A challenge in many permeable, water-sensitive, subhydrostatic reservoirs is avoiding the loss of completion fluid when completing or working over wells. 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 degrade into organic acids. The VDPM reduces the effective permeability to 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 limitations of many conventional fluid-loss-control pills have resulted in the development of a number of solids-free fluid-loss-control pills. In high-permeability reservoirs, a highly crosslinked gel is needed to achieve good fluid-loss control. Polysaccharides, such as guar, have been widely used for this application because of their low cost and availability. These guar-based fluids are typically crosslinked with borate or organometallic crosslinkers. The viscosity of crosslinked guar decreases significantly at temperatures greater than 200°F because of the limited thermal stability of the polymer. For higher-temperature applications, polyacrylamides can be used to form crosslinked gels.
A limitation of crosslinked polysaccharide and polyacrylamide polymers is that they require an internal or external breaker. The breaker is required to break the crosslinked polymer and lower the viscosity of the fluid so the broken gel can flow out of the formation matrix. Even when using an internal breaker, some polymer remains in the pore spaces, effectively reducing and damaging the permeability of the formation.
To overcome the limitation of residual-polymer damage, hydroxyethylcellulose (HEC) has been used extensively because of its low residual-solids content. However, linear HEC polymer solutions do not form rigid gels but control fluid loss through viscosity and gradual filtration. This means that, as the linear fluid penetrates deeper into the formation, the shear rate decreases and the apparent viscosity increases. Permeability damage has been shown to increase with increasing penetration of viscous fluids, not only with HEC.
Despite all the advances made regarding the design of fluid-loss-control pills, the greatest challenge remains the same, which is to have a fluid that prevents the loss of water-based fluids into the reservoir but does not limit the production of crude out of the reservoir when the well is put on production....
Controlling Losses When Recompleting Low-Pressure Reservoirs
05 January 2016