Improved Oil Recovery Via Polymer Nanogel Flooding from Fractured, Oil Wet Iranian Carbonate formation

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1 Improved Oil Recovery Via Polymer Nanogel Flooding from Fractured, Oil Wet Iranian Carbonate formation Mahmood Torabi Angaji1, Ali Karimi2 1. Chemical Engineering Dept, Faculty of Engineering TEHRAN University 2. Engineering &Process Development Division, NIOC. Research Institute of Petroleum Industry (RIPI) Abstract: Polymer flooding can yield significant increase in percentage recovery when compared to conventional water flood projects in certain reservoirs. The addition of a polymer yields two benefits 1) reduces the total volume of water required to reach the ultimate residual oil saturation and 2) increases sweep efficiency due to improved mobility ratio. Many successful Projects have reported increased recoveries of 5-15%. However many unprofitable projects were the result of inadequate reservoir description or problems with the polymer system being used. In recent years the polymer gel flooding technology has been field tested extensively and can be classified as proven technology. However, there are limitations with the existing polymer gel technology such as degradation (thermal, mechanical, bacterial, shear ) or Polymer rheological. In order to overcome the weakness and limitations of present Polymeric gels. a novel Nanogel based on polyacrylamideclay (MMT) has been developed and its' Properties have been experimentally tested as a suitable candidate in polymer flooding for oil field projects. The experimental results shows a good mechanical, thermel and Rheological properties as compared to the exsiting polymer gel available in market. Introduction: - Many reservoirs in IRAN are fractured and much oil remains in these fractured, carbonate oil reservoirs after water flooding or gas injection. The high remaining oil saturation is due to a poor sweep in fractured reservoirs. Sweep efficiency is an important factor in oil recovery, especially in fractured formation. Fractures systems generally have a broad distribution of fracture widths. The wide fracture will act as thief zones for the injected fluid and little of the injected fluid will reach the narrow fractures. Another reason for high remaining oil saturation in fractured oil-wet, carbonate formation is that the matrix will retain oil where the upward oil film flow path is interrupted by fractures. As a result, oil is retained in oil-wet pores by capillarity. The basic principle behind the use of flooding polymers as an oil recovery technique is to reduce the mobility of the injected water thereby reducing the tendon eye of water to. beak past the oil and assisting to give an even sweep through the reservoirs. The objective is to improve the total recovery of oil within a reduced time frame and improve the Produced oil/ water ratio. In the case of a reservoir that is nonhomogenous the effects of high oil viscosity are magnified. The more mobile water will tend to break rapidly via the zones of higher Permeability.

2 The objective in this paper is to identify the important factors affecting the improved oil recovery by polymer Nanogel flooding technology from fractured, oil wet Iranian carbonate formation. Profile Modifications Using Polymer Nanogels Use of polymeric systems for profile modification or plugging is increasing. Currently, Cement or mechanical systems are the most widely used methods for permanently Controlling water flow. However, Cement and mechanical methods are effective only in the well bore area. In reservoirs with good vertical permeability and no intervening shales, these profile Control methods are not successful. Cement s greatest disadvantage is that it can not penetrate the formation matrix, so it s effect is limited to within the drilled hole. Fig. (1) illustrates water by passing a cement plug back. There are three types of water control problems in water flooding systems: 1) Open flow paths-this kind of problems is associated with the presence of fissures, fractures or faults or to oil being confined to the rock matrix. Fig (2). 2) Fluid fingering or viscous fingering (edge water) is caused by post aerial sweep which is common in heterogeneous reservoir. 3) Conning water or bottom water problem- where oil is being produced near the oil water contextures, too much water is also being produced. Fig (3). All these three problems can be handlled by polymer Nanogel injection to eliminate or minimize its effects, Fig(4), Fig(5). Development of polymer Nanogel Technology can be used to solve a number of production problems associated with water flooding and improved recovery technologies in the oil industry. These Nanogels can be formulated to range in consistency from a slightly thickened water solution to an elastic solid, and can be function as blocking agents by invading naturally occurring fractures and diverting flow into other parts of the reservoir improving the areal conformance of the flood water Fig(5). Also, a different formulation with the same chemistry can be used for total zone shutoff. Controlled or regulated water injection using Nanogel can create a much more uniform flood front and oil bank, and can result in a much better sweep efficiency. Injection Nanogel will allow all zones in a formation to be processed simultaneously rather than individually. Problems and limitations associated with existing polymer gel Technology: The polymer gel flooding process has been field tested extensively and can be classified as proven Technology. However the following Problems are associated with existing polymer gel Technology: 1) Low Mechanical properties 2) Bacterial degradation 3) Polymer slug break down due to high crosslinking density and the polymer debris will be coming out of the producing well. 4) Degradation of polymer gel due to the following parameters:

3 - Temperature above 80ºc - Presence of free radicals - PH - Shear forces - Salinity-presence of ions in reservoirs. 5) High Shrinkage Some of interesting properties of Nanogel polymer for IOR 1- Higher mechanical properties as compared to the ordinary polymer gel injection used in past. (higher mechanical modulus with lower cross linking density) 2- Excellent elongation recovery (rubber like) 3- High Temperature resistance (350ºc -400ºc) 4- Low shrinkage 5- Better control of gelation time. 6- Resistance to Ionic properties of the oil/ water mixture 7- Low concentration is used (400ppm- 500ppm) Some of parameters affecting formulation of Nanogel for injection 1- Nanoclay type 2- Nanoclay concentration 3- Temperature 4- PH 5- Rate of mixing 6- Ionic strength of the solution 7- pressure Experimental Crosslinking polymer solutions to form a gel provides better physical plugging of pores, This is achieved with polyacrylamide-clay solutions by adding multivalent ions that form a three- dimensional polymer structure. Polyacrylamide gel systems are in common use, this system is based on the controlled release of multivalent metal ions, which react with a polymer to form a gel. In this process, the chromium Redox process, Consist of injecting a polymer slug containing chromium (VI) Cations, a chemical reducing agent. and a nanoclay(mmt). The reducing agent reduces the chromium (VI) to chromium (III). Chromium (III) reacts with the polyacrylamide to form the Nanogel. Solution Nanogel (and pump) time depends upon the rate of chromium (VI) reduction. Concentration and the type of reducing agent can be varied to control gelation times from a few minutes to several days. It is necessary in all IOR polymer flooding products that the laboratory quality control be of the highest standards. Polymer flooding products are needed to an extremely rigorous test procedures prior to injection. For Quality control the following parameters are analyzed: viscosity. Screen factor- this factor can be used to determine the effect of shear degradation on site before and after injection.

4 Filter ratio. Percent hydrolysis Free monomer content Mechanical and thermal properties of Nanogel formed Rheoloyical properties References: 1. An Overview and Evaluation of the North-Burbank unit Block A Polymer Flood Project, Osage County, Oklahoma SPE: The Polymer: Aluminum Citrate Polymer 2. Performance analysis of Several Polyacrylamide Floods in North German Oil Fields SPE: The Polymer: Polyacrylamide 3. Profile Modification in Water Injection Wells by Polymer Treatments SPE: The Polymer: Hydroxyle-Ethyle-Cellulose Actual Field Data Show that Production Costs of Polymer Flooding can be lower than water flooding

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