Evaluation of an Environmentally Acceptable Colloidal Silica- Based Conformance System
Authors: Patil Prajakta Ratnakar , Boye Bjorn , Devaraj Murugan 
Published: March 2015

Abstract
Excessive water production greatly affects the economic life of producing wells. Though a variety of chemicals are used by the industry to control water production, most of them are not accepted in the regions with strict environmental regulations. Based on this background information, a new, green, environmentally acceptable conformance sealant has been evaluated for its performance. This conformance sealant is a two-component system that incorporates colloidal silica and an activator.
This work presents the results of laboratory experiments conducted to select an appropriate activator that could provide adequate gelation times. Sodium chloride brine with a specific gravity (SG) of 1.04 was identified as a better activator than the others considered. Static and dynamic gelation times were evaluated at different temperatures up to 150°C. The effects of pH and temperature on gelation times of the new conformance system were also studied. In the pH range of 5 to7, particle collision predominates and leads to faster aggregation and formation of gel. Hence, the gelation time is minimized in this range. At pH above 7, silica particles exhibit charge repulsion resulting from surface ionization in alkaline solution, which leads to longer gelation times. An increase in temperature causes an increase in the particle collision, leading to shorter gelation times.
Carbonate core tests and dynamic sandpack-flow experiments were conducted to evaluate the treatment effectiveness and thermal stability of the system due to ageing. Results showed that this system can provide effective permeability reduction in sandpacks for extended periods of time. The conformance sealant forms a gelled mixture while passing through a carbonate core, plugging the pores and voids. Laboratory experiments also proved that the sealant can be easily and effectively viscosified using a xanthan-based polymer. This viscosified version could help in providing viscous diversion to cover longer intervals.

Introduction
Water production from oil wells is highly undesirable and is about four to five times higher than the worldwide oil production (Shafian et al. 2010). High water cut largely affects the economic life of producing wells and is also responsible for many oilfield-related damage mechanisms, such as scale deposition, fines migration, asphaltene precipitation, corrosion, etc. (Shafian et al. 2010; Curtice et al. 2008; Smith and Ott 2006). This also leads to increased operating costs to separate, treat, and dispose of the produced water according to environmental regulations. Throughout the years, the industry has developed different techniques to control water production, including mechanical isolation, squeeze cementing, and different chemical treatments. These techniques are commonly referred to as conformance-control techniques (Thomas et al. 2000; Zhao et al. 2006; Vasquez et al. 2005). The general approach of any chemical-treatment technique has been to inject a mixture of reagents, initially low in viscosity, into the high-permeability, water-producing zones. After sufficient time and at elevated temperatures, the mixture of reagents forms a barrier that partially blocks the flow of water and or gas. Polymers have been widely used as conformance-control materials since the 1960s. Conformance control includes injection of aqueous solutions of polymer and activators into high-permeability flow paths whereby polymers are gelled and crosslinked therein (Mack and Smith 1994; Ortiz et al. 2004). However, effective use of polymers requires resolving technical problems, such as difficulty in controlling gelation kinetics, mixing of polymer-crosslinker, and undesirable gel-phase separation in heterogeneous reservoirs. Though a wide range of conformance chemicals are available, most of them are not accepted in environmentally stringent regions, such as the North Sea. Hence, there is always a growing need for environmentally acceptable conformance products in the North Sea region.