Mitigation of Silica Deposition in Wellbore Formation in Malitbog Sector, Tongonan Leyte, Philippines

Abstract

Brine being injected in Malitbog is oversaturated with respect to silica. While maintenance of pipelines in 1999 - 2003 showed minimal silica scaling (3.0-8.0 mm total), the injection wells have recurring injection capacity declines. Well 5R1D, after being mechanically cleared, barely gained capacity from 98 to 123 kg/s, but showed increased capacity from 44 to 137 kg/s after acidizing. Well 5R4, after acidizing, regained full capacity from 76 to 201 kg/s. Injection capacity decline indicates silica deposition is occurring in the wellbore formation.
Tests were conducted to determine the effectiveness of Geogard SX (GSX) and pH Modification in controlling silica scaling at low excess silica in the brine (~ 50 - 120 ppm silica; SSI = 1.10 - 1.18). A pilot set-up mimicking the pipelines and wellbore formation incorporated retention and formation vessels. Induction time (IT) in a polymerization vessel of the untreated brine (pH = 6.8) is 45 - 55 minutes, while in the acid treated brine (pH = 5.3 - 5.6), monomeric and total silica were maintained for at least 75 minutes, which was the end of the induction test based on the available sampled brine in the polymerization vessel. The residence time (RT) of the brine to the wellbore loss zone is 23 minutes, confirming that silica will deposit in the formation and not upstream.
In the untreated brine test, silica deposition occurred in the inspection pipe (31.3-50.7 mm/yr) and formation materials, while in the pH Modification test, no silica deposition occurred anywhere in the line. These were confirmed by inspection, flowrate, formation weight, petrologic analysis and brine chemistry monitoring. However, in the acid treated test, corrosion rate was relatively high and attributed to overdosing resulting to brine pH sometimes way below the target pH. In the GSX test at 0.5 ppm dosing, silica deposition still occurred in the inspection pipe and formation materials but at very low thickness and deposition rates (2.4-11.9 mm/yr). These were also confirmed by inspection, flowrate, formation weight, petrologic analysis and brine chemistry monitoring.
Of the two methods tested, pH Modification is the most cost effective mitigating solution to silica deposition in Malitbog. However, GSX inhibition (up to 2.0 ppm) is still cheaper than work-over and acidizing.