An Experimental and Numerical Study of Polymer Action on Relative Permeability and Capillary Pressure
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Cited by 3 publications
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Candidate and Chemical Selection Rules for Water Shutoff Polymer Treatments
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWater shutoff treatments with polymers that selectively reduce the water permeability can be effective to reduce water production from hydrocarbon wells that cannot be treated with conventional approaches. In this paper we present an analysis that starts from the principal operative questions and defines the main physical-chemical issues to be understood in order to develop a reliable technology. These issues are discussed by showing results from a number of experimental and simulation studies conducted in our laboratories with the goal to link the understanding of basic mechanisms to the development of operational rules. This work shows the elaboration of understanding-based rules for candidate and chemical selection to be an effective strategy for the application of this technology in the field.
Candidate and Chemical Selection Rules for Water Shutoff Polymer Treatments
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWater shutoff treatments with polymers that selectively reduce the water permeability can be effective to reduce water production from hydrocarbon wells that cannot be treated with conventional approaches. In this paper we present an analysis that starts from the principal operative questions and defines the main physical-chemical issues to be understood in order to develop a reliable technology. These issues are discussed by showing results from a number of experimental and simulation studies conducted in our laboratories with the goal to link the understanding of basic mechanisms to the development of operational rules. This work shows the elaboration of understanding-based rules for candidate and chemical selection to be an effective strategy for the application of this technology in the field.
Adsorption of water-soluble polymers in porous rocks is known to reduce water permeability much more than oil permeability. This effect is often referred to as "Disproportionate Permeability Reduction" (DPR) and can be used in production well treatment to reduce the water cut. This paper deals with nonionic polyacrylamide adsorption on carbonate rocks having different wettability properties. Cores were used first in their native water-wet condition, and after a wettability modification, which was obtained by aging cores saturated with a polar crude oil at 60°C for 6 weeks. Efficiency of the treatment was attested and quantified by strong changes in the wettability index as measured using Amott tests. Drainage and imbibition cycles were performed on these carbonate samples before and after polymer treatment. Polymer adsorption and oil/water relative permeabilities were compared for both media. While the quantity of adsorbed polymer is almost the same on water-wet and on wettability-modified cores, adsorption rates, estimated from viscosity profiles of effluents, are significantly different, suggesting that the polymer slowly restores part of the water-wet character of the native core. All our results indicate that polyacrylamide adsorbs on the rock whatever being the wetting conditions. While disproportionate permeability reduction is always observed, DPR is greater in low-permeability cores. Introduction. Excessive water production is a problem of central importance for field operators. High water cut can lead to stop production for economical reasons. Among the solutions proposed to circumvent this problem, direct injection of polymer in the surroundings of the wellbore has been shown to be an efficient one. Polymer or gel injections in producing wells are able to lower the water cut by selectively reducing water relative permeability of the rock with respect to oil relative permeability. From a physical point of view, adsorption of water-soluble polymer on pore walls is known to modify two-phase flow properties of a porous medium. Several mechanisms involved in the action of polymers or gels have been reported in the literature and put forth to explain what is often referred to as "Disproportionate Permeability Reduction" (DPR):Shrinking/swelling of polymer depending on phase flow (Menella et al.1). A possible explanation lies in the fact that adsorbed polymer shrinks while oil flows, and swells in the presence of water. This mechanism is consistent with the fact that the stress applied on the polymer layer by the flow is strong enough to induce a significant deformation of the layer. However, more experimental work is required to confirm the evidence of this phenomenon.Segregated pathways. This hypothesis put forth by Liang et al.2, and more recently by Nilsson et al.3 suggests that water and oil are flowing in two different pore networks. Consequently, a hydrophilic polymer flowing preferentially through the water network is able to reduce water permeability much more than oil permeability. On this basis, polymer injection was performed into rocks having different wettability properties and Nilsson et al.3 found that DPR was greater for porous media of mixed-wettability, which is consistent with the fact that water and oil pathways are better separated in this case.
