Selective Placement of Fluids in a Fracture by Controlling Density and Viscosity
Authors: Fredrickson S.E. , Broaddus G.C. 
Published: December 2007

An investigation was initiated to study the effects of varying densities and viscosities of adjacent fluids as they are pumped into a vertical fracture in a stage-treatment manner. It is shown that acid can be placed selectively in a vertical fracture by controlling the density and viscosity of the preflush.

Introduction
Preflushes and overflushes are commonly used in Preflushes and overflushes are commonly used in fracture acidizing. Preflushes are useful for cooling the fracture, improving fracture geometry, and lowering the fluid loss of the acid solution that follows. While attention is generally given to viscosity in treatment design, fluid density has not been considered previously. This paper presents information obtained from model studies paper presents information obtained from model studies on the effect of fluid density using a vertical-fracture flow cell and from field treatments. These studies show that acid can be placed selectively in a vertical fracture by controlling the density and viscosity of the preflush.

Background
This study was initiated to investigate the effects of different densities and viscosities of adjacent fluids as they are pumped into a vertical fracture in a stage-treatment manner. With immiscible fluids such as kerosene and water, a slight overriding or underriding effect would be expected to occur because of density differences. However, this was not believed to be the case when two miscible fluids, such as different densities of salt water or acid and water, were used. Very little information is available on the mixing of two miscible fluids of different densities. However, it is recognized that a large amount of agitation is necessary to achieve uniform mixing of miscible fluids (for example, adding water to concentrated hydrochloric acid).
While most wells will be completed in zones of best possible production, it may be difficult at times to keep possible production, it may be difficult at times to keep a created fracture from extending vertically above or below this preferred section. There are also instances where the created fracture will stay in the preferred section but the acid will not flow through the entire vertical area.
At number of fracture acidizing treatments have been run using a hydrocarbon-water (rho = 0.8) emulsion as a viscous preflush followed by acid (rho = 1.075 to 1.15). In some of these treatments it appeared that acid penetreated primarily the lower section of the fracture penetreated primarily the lower section of the fracture extending into a water zone, since the water-oil ratio was much higher than when no preflush had been run. In some other fracture acidizing treatments where a high-viscosity, gelled-water preflush was pumped ahead of the acid, the results again seemed to indicate that the acid had penetrated the lower part of the created fracture.
After reviewing many of these results, it was concluded that ah attempt should be made to answer questions about the flow of fluids with different density and viscosity in a vertical fracture. The major questions were the following:Does this possible fluid separation and consequent acid underrunning occur only with the immiscible fluids?Does it also occur in the miscible fluids because of the density difference?How does viscosity difference affect this possible vertical fluid movement?
JPT
P. 597