The formation damage that results from the incomplete back-production of viscous, fluid-loss control pills can be minimised if a slow acting internal breaker is employed. In particular, core-flow tests have indicated that combining a succinoglycan-based pill with a hydrochloric acid internal breaker enables a fluid-loss system with sustained control followed by delayed breakback and creates only low levels of impairment. To describe the delayed breaking of the succinoglycan/hydrochloric acid system, a model, bassed on bond breaking rate, has been used. With this model, it is possible to predict the change of the rheological properties of the polymer as a function of time for various formation temperatures, transition temperatures of the succinoglycan and acid concentrations. Hence, the model can be used to identify optimum formulations of succinoglycan and acid breaker on the basis of field requirements, such as the time interval over which fluid-loss control is needed, the overbalance a pill should be able to withstand and the brine density required.
Introduction
The loss of fluid into a drilled formation often has to be controlled to ensure the success of well completions or workover operations. An excessive loss of completion fluid into the formation can damage the formation, delay production and exact high costs in the case of heavy brines. To control such fluid loss, the industry relies on a variety of materials. Graded salts or sized carbonates, for example, reduced fluid loss by forming a low-permeability filter cake at the formation sand face or at the entrance of the perforations. When used during gravel packing (especially after pre-packing), however, these materials may fill unpacked perforations with solids that are difficult to remove, thereby resulting in impaired wells.
For that reason brines that have been viscosified with water-soluble polymers are preferably used to control fluid-loss during gravel packing and other workover operations, especially in perforated wells. The viscous brine is pumped down the hole and allowed to leak off slowly into the formation. The leakoff of fluid is retarded by the increase of its apparent viscosity during radial flow - a feature explained by the shear-thinning that characterises these water-soluble polymers. If poorly formulated, however, highly viscous pills of this kind can cause formation impairment that reduces the productivity of wells1,2,3.
Some properties of commercially available brine viscosifiers are discussed in this paper. Our intent is to explain how one can obtain the best viscous-pill formulation. Our emphasis is placed on the optimisation of fluid-loss control pills based on succinoglycan, since this biopolymer exhibits some unique features.
Brine Viscosifiers
Hydroxyethylcellulose.
Hydroxyethylcellulose (HEC) is the most commonly used polymer for viscous fluid-loss control 4. It is available not only as a dry powder but also as an activated liquid5 that is suitable for viscosifying heavy calcium chloride/bromide brines. Advantages of HEC are that it is cheap and that the liquid versions are readily soluble in high-density brines. It also has certain disadvantages, though. Improperly prepared HEC solutions can contain "fisheyes" of poorly hydrated particles that can cause formation damage. The viscosity of HEC also steeply declines with increasing temperature, resulting in poor control of fluid losses into the formation (see Fig. 1). Quite often HEC is used in combination with xanthans to enhance its rheological performance; those pills are referred to as multi-polymer pills.
Hydroxyethylcellulose.
Hydroxyethylcellulose (HEC) is the most commonly used polymer for viscous fluid-loss control 4. It is available not only as a dry powder but also as an activated liquid5 that is suitable for viscosifying heavy calcium chloride/bromide brines. Advantages of HEC are that it is cheap and that the liquid versions are readily soluble in high-density brines. It also has certain disadvantages, though. Improperly prepared HEC solutions can contain "fisheyes" of poorly hydrated particles that can cause formation damage. The viscosity of HEC also steeply declines with increasing temperature, resulting in poor control of fluid losses into the formation (see Fig. 1). Quite often HEC is used in combination with xanthans to enhance its rheological performance; those pills are referred to as multi-polymer pills.
