Characteristics of Microgels Designed for Water Shutoff and Profile Control

Abstract

Abstract Two microgel samples were prepared by cross linking an acrylamide-based terpolymer solution with a non-toxic zirconium crosslinker under controlled shear flow. The characteristics of these two microgel samples were determined by using different laboratory techniques in order to evaluate their performances for water shutoff or profile control operations. The microgel size was measured by Photon Correlation Spectroscopy (PCS) in dilute regime and at small scattering vector. As predicted by our theoretical model, microgel size decreases as the −1/3 power of applied shear rate. The size of these microgels is almost independent of pH, salinity and temperature in the domain investigated. Their mechanical and thermal stability is also satisfactory. In addition, such microgels can be easily injected into porous media without any sign of plugging. All these results suggest that these microgels should be good candidates for water shutoff and profile control operations. However, a further investigation is required to optimize their preparation to obtain the desired properties for a given application. Introduction Continuous extraction of hydrocarbons from a subterranean reservoir leads to a reduction of the pressure available to further produce the remaining oil. To maintain or increase the driving force, water is usually injected as a displacing fluid. Nevertheless, oil-bearing rocks generally comprise many layers of varying permeability so that the injected water, which is usually less viscous than oil, tends to follow the more permeable paths and finally water often by-passes oil-bearing zones. These high permeability layers act as channels transporting a large fraction of injected water so that the "tighter areas" where the residual oil remains trapped cannot be swept efficiently. In addition, a strong heterogeneity of the injection profile is often observed owing to the drink-in of the injected fluid by the high permeable streaks. These unwanted water production and poor homogenous injection profile result in operational and economic problems such as decreased oil production and increased water amount in producing fluid, extra cost for oil/water separation, water treatment and transportation. Other problems may include corrosion and an increased tendency for the formation of emulsions and scale in the reservoir. It is thus highly desirable to control the unwanted water production (known as water shutoff) [1,2] and to homogenize the flow distribution in the injection wells (so-called profile control) [3]. An often-used procedure is to treat the more permeable zones with a gelling fluid, i.e., a water-soluble polymer mixed with an organo-metallic crosslinker, which is expected to reduce the water income from high permeability layers and thus cause a fluid flow diversion. This technique has been investigated at both laboratory and field scale over the last three decades [4–7]. Conway et al [8] reported in early 1980s that water-soluble polymers can be gelled with 22 metal ions among which chromium is the most frequently employed. In oil production, chromium is widely used in spite of the notorious toxicity of Cr-based compounds particularly Cr(VI) which was reported to be carcinogenic [10] and thus environmentally unacceptable [10–13]. A recent paper [14] pointed out that relatively poor thermal stability and gel syneresis are main disadvantages of chromium-based gels. These deficiencies, along with more severe regulations [15,16] for the use and discharge of oilfield chemicals, stimulated investigations to find alternative crosslinkers with better performance and environmental acceptance. Zirconium has been identified as such a promising candidate [10,17–19]. Compared with chromium, zirconium forms more thermally stable metal-oxygen bond [20,21], has a much lower toxicity [10,20] and a much better biodegradability [22]. In addition to be attractive candidates as crosslinkers for water shutoff [10,23,24] and profile control [23,25], zirconium chelates have been already widely used to crosslink water-soluble polymers during hydraulic fracturing operations [26–33], in drilling fluids [11,21,22,33–37] and for clay stabilization [38,39].