RIASSUNTO
ABSTRACT
The ability of a fluid to flow adequately into the formation during gravel-pack treatments is critical to achieving a good pack. Recent studies have indicated â??fish-eyesâ?¿ and/or â??microgelsâ?¿ present in many polymer gelled carrier fluids will plug pore throats, leading to impaired leakoff and causing formation damage. Intensive manipulation of the polymer gelled fluid using shear and filter devices will help remove the particles, but it adds to the cost of the treatment in terms of equipment and manpower. Excessive shear will degrade the polymer leading to poor gravel suspension, while too little shear will cause filtration problems.
A gelled carrier fluid using a viscoelastic surfactant system has been found to leak off very efficiently to the formation, and cause no formation damage, without the use of shear/filter devices. Viscoelastic surfactant-base gelled fluids develop viscosity because of the association of surfactant molecules into large rod-shaped aggregates. There is no hydration of polymer involved, so fish-eyes and microgels will not be formed in the viscoelastic fluid. A surfactant-base system having a yield point allows the gravel carrying properties to be much better than fluids gelled with conventional polymer systems (hydroxyethylcellulose [HEC]). For example, a gravel carrier fluid gelled with 80 lb HEC/1,000 gal has a viscosity of about 400 cp at 170 sec-1 a viscoelastic surfactant-base system having only one-half the viscosity still flows into cores about four times more efficiently than the HEC-base fluid. The rheology, leakoff, formation damage and mixing properties of a viscoelastic, surfactant-base, gravel-pack carrier fluid are discussed.
INTRODUCTION
Permeability reduction of formation core samples has been attributed to exposure of the cores to common gravel-pack gelled fluids.1 The damage is generally believed to be caused by poorly hydrated polymer fluids which contain insoluble particulate â??fish-eyes.â?¿ Unhydrated polymer is trapped within the core matrix and is not removed by fluid flow. Note that the fish-eyes also are not broken by breakers added to the gel. Several papers have been published during the last few years2,3 addressing methods to remove the damaging unhydrated polymer, so formation damage will not occur as a result of gravel-pack operations. Tests indicate shear/filter devices may improve the fluid properties to the point that little formation damage will occur.
A clean, nondamaging carrier fluid prepared on location without the need for the shear/filter operations has been developed using viscoelastic surfactant gelling agents. The criteria used when developing the new gravel-pack carrier fluid included improvements in several areas. The fluid must be nondamaging to the formation. The benefits of a well-designed and executed gravel pack will be lost if the formation is damaged. To obtain a complete pack in the perforation tunnel area, the fluid must leak off easily to the formation. Gravel suspension properties of the fluid must be improved. Gravel suspension is particularly important in gravel packing deviated wells. A new gravel-pack system must be superior to HEC in handling and mixing characteristics. The effect of the surfactant on wettability of sandstone formations also must be understood.