The use of polymer solutions to enhance oil-displacement efficiency by seawater injection in North Sea oil reservoirs has been investigated. We have evaluated over 140 polymers for viscosity retention and porous media flow performance under high temperature (90 deg. C), high salinity, and high pressure. Scleroglucan polymers give the best performance in our tests. Polyacrylamides (PAAm's) are particularly unsuitable for mobility control. Using polymers to enhance seawater injection and waterflooding processes is not practical in North Sea reservoirs, but selective injection may improve local sweep efficiencies. Introduction North Sea Waterflooding With 95% of Ne crude oil reserves of Western Europe and 90% of the current crude oil production coming from deposits lying under the North Sea bed, oil producers have been prepared to exploit them by making the high capital investment in the new technology of deepwater production platforms. Seawater injection schemes have been introduced early in the life of many/ North Sea fields, and are featuring in Middle East and North and South American offshore field development programs. Most North Sea oils are fairly light, and many can be produced at high rates from thick oil zones in good permeability sandstone reservoirs. The principal aim of the injection schemes has been to maintain reservoir pressure with peripheral injectors positioned mainly below the oil/water contact. Until now, the main problem has been to keep the seawater injection rates high enough. With the incentive of producing more of the North Sea oil reserves, research is being done to ameliorate some other foreseeable problems. One major problem is the severe channeling of injection water, leading to seawater breakthrough into production wells, and the likelihood of barium sulfate scale formation. Channeling resulting from mobility ration effects may be through high-permeability layers (most North Sea reservoirs are very heterogeneous), fractures, or viscous oils. Another factor reducing efficiency is the general rise of the oil/water contact, causing the producing wells to cut excessive quantities of water. Selectively placed polymer injection treatments may reduce channeling, and polymer squeeze treatments may restrict water production. Polymers and other chemical additives need to have adequate chemical stability in the high-salinity, high-temperature environment of North Sea oil reservoirs. Accurate prediction of reservoir performance of enhanced oil recovery (EOR) techniques requires precise data on the behavior of crude oils and relevant aqueous systems in porous media at reservoir conditions. This paper reports thermal stability and porous media test results for a range of polymer types and discusses their possible use to augment North Sea waterflooding. Experimental Polymers Tested. We screened more than 140 polymers, which we classify as polyacrylamides (PAAm's), polyvinylpyrrolidones (PVP's), hydroxyethylcelluloses (HEC's), cellulose sulfate esters (CSE's), guar gums, xanthans, and scleroglucans. Solution Preparation. Solutions were made up in the manner of Hill et al. in seawater (0.45 um filtered) obtained from Chesil beach on the English southwest coast. The seawater contained residual (less than 0.2 ppm) hypochlorite biocide, from a treatment added on collection. Polymer solutions were characterized by filtration profiles through 5-um Millipore filters (at 0.069-MPa driving pressure, and following prefiltration) and by Brookfield ultralow viscometer measurements at 25 and 55 deg. C, with parameters to represent the solution viscosity at high and low shear rates. SPEJ P. 353^
In view of the high permeability, fairly high oil viscosity, moderate reservoir temperature, low salinity formation water and availability of low salinity injection water, the Marmul Al Khlata formation was considered a good candidate for polymer flooding. A pilot consisting of an polymer flooding. A pilot consisting of an unconfined 5-spot on a 200 m spacing was initiated in May 1986. After the injection of a water preflush of 0.23 PV, a polymer slug of 0.63 PV and preflush of 0.23 PV, a polymer slug of 0.63 PV and a water postflush of 0.34 PV, the total oil production was 59% of STOIIP. The injection of production was 59% of STOIIP. The injection of polymer resulted in a sharp increase in oilcut in polymer resulted in a sharp increase in oilcut in all four wells. From the viscosity/concentration relationship of the produced polymer and from a detailed rheological characterisation it was shown that no significant degradation of the polymer has occurred. A satisfactory history match of the oilcut response and of the polymer production was obtained with a numerical simulator. From the simulation results and from analytical calculations it was deduced that the reduction of oil in place in the pilot area since the beginning of the preflush was some 50% of STOIIP. This indicates that a pattern polymer flood in the pilot region offers scope for an increase In pilot region offers scope for an increase In ultimate recovery of some 25% to 35% of STOIIP compared to primary production with aquifer support. The corresponding amount of polymer is about 2.3 to 3.2 kg of active matter per additional m of oil (0.81–1.1 lb/bbl). In a forthcoming pilot extension project polymer flooding will be carried out in an unconfined 5-spot on the original 600 m Marmul well-spacing. Introduction The Marmul field, situated in the southern part of Oman was discovered in 1956 by Dhofar City part of Oman was discovered in 1956 by Dhofar City Services but relinquished as non-commercial soon afterwards. Appraisal drilling was initiated by Petroleum Development Oman (PDO) in 1976 and the Petroleum Development Oman (PDO) in 1976 and the field was brought on stream in 1980. The stock tank oil initially in place (STOIIP) is estimated at 390 × 10 m of which 60% is contained in the Al Khlata formation, a complex sequence of very heterogeneous glacial deposits. The oil in this high permeability formation (1 -20D) has a viscosity of 80 mPa. s. The Al Khlata is currently producing around 5000 m/d of oil from some 90 wells at an average watercut of 50%. There is a natural flank waterdrive which in some areas is quite strong. It was recognised that in view of the unfavourable water-oil mobility ratio (M=45), there is considerable scope for EOR by polymer flooding and steam-flooding. It was decided to pilot test both processes.) A small pilot was recommended as a first stage in the testing of polymer flooding.) Its results would serve as guidelines for a possible larger scale pilot. This paper is concerned with the performance and analysis of this first pilot which is also the first polymer flooding project undertaken in the Middle East. DESIGN Objectives The objectives of the pilot were defined as follows:To obtain information on improved sweep efficiency by polymer flooding.To obtain experience with the make-up of a polymer solution in the harsh Marmul desert environment. P. 385
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