Lessons Learned From Four Selective Water-Shutoff Treatments in Mature Reservoirs in Maracaibo Lake

Abstract

Some of the most important oil reservoirs located at the Maracaibo Lake Basin are produced by a waterdrive mechanism. After four decades since the beginning of production, oil rates have declined mainly due to a continuous increase in water production. These reservoirs are characterized by highly heterogeneous sandstone formations in which water channeling through high permeability strakes is a very common cause of water production. Another type of problem affecting production in these reservoirs is behind casing communication between producing sand and nearby water sands. Both problems have caused a rapid increase in the water-oil ratio of the wells, which in most of the cases were abandoned affecting considerably the oil recovery of the reservoirs. This paper describes four successful selective water shutoff applications in producing wells from mature reservoirs at Maracaibo Lake Basin performed during the last seven years. The aim was to correct problems of channeling and behind casing communication. An elastic gel system of variable consistency with easily controlled gelling times was used. In some applications a gel-cement combination was necessary to improve results. Reservoir and well characteristics, diagnostic analysis and the different types of selective placement used are shown as key factors for success. Using the elastic gel system the water production has been reduced to a minimum, obtaining an increase in wells productive life-time, which in global terms reaches a volume higher than 2 millions barrels of additional oil recovered. Introduction In mature oil reservoirs, produced by waterdrive mechanism and with strong vertical variability in permeability, water channeling problems are very frequent, being the main cause of wells abnormal oil production declining. If this problem is not properly identified and corrected can lead to premature abandonment which compromises the reservoir recovery and causes an increase in field development investments because the need of drilling new wells to access bypassed oil reserves. In Maracaibo Lake Basin the biggest light-oil reservoirs have been produced over the last 40 years. Main recovery mechanism of these important oil accumulations is waterdrive, in some cases supported by flanks water injection. Current oil remaining reserves are no higher than 9% of total oil recoverable volume. These consolidated sandstone reservoirs of fluval-deltaic origin are characterized by highly complex stratigrafy, which determines its lenticularity. The reservoirs are compound by several sand layers separated by shales of variable thickness. Permeability values can vary between 200 and 1000 mD, favoring water channeling and a rapid increase in wells water production. Additionally, behind casing communication problems due to deficiencies in primary cementing are also very frequent, affecting considerably wells production behavior. Traditional water control technique applied in wells with high water-oil ratios (WOR) had consisted on setting a plug in the well to abandon watered-out intervals and perforating upper prospective sand layers to increase oil production. This operational practice very frequently used in development stage had caused severe oil bypassing, recoverable only with new wells. To increase wells productive life-time and improve oil recovery in these today's mature reservoirs, a water shutoff project has been carried out during the last 7 years of production[1]. Water shutoff applications consisted on a gel system injection based on a previous diagnostic of the type of problem present and precise identification of offensives and productive intervals. Selective placement techniques were considered necessary to protect wells productivity[2]. This paper presents four successful water shutoff field experiences showing the evolution in time of selective placement technique used to perform the gel injection. The results in terms of reduction in WOR and additional oil recovery are also presented. Finally, some learned lessons form these experiences are included.

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