Effect of Bridging Agents and Carrier Fluids on Diverting Efficiency
Authors: Dill W.R. 
Published: December 2007

Tests show that effective bridging is more dependent upon particle size distribution than upon the type of bridging particle size distribution than upon the type of bridging agent, and that the efficiency of the diverting material is influenced by the carrier fluid.

Introduction
Diverting materials have become a prominent component of oil, gas, injection and disposal well treating operations. They function to distribute treating fluids more uniformly through a zone or from one zone to another, and the effectiveness of the diverting material is dependent on the efficiency of the bridging agent they contain.
To treat effectively a zone having a permeability differential, aliquots of the total treating volume are separated by a volume of diverting material. The diverting material is usually composed of a viscous carrier fluid and a bridging agent.
Rock salt has been used extensively as the bridging agent in diverting materials. Perfpac balls (rubbercovered perforation sealers) and packers also have been used for the placement of treating fluids. Each of these has disadvantages. Rock salt does not have the particle size distribution required for effective fluid control. Perfpac balls depend for their success on the specific gravity and viscosity of the treating solution, on the pump rate, and on the roundness and smoothness of the perforations. A packer may require too much time to place, and as a result be less economical than the other methods. Sometimes there is communication behind the pipe, so that packers or balls cannot function effectively.
The problem of selecting bridging agents to divert treating solutions is similar to the problem of designing a gravel pack to control unconsolidated sand, but the selection requires that the particle size distribution be extended to include fine particles.
The problem of selecting bridging agents is similar also to the problem of designing an effective lost circulation material. Because of this similarity, the procedures presented by the API for testing bridging procedures presented by the API for testing bridging agents for list circulation were used as a basis for determining the efficiency of diverting materials for treating solutions. Tests were designed to simulate down-hole conditions as nearly as possible.

Comparison of Bridging Properties of Salt and Sand

Equipment and Procedure
Initial tests were conducted at 76 degrees F to evaluate and compare the bridging and sealing properties of salt and sand of various sieve sizes (U. S. Standard Sieves).
The tests were conducted using a modification of the API BB Bed Test (Fig. 1). A lucite tube 356 mm long, having a 50.8 mm diameter, was filled to provide a 178-mm column of BB's (2,400 gm), using brass-clad BB shot having a diameter of 4.35 mm. The remaining volume (360 mi) was filled with a gel of saturated salt (NACl) water, gelled with the equivalent of 300 lb of guar gum per 1,000 gal of water. The gel contained the equivalent of 4 lb of salt or sand per gallon of water.
The bridging agent was mixed uniformly into the gel and agitated until sufficient viscosity developed to maintain a uniform dispersion of bridging material.
JPT
P. 1347