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Gel treatment is a cost-effective method for oilfield conformance control. Traditionally, in-situ gels formed by the reaction of polymer and crosslinker at reservoirs have been used widely to control conformance. However, a newer trend is to apply preformed particle gels (PPGs) for this purpose because they are formed at surface facilities before injection and they overcome some distinct drawbacks inherent in in-situ gelation systems, such as lack of control over the gelation time, gelling uncertainty due to shear degradation, chromatographic fractionation or change of gelant compositions, and dilution by formation water. PPGs are characterized as having robust gel chemistries and as being highly insensitive to petroleum reservoir environments and interferences. They can resist temperatures up to 120 ºC (250 ºF) and are compatible with any kind of formation water. In this paper, we describe and discuss our extensive laboratory and field experiences with the widely applied and successful PPG technology. Highlights of illustrative PPG field applications and results are presented. An overview of what over a decade of experience in applying the PPG technology has taught us is discussed. This includes a discussion of classifying and distinguishing conformance problems and treatments, attributes of good candidate wells, requirements that must be met in candidate wells, gel treatment elements that must be implemented successfully to achieve success, and the guidelines as to where PPG conformance treatments are applied most successfully.
This paper introduces a new diverting agent that has been developed to control water production in high salinity, high temperature reservoirs. Several dry gels have been crushed and sieved to obtain different cuts of gel particles. The dry powdered gel particles swell in water to give a stable suspension. The swollen pre-gelled (PG) particles do not dissolve in water and can move inside the porous media. Results presented here include test pilots and laboratory experiments. PG particles have been successfully used in two recent pilot tests in Shengli Oilfield, China. Micromechanisms of particle motion and oil mobilization have been studied through transparent glass micromodels experiments. Core tests have also been conducted to verify the selective placement of the particles. According to the experimental laboratory results we can conclude that the mechanisms responsible for the enhancement of oil production are the following:
After swelling and under a high pressure gradient (near the wellbore), the pre-gel particles can deform to pass through small pore throats, ensuring displacement of the residual oil. When the pressure gradient is too small (away from the wellbore) the particles plug the pore throats changing the flow pattern inside the reservoir.
In addition, as the gelation is accomplished before the injection, the PG particle technique overcomes some of the most important problems which can be encountered by classic gel treatments: lack of control of the gelation time, lack of control of the stability of the gel formed or ungelation due to adsorption, dilution or degradation of the polymer or pH change. Furthermore, due to these specific characteristics, this process can be used in oilfield environment that would prevent the in-situ gelation of a classical gelant solution.
We investigated how different types of gels reduce permeability to water and gases in porous rock. Five types of gels were studied, including (1) a "weak" resorcinol-formaldehyde gel, (2) a "strong" resorcinol-formaldehyde gel, (3) a Cr(III)-xanthan gel, (4) a Cr(III)-acetate-HPAM gel, and (5) a colloidal-silica gel. For all gels, extensive coreflood experiments were performed to assess the permeability-reduction characteristics and the stability to repeated water-alternating-gas (WAG) cycles. Studies were performed at pressures up to 1,500 psi using either nitrogen or carbon dioxide as the compressed gas. We developed a coreflood apparatus with an inline high-pressure spectrophotometer that allowed tracer studies to be performed without depressurizing the core. We noted several analogies between the results reported here and those observed during a parallel study of the effects of gel on oil and water permeabilities.
This work constructed transparent fracture models to visually track swollen preformed-particle-gel (PPG) propagation through open fractures and water flow through PPG placed in the fractures. During injection, PPG propagated like a piston along a fracture and a gel pack was formed in the fracture. When water broke through the particle-gel pack after PPG placement, several channels were created that discharged water from the outlet while water was being injected. Investigation of factors that influence PPG injectivity and plugging efficiency revealed that PPG injectivity increases with fracture widths and flow rates but decreases with brine concentrations (on which the PPG swelling ratio depends). PPG can reduce the permeability for the fractures with different widths to the same level. Full-factorial experimental design analysis was performed to rank the influence of injection rate, fracture width, and PPG swelling ratio on pressure response, resistance factors, and injectivity.
Many conformance control treatments rely on the ability of gels to extrude through fractures during the placement process. This paper describes an experimental investigation of the mechanism for propagation of a Cr(III)-acetate-HPAM gel through fractures. When large volumes of this gel were extruded through a fracture, progressive plugging (i.e., continuously increasing pressure gradients) was not observed. Effluent from the fracture had the same appearance and a similar composition as those for the injected gel, even though a concentrated, immobile gel formed in the fracture. The concentrated gel formed when water leaked off from the gel along the length of the fracture. The driving force for gel dehydration (and water leakoff) was the pressure difference between the fracture and the adjacent porous rock. During gel extrusion through a fracture of a given width, the pressure gradients along the fracture and the dehydration factors were the same for fractures in 650-md sandstone as in 50-md sandstone and 1.5-md limestone. A simple model was developed that accounted for many of the experimental results.
Using wide ranges of gel age, gel velocity, and fracture conductivity or tube diameter, Cr(III)-acetate-HPAM gels were studied as they extruded through fractures and tubes. Gels exhibited shear-thinning behavior in fractures and tubes that correlated with the gel superficial velocity and the fracture width or tube diameter. In fractures with sufficiently small widths, gels dehydrated during extrusion, thus reducing the rate of gel propagation. This effect was more pronounced as the fracture width decreased. Using the experimental results, a numerical study was conducted to compare placement of preformed gels and water-like gelants.
Injecting 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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