Fluid Loss Control Agents
In fracture acidizing, fluid loss control helps to extend fractures by minimizing leakoff of the treating fluid through the fracture face. Fluid loss control is difficult to obtain with acid solutions on carbonate formations because the acid is reacting on the surface where the fluid loss additive is being deposited.
Fluid loss additives often are composed of two agents: an inert, solid particle that bridges at the fracture surface and a gelatinous material that plugs the pores in the solid granular material. Commonly used fluid loss additives are listed in Table 1 .
| Fluid Type | Solid Additives | Gelatinous Additives |
|---|---|---|
| Aqueous pad | Silica flour | Guar |
| Calcium carbonate | Cellulose | |
| Organic polymer | Polyacrylamide | |
| Inert solid coated with guar type material | ||
| Hydrocarbon pad | Inert solid coated with organic sulfonate | |
| Acid | Acid swellable solid | Guar |
| Organic resin | Karaya | |
| Silica flour | Cellulose | |
| Organic polymers | Polyacrylamide | |
| Polyvinyl alcohol |
In aqueous pad fluids, a combination of solid additive and polymer is usually selected. When water without a polymer additive is used, an inert solid coated with guar-type material may be chosen. Acid additives include acid-soluble organic polymers that swell and become soft when contacted by acid, thereby giving both solid and gelatinous characteristics. Other additives are similar to those chosen for use in water-base pad fluids.
We now focus on matrix fluid loss, fluid loss in natural fractures, fluid loss in wormholes, and viscosity effects.
Matrix Fluid Loss: Many carbonates contain little permeability within the matrix of the rock. Because acid is a reactive substance, however, it creates more surface area as it travels during leakoff. It is very important, therefore, to confine the acid to the fracture. Fluid loss additives are usually sufficient to control matrix leakoff. Ideally, they should be either degradable or slowly soluble in the acidizing solution or in produced fluids.
Fluid Loss in Natural Fractures: Some limestone formations have unusually high permeability in the form of natural fractures or vugular flow channels. Fluid loss control in this type of formation cannot be achieved with ordinary fluid loss additives. A larger diverting material is usually required to control the fluid lost to natural fractures.
Fluid Loss in Wormholes: Fluid lost to the matrix will react and, if lost in a highly reactive part of the formation, may cause wormholes. These are large, highly conductive channels which usually start when a large pore or vug grows at a rate substantially higher than that of smaller pores and receives an increasingly larger amount of the acid as new surface area is created. From a fluid-loss standpoint, wormholes are similar to natural fractures since they require larger particles for control. If little fluid leakoff occurs to the matrix, few wormholes are likely to occur. For a more detailed discussion of wormhole formation and growth, see Economides et al. (1995).
Viscosity Effects: The rate of leakoff is directly related to viscosity. A thicker fluid results in less fluid lost to the formation and, therefore, improved fluid efficiency. If efficiency increases, more live acid can be pumped deeper into the formation, thus increasing fracture flow capacity. Viscosifying the acid can be one of the most important steps in providing fluid-loss control in fracture acidizing treatments.
