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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.

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Current experimental methods used to determine pore size distributions (PSD) of porous media present several drawbacks such as toxicity of the employed fluids (e.g., mercury porosimetry). The theoretical basis of a new method to obtain the PSD by injecting yield stress fluids through porous media and measuring the flow rate Q at several pressure gradients ∇ P was proposed in the literature. On the basis of these theoretical considerations, an intuitive approach to obtain PSD from Q(∇ P) is presented in this work. It relies on considering the extra increment of Q when ∇ P is increased, as a consequence of the pores of smaller radius newly incorporated to the flow. This procedure is first tested and validated on numerically generated experiments. Then, it is applied to exploit data coming from laboratory experiments and the obtained PSD show good agreement with the PSD deduced from mercury porosimetry.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractInjecting stable, preformed microgels as relative permeability modifiers to reduce water production minimizes the risk of well plugging or the absence of efficiency inherent to a technology based on in-situ gelling. Recent investigations showed that microgels formed by crosslinking a polymer solution under shear are soft, size-controlled, quasi-insensitive to reservoir conditions, stable over long periods of time and can control in-depth permeability by adsorbing onto all types of rock surface. The new laboratory studies reported in this paper aimed at knowing how to control the kinetics of crosslink formation by ionic strength and at determining the role the interactions between microgels on their propagation in porous media. The reported experiments include: 1) gelling tests at different ionic strengths, 2) measurements of viscoelastic properties of solutions, 3) determination of both microgel density and microgel-microgel interaction parameter for different conditions of stabilization, 4) the relation between the interaction parameter and the mode of adsorption of microgels. Partly attractive microgels were found to adsorb by forming multilayers and thus to induce drastic permeability barriers. Fully repulsive microgels adsorb as a monolayer and propagate easily in porous media at long distances depending only on the quantity of microgel injected. Thus, by controlling both gelling and stabilization processes, microgels can be produced to be either diversion agents or disproportionate permeability reducers to control water permeability at long distances from the wells.

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