Theme: 10. Characterization, Fate and Transport

Topic Area: 10b. Advanced Sampling and Analysis Tools and Techniques

Formatted Title:

Preventing Vertical Water Flow in Well Annulus Using Silica Gels to Enable Depth-Discrete Contaminant Sampling

Background/Objectives:

Many long-screened wells have been constructed for monitoring purposes, such as those at the U.S. Department of Energy (DOE) Hanford site located in southeast Washington State. Vertical water flows in the well annulus during groundwater sampling from the wells result in biased results. Blocking the vertical water flow in the long-screened well annulus and sealing the groundwater flow from the target aquifer layers into wells are needed to obtain accurate concentrations of contaminants from sampling. Attributed to the flow and gelation characteristics of colloidal silica (CS) suspensions and their environmental benignity, these suspensions are auspicious candidates for the vertical water flow blocking application in which the suspensions are injected into to target locations to form impermeable silica gels in the well annuli.

Approach/Activities:

In this research, we studied the viscosity, rheology, and gelation of micron-sized fumed powder silica suspensions and nanosized silica suspensions recovered from geothermal fluids. We evaluated the application of CS suspension injection and sequential gelation to block the vertical water flow in the annuli of long-screened wells through batch, column, and sandbox studies.

Results/Lessons Learned:

Fumed silica suspensions showed shear thinning, while nanosized silica suspensions exhibited Newtonian flow behavior. During the gelation process, the nanosized silica suspension changed from a Newtonian fluid to a shear thinning fluid while increasing its overall viscosity. At comparable concentrations, the nanosized silica suspensions have much lower viscosity than that of the fumed silica suspensions. Increases in the salinity and silica particle concentration in these suspensions shortened the gelation time. Silica suspension gelation in sand columns completely blocked the water flow and sustained the injection pressure up to 50 psig. A silica suspension was successfully injected into the target zone in the annulus of a bench-scale sandbox mimicking long-screened wells in the field. The silica gel formed in the annulus effectively blocked chemical transport through the gelled zone. Our research reveals that a process using silica suspension injection and sequential gelation technology is promising for blocking the vertical water flow and chemical transport through the filter pack in well annuli.

Sarah Saslow

Affiliation

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(Group 2, Poster Board #134) Preventing Vertical Water Flow in Well Annulus Using Silica Gels to Enable Depth-Discrete Contaminant Sampling