Candidate and Chemical Selection Guidelines for Relative Permeability Modification (RPM) Treatments
Summary Water shutoff treatments with polymers that selectively reduce the water permeability can be effective at reducing water production from hydrocarbon wells that cannot be treated with conventional approaches. This paper describes the elaboration of understanding-based rules for candidate and chemical selection to guide the application of this technology in the field. These rules are based on an analysis of the principal operative issues and on experimental and simulation studies that explore the physical and chemical mechanisms governing treatment efficacy. Introduction Excessive water production from oil and gas wells is a widespread problem with important economic consequences. High water rates reduce well productivity, increase operating expenditures, and can completely block production from gas wells. The need for effective water shutoff technologies in the oil industry will become more urgent in the future, as mature fields will be affected by increasing water cuts. Development of fields in new exploration frontiers, such as offshore deep water, marginal fields, and regions with severe environmental restrictions, will require improved capabilities to reduce and to manage efficiently the water produced. Excessive water cuts can be reduced by recompleting the well or by placing mechanical devices to isolate water-producing zones. These solutions, however, are expensive and in microlayered formations can cause the loss of substantial volumes of hydrocarbons. Polymer gels often represent a valid and economic alternative to mechanical isolation, but their application requires that water zones are identified and isolated. Therefore, in microlayered formations and/or gravel-pack completed wells (i.e., where zone isolation is not feasible) excessive water rates can determine a premature abandonment when no efficient and low-risk water-control technologies are identified. Relative permeability modification (RPM) bullheading treatments are an option to consider when conventional approaches cannot be applied. These consist of the injection, into all open intervals, of chemicals that selectively reduce the permeability to water.1–7 Systems most commonly used for this purpose are solutions of water-soluble, high-molecular-weight polymers that adsorb onto the pore surface and change the flow properties of the porous media. The main advantages of this approach are its low cost, because the chemical is used in limited quantities and the treatment does not require zone isolation; low risk, because the polymer reduces the water permeability without plugging the formation; and low environmental impact. Although application of these treatments in the field is relatively simple, field tests generally have been carried out in the absence of reliable criteria to select candidate wells and chemicals. This trial-and-error approach is responsible for the moderate success ratio and for the difficulties in interpreting field-test results.5 This paper reports results of research studies conducted in the Eni Group that were aimed at the application of polymer treatments for controlling water production from hydrocarbon wells. Starting from the main operative questions, we have defined and analyzed key mechanisms that operate at the different scales in the reservoir; from the results obtained it has been possible to derive some guidelines that subsequently have been applied successfully in a field test in a gas reservoir in southern Italy. RPM Treatments With Polymers: Key Operative Questions From the operator's point of view, one of the most important needs in the field application of RPM treatments is the ability to formulate reliable answers to these questions:Will this well respond to an RPM treatment?What chemical system and operating conditions (i.e., volumes, flow rates, etc.) will give optimal results? This implies the availability of rules to be applied in each particular case on the basis of field data. Development of such reliable criteria requires systematic study of key physical and chemical mechanisms operating in the different phases of a treatment (i.e., selection of the chemical, injection, and posttreatment). Fig. 1 illustrates some relevant mechanisms that must be considered in the development of candidate and chemical selection rules. This paper discusses these mechanisms with particular reference to the application of these treatments in gas wells with sandstone formations; the last section presents some guidelines for candidate and chemical selection that were deduced from the results obtained and highlights areas that need further development. Physical Mechanisms Role of the Water-Production Mechanism. The first aspect to be considered during the selection of a candidate well for an RPM treatment is the mechanism by which water arrives at the well, as illustrated in Fig. 2. Early field experience suggested that RPM treatments are most effective in multilayered matrix formations in which one or more layers are still saturated by hydrocarbons8,9 as shown in Fig. 2c. To understand the most favorable cases for these applications, we performed some reservoir simulations that helped us to draw some general conclusions. The simulations were run on a model reservoir with geometry and production data from an Italian gas well that produces from a 3 m-thick pay subdivided in two sand layers separated by a shale barrier. Because the main objective of these simulations was to assess the impact of the water-production mechanism on the effect of an RPM treatment, we modified petrophysical parameters, capillary pressure, and relative permeability curves to build two different scenarios qualitatively consistent with the available production data and illustrated in Fig. 3. The scenario depicted in Fig. 3a shows water arriving only from the upper layer. The scenario depicted in Fig. 3b shows mobile water and gas in both layers. The effect of the RPM treatment was simulated by applying a reduction factor to the relative permeability curves in the near-wellbore region, approximately 5 m deep in the formation. The relative permeability to water was reduced by a factor of 90% while the relative permeability to gas was left unchanged.
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