A New Gel-Foam System for Water Shut-Off purposes in Wormhole Reservoirs

Abstract

Abstract Oil reservoirs in the Lloydminister region, Canada, are produced through the process of Clod Heavy Oil Production with Sand (CHOPS). Implementation of CHOPS in these fields causes the formation of wormholes, which leads to production of excessive water and abandonment of the wells. In order to reduce the excess water production from wormholes, variety of techniques have been suggested and tested with minimal success. Blocking the wormholes by the means of clay, polymer-gel systems, and gel-foams are among the proposed methods. This paper presents the results of a series of experiments conducted in order to develop and test the applicability of a new gel-foam system for blocking wormholes in oil reservoirs. As a result, a new gel-foam system was invented that creates stable foam in wormholes for the duration of gelation time. The composition of the gel-foam was based on polyacrylamide cross linked with chromium(III), plus a mixture of solutions of two commercially available surfactants. These commercially available surfactants were tested at various concentrations and ratios to develop the most stable gel-foam system. An experimental set up was designed and built which allowed for two layers of unconsolidated sand as the matrix separated by a high permeability wormhole between them. The effect of various sizes of wormholes on the performance of this gel-foam system was tested. All experiments were conducted at atmospheric pressure and constant temperature of 30°C, with some experiments in the presence of residual oil. For all the experiments the residual resistance factors to the flow of water in high permeability pathways were in the range of 25 to about 200. The results obtained are clear indication of the effectiveness of this newly developed gel-foam system for blocking wormholes and other high permeability pathways in reservoirs. Introduction There are vast heavy oil resources in Saskatchewan and Alberta. In Lloydminister region, the dominant mechanism for heavy oil production is Cold Heavy Oil Production with Sand (CHOPS). This method is applied in unconsolidated sand heavy oil reservoirs, where instead of blocking sand ingress, by screens or gravel packs, sand production is encouraged by aggressive perforations and swabbing strategies. For these reservoirs, if the sand production is prevented, oil production rates drop to uneconomic level(1). It is believed that sand production in these wells leads to higher oil production due to the following reasons(2):If sand can move, the basic permeability to fluids is enhanced.As more sand is produced, a growing zone of greater permeability is generated around the wellbore.Continuous sanding means that asphaltene or fines plugging at the near wellbore cannot occur to hinder oil flow. Over 280,000 barrels of oil per day is produced under CHOPS process in Canada. If sand is not allowed to be produced form these reservoirs the ultimate oil recovery will be less than 5% OOIP, but CHOPS allows oil producers to recover over 12% OOIP from these reservoirs. Typically, a well placed on CHOPS will initially produce a high percentage of sand, up to 40%; however, this generally decreases to 0.5% to 5% sand by volume after a few months. As a result of producing sand from these reservoirs, pathways of extremely high permeability are generated in oil producing formations. These high permeability pathways are known as "wormholes"(3). As the sand production is continued, wormholes grow larger and extend throughout the reservoir. Although there is no clear theory about the shape and pathway(s) of these wormholes, if they reach a water source, water will flow through these wormholes and get into the production wells. At this time, the production well becomes watered-out and it will be abandoned. Many wells in Lloydminister area are abandoned because of this problem. However, shutting off the watered-out wormholes can lead to additional oil production from these abandoned wellbores and improve the economy of implementing CHOPS.