The performances of new microgels specifically designed for water shutoff and conformance control were extensively investigated at laboratory scale. These microgels are preformed, stable, fully water soluble, size controlled with a narrow size distribution, and non-toxic. They reduce water permeability by forming adsorbed layers soft enough to be very easily collapsed by oil-water capillary pressure, so that oil permeability is not significantly affected. Since the manufacturing process of these new microgels make possible to vary chemical composition, size and crosslink density, they can be designed as desired to meet the requirements of a given field application. The laboratory results reported in this paper concerns mainly three microgel samples having significantly different crosslink densities. We describe the relevant laboratory methods used to determine main microgel characteristics. The microgels have remarkable mechanical, chemical and thermal stability. Their behavior in porous media have been investigated extensively, showing that:their propagation distance is only limited by the volume injected,their injectivity is facilitated by a shear-thinning behavior andwater permeability reduction can be achieved as desired by controlling the thickness of adsorbed layer. Thus, this new microgels, now available at industrial scale, look as very promising tools, not only for water shutoff but also for conformance control in heterogeneous reservoirs. Introduction Background In a global context of growing energy needs with a perspective of depletion of oil and gas resources, extending the life of hydrocarbon reservoirs is a real challenge for the decades to come. In that situation, as well as for environmental reasons, reducing significantly water production and improving oil recovery efficiency is an important goal for oil industry. Thus the development of more reliable techniques using "green" products for water-shutoff, conformance, and mobility control is of crucial interest. Among the methods available to reduce water production [1], injecting a gelling system composed of a polymer and a crosslinker has been widely used [2–5]. In this process, the gel is formed in-situ. Since gelling properties have been found to depend on many factors [6–11], the gelling time, the final gel strength and also the depth of the gel penetration is quite difficult to predict. This difficulty results from the uncertainties concerning different factors: shear stresses both in surface facilities and in near-wellbore area and also physico-chemical environment around the well (pH, salinity and temperature). Moreover, both polymer and/or crosslinker adsorption in the near-wellbore region and dilution by dispersion during the placement can affect the effectiveness of the treatment. To overcome these severe drawbacks, different authors have recently proposed new methods, aimed at improving the process by injecting preformed gels particles or dilute gelling systems. Bai et al. method [12,13] consists in drying, crushing and sieving polymeric gels prior to injecting them. Mack et al. [14,15] method consists in obtaining "colloidal dispersion gels" (CDG) by crosslinking low concentration polymer solutions with low amounts of chromium acetate or aluminium citrate. This process slows down the gelation kinetics, so that, on a well injection time scale, those systems only form separate gel bundles, thus making possible to enter the matrix rock. However, the in-depth propagation of these two of gels remains questionable. In 1999, Chauveteau et al. introduced [16] a completely new concept which consists of injecting fully water soluble, non-toxic, soft, stable and size-controlled microgels into the reservoir. A first type of microgels, using an environmentally friendly zirconium crosslinker, has been extensively studied in the past years, regarding both the understanding of gelation mechanisms and the transport properties in porous media [16–23]. More recently, a second type of microgels, which are covalently crosslinked, was introduced [24]. These microgels, now available at industrial scale, have been shown to have very attractive properties for both water shutoff and conformance control operations.

The present paper describes the first field application of a new water shutoff technology using size-controlled microgels for the treatment of a gas storage well. The treatment was designed from an integrated study combining laboratory coreflood experiments and near-wellbore reservoir simulations. The candidate well (open hole completion with liner) was drilled in a sandstone reservoir formation made of a succession of layers with different petrophysical quality all connected each other. The presence of a thin high-permeability streak located in the bottom part of the open interval was assumed to be the main factor of excessive water production. A bullhead option was chosen for the treatment. Microgel size (around 2 micron) favored their placement in the high-permeability layer, whereas the penetration in the rest of the reservoir was expected to be very superficial. The treatment was performed in June 2005. Due to pressure constraints, the volume of the treatment had to be reduced to 26 m3 only. A backflush of gas was necessary to recover gas injectivity. The well was kept on production during the whole winter season 2005–2006. Water production was significantly reduced thus enabling higher gas rate production. A positive impact on sand production was also observed. Well behavior was in good agreement with model predictions. Introduction Water intrusion from aquifer in underground gas storage wells affects well performances more especially when the Gas-Water Contact is high in the reservoir and the gas rates due to peaks of demand are intense. Some pioneer field operations have shown that polymer technology can be an efficient way to solve this problem.1,2 The principle of the treatment is shown in Figs. 1 and 2. The polymer injected into the open interval invades the different layers surrounding the wellbore with a deeper invasion of the higher permeability layers, which are frequently the main water producing zones. This type of product adsorbs on the formation rock almost irreversibly and induces a selective reduction of the relative permeability to water with respect to the relative permeability to oil or to gas. This unique property of water soluble polymers and gels is well documented in the petroleum literature, as "RPM" (Relative Permeability Modification) or "DPR" (Disproportionate Permeability Reduction). Indeed, although the origin of this phenomenon remains controversial, there is a large consensus about the reality of this property, which has been observed during either water/oil or water/gas two-phase-flow coreflood experiments with many different core materials and polymer/gel species.3–12 The combination of a polymer/gel favorable placement in the water-producing layers together with RPM effect induces lower water cut production after treatment. If the loss of Productivity Index can be compensated by higher drawdown, the treated well can produce more oil or more gas. Liang, Lee and Seright pointed out that RPM effect is not enough to guarantee a successful treatment, especially in radial flow conditions.13 In fact , a perfect RPM product, which reduces the relative permeability to water without inducing any change in the curves of relative permeability to oil or to gas, may affect the effective permeability to oil or to gas through an increase in water saturation. This occurs when the fractional flow of water from the oil producing layers is significant. The water blocking effect increases with the depth of penetration of the polymer/gel in the layer and with the intensity of the permeability reduction to water. Therefore, preventing a deep penetration of RPM products into the oil/gas layers can be considered as mandatory to achieve a good water shutoff treatment.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe performances of new microgels specifically designed for water shutoff and conformance control were extensively investigated at laboratory scale. These microgels are preformed, stable, fully water soluble, size controlled with a narrow size distribution, and non-toxic. They reduce water permeability by forming adsorbed layers soft enough to be very easily collapsed by oil-water capillary pressure, so that oil permeability is not significantly affected. Since the manufacturing process of these new microgels make possible to vary chemical composition, size and crosslink density, they can be designed as desired to meet the requirements of a given field application. The laboratory results reported in this paper concerns mainly three microgel samples having significantly different crosslink densities. We describe the relevant laboratory methods used to determine main microgel characteristics. The microgels have remarkable mechanical, chemical and thermal stability. Their behavior in porous media have been investigated extensively, showing that: 1) their propagation distance is only limited by the volume injected, 2) their injectivity is facilitated by a shear-thinning behavior and 3) water permeability reduction can be achieved as desired by controlling the thickness of adsorbed layer. Thus, this new microgels, now available at industrial scale, look as very promising tools, not only for water shutoff but also for conformance control in heterogeneous reservoirs.

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