Reservoir Damage During Workover Operations in Ecuador

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Abstract In Ecuador, the principal reservoirs are subhydrostatic, with permeability ranging from 100 to 2,000 mD and significant clay content. The crude oils are prone to form emulsions in contact with completion fluids. After workover operations—pulling electric submersible pumps (ESP) or recompleting a well—it is common to lose more than 1,000 bbls of completion fluid, resulting in a 20% to 50% reduction in production. Fluid loss control pills containing sized particulate, such as calcium carbonate or sized salt, are frequently used to control the fluid loss; however, a further treatment is required to remove the solids. Using this technique, 25 wells producing a total of 8,000 BPD were worked over, after which the production decreased to 5,000 BPD. The main challenges in developing a solids-free fluid loss control pill to control losses of completion fluid during a well intervention are: a) the low, subhydrostatic (0.16 to 0.36 psi/ft.) reservoir pressure, b) high matrix permeability, and c) cleanup when well is put on production. To overcome this, a highly viscous, polymer crosslinked fluid with an internal breaker was developed to temporarily isolate the reservoirs. Using this fluid, it is possible to work over a well without losing fluid into the reservoirs and having the associated loss of production. In 15 wells worked over using the fluid loss control fluid combined with modified workover procedures, fluid losses were controlled and production was maintained after the workovers. This technique also made it possible to selectively stimulate certain intervals, increasing production. The total production prior to the interventions was 7,300 bpd. Afterward, it increased 27% to 9,250 bpd. The fluid-loss control fluid, together with the new workover procedures, has now been adopted as a standard. It has proved an effective means to protect the subhydrostatic reservoirs in mature fields during workover interventions.