The challenge in many permeable water sensitive subhydrostatic reservoirs is to avoid the loss of completion fluid when completing or working over wells. It is not unusual to lose up to 1,000 bbl of completion fluid, resulting in a 20% to 50% reduction in production.
Several different approaches have been used to prevent these losses. The most common is viscous fluid-loss control pills with sized particulates, such as calcium carbonate, which often requires a further treatment to remove the solids. An alternative is a solids-free crosslinked viscous polymer pill, which must be spotted and kept in position across the reservoir prior to it crosslinking.
To overcome the limitation of conventional fluid-loss control pills, a low-viscosity system was developed. The system is composed of a viscous disproportionate permeability modifier (VDPM) with sized synthetic polymer particles and fibers, which, with time and temperature, degrade into organic acids. The VDPM reduces the effective permeability to the water-based fluids, and the sized particles create an impermeable filter cake. When the particles degrade, the organic acid acts to break any remaining polymer. The system can either be bullheaded or spotted in the wellbore prior to perforating and effectively seals the formation for several days.
The fluid-loss pill has been used successfully in more than 25 wells. The volume of fluid lost into the formation is often less than 1 bbl, and production is maintained after the workover. The technique has made it possible to selectively stimulate new intervals or recomplete wells with an average increase of more than 240 BOPD, whereas, historically, a decline in production was not uncommon.