Success of a High-Friction Diverting Gel in Acid Stimulation of a Carbonate Reservoir - Cornell Unit, San Andres Field
Authors: Wiley Charles B. 
Published: December 2007

Summary
High-friction gel (HFG) is a fluid that exhibits unusual shear stress characteristics that make it well suited as a diverting agent for well stimulation procedures. It is being pumped into the formation of the Cornell Unit to divert acid to other portions of the formation. Injection wells that have been acid stimulated using HFG as a diverting agent have displayed better vertical sweep efficiency than those using straddle packers or benzoic acid flakes to divert acid.

HFG
HFG is a non-Newtonian, Bingham plastic fluid consisting of 36.3 kg (80 lbm) of guar gum, 11.3 kg (25 lbm) of borax, and 1.4 kg (3 lbm) of enzyme per 3.8 m3 (1,000 gal) of fresh water. Its active life is approximately 24 to 36 hours and, as it is used for the subject of this paper, consists of two phases. During the first phase HFG exhibits the drag-reducing shear stress characteristics of a power-law fluid which permit it to be pumped into place within the formation. This is accomplished by imparting high shear rates to the HFG by keeping it in continuous motion from the initial mixing through the surface pumping equipment, wellbore tubulars, and formation. The second phase begins when the motion of the HFG ceases, and it assumes the high-drag shear stress characteristics of a non-Newtonian, Bingham plastic fluid. Following the mixture's active life, it is broken completely by the enzyme breaker and ceases to exist as a gel.During the period that the HFG acts as a non-Newtonian, Bingham plastic fluid, the high shear stress, or yield point, of the HFG must be exceeded for it to start to flow. In addition, high friction losses are experienced when moving the HFG because, after yield point, equal increments of additional shear stress will produce equal increments of a shear rate in proportion to the plastic viscosity of the HFG.It can be seen, therefore, that HFG effectively would divert acid to less permeable parts of a formation if it could be placed in the more highly permeable portion of a formation. Then, the pressure necessary to break down less permeable parts of the pressure necessary to break down less permeable parts of the formation could be reached before the yield point of the HFG in the permeable portions was exceeded and began to move.

Acid Stimulation Using HFG
The mixing and pumping equipment layout is shown in Fig. 1. Before the job is begun, fresh water is drawn from the transport by Pump A on the blender truck and discharged through the jet mixer below the hopper into the mixing tub. Guar gum is added at the hopper and drawn into the water by the jet mixer. The guar gum and water mixture is drawn from the mixing tub by Pump B and discharged back into the transport. This blending is continued until the proper concentration of guar gum is attained for the given volume of gel to be used in the stimulation treatment. Meanwhile, the borax complexer and enzyme breaker are mixed with fresh water in a separate auxiliary mixing tank agitated by air. When all the mixing is completed, the treatment is ready to begin.
JPT
P. 2196