Mineral scale formation is a problem for oil and gas operators that can result in deterioration of assets, increased lifting costs and lost production.1 Scale which forms in or at the perforations can only be prevented by placing scale inhibitor in the producing formation via some type of reservoir displacement technique commonly referred to as a squeeze. Operators desire inhibition that will last as long as possible and minimize the production interruption for treatment. In reservoir formation types that exhibit known scaling problems, scale inhibition treatment can be initiated during the completion stage using a solid inhibitor integrated into the hydraulic fracturing program. Treatments of this type can result in deep reservoir placement of inhibitor without large amounts of introduced water frequently needed with traditional squeeze treatments. Data presented in this paper illustrate the superior performance and longevity of these types of treatments.
Scale formation is a problem throughout the oil field. Traditionally, operators have primarily treated for scale with liquid chemical inhibitors. These chemicals can be administered through batch, continuous or squeezed applications. The choice of treatment choice depends on where the scale is occurring. If scale is forming at or before the formation perforations, a scale squeeze has traditionally been the only treating option. In a squeeze treatment a chemical scale inhibitor is pushed back into the formation and the chemical either adsorbs or precipitates onto the formation surface. As the well produces water the precipitated scale inhibitor slowly dissolves and is available to prevent scaling.
For an inhibitor to be effective at preventing scale it must be present at a level above the minimum effective concentration (MEC) prior to the onset of conditions that cause scaling (compositional, temperature, pressure or pH changes). 2 The effectiveness of the scale inhibitor is determined by analyzing the produced water for evidence of the scale inhibitor. This is called a residual test. When the scale inhibitor residual falls below the MEC, the operator is informed that the well must be re-squeezed. In the case of batch or continuous treatment the residuals are not as critical since the well undergoes treatment on a much more frequent basis (usually for batch treatments the first few treatments are measured for residuals in order to ascertain the treating frequency). Experientially, wells that have been squeezed maintain an acceptable residual level for approximately one year or less.
A special class of treatment candidates is represented by new wells that require stimulation and for which there is an expectation of water production with associated scaling problems. Stimulation by hydraulic fracturing often utilizes a slurry of solid proppant. The proppant fills the voids created by hydraulic fracturing and provides a permeable conduit for production fluids to reach the wellbore. Although the wells selected to date have been new wells, there is no reason that existing wells that are amenable to hydraulic stimulation could not also be considered candidates. For these wells a new approach to scale inhibition is available.
