Well Stimulations and Scale Inhibitors
Authors: Vetter O. J. , Lankford S. , Nilssen T. , Shelton M. 
Published: December 2007

Three major well or reservoir stimulation methods or techniques are concerned with scale inhibitor applications:

Acidizing. Fracturing. Acid Fracturing.

Scale inhibitors are frequently used during these three types of well and/or reservoir stimulation jobs. The main objective for the use of scale inhibitors concurrently with the stimulation fluids is the same in all three of these stimulation techniques.

Scale inhibitors are often added to the stimulation fluids to prevent further scale formations within the well or reservoir during future well or reservoir operations after the stimulation job is completed, i.e., after the well is returned to production. Obviously, the technical and economical advantages of this particular scale inhibitor application, i.e., adding of scale inhibitor to the stimulation fluid, are rather clear and obvious assuming these scale inhibitor application methods will work at all.

Some recent lab and field work performed for some California oil fields revealed some rather unexpected problems and risks related to some inhibitor additions to a number of stimulation fluids. These problems are different for different stimulation jobs:

Acid jobs: None of the tested inhibitors can work in the low pH environment that exists in the well and/or reservoir for extended times after the acid job is performed. Much of the injected inhibitor is wasted because it can not adsorb onto the rock at the low pH values and, thus, the inhibitor is not properly retained in the reservoir. Frac jobs: Aqueous Frac Fluids: 36 scale inhibitors were examined for their chemical compatibility with two water-based frac fluids (containing cross-linkers). All of the inhibitors prevented the frac fluids from acquiring any gel strength even if the inhibitor concentration was only 5 ppm or even less. Each of the tested inhibitors acts as a poison for the already gelled (cross-linked) frac fluids. Near complete breakage of the gels occurs within 30 seconds upon contact between inhibitor and gel. Gel strength in the presence of a scale inhibitor can be provided by increasing the cross-linker concentration. In this case, some of the excess cross-linker becomes sacrificial. However, a rather unexpected result of these countermeasures is that the gel becomes extremely unstable upon application of mechanical shear. Emulsion Frac Fluid: Emulsion frac fluids are not as sensitive to scale inhibitor additions as aqueous frac fluids. The sorption characteristics of the emulsifier and scale inhibitor may clash under reservoir conditions. Oil Frac Fluids: The main problem is to place the inhibitor in the oil. Oil soluble scale inhibitors should not be used for the obvious reason that scale forms from the water phase and not from the oil phase.

Presently, the industry seems to be at a loss when it comes to scale inhibitor applications during and after stimulation jobs. Numerous techniques were tried to overcome the compatibility problems. Some of these new techniques and our own experience with these methods are described. New laboratory methods were designed and used to screen scale inhibitors for their potential use during and immediately after stimulation jobs. All attempts to come up with a reasonable compromise for field applications failed. Despite the frequent use of scale inhibitors during well stimulations we believe that these jobs either may not yield a proper scale inhibition or the efficiency of the stimulation job is greatly reduced.