Cantarell, which is an offshore complex of fields in the Bay of Campeche, is the most important complex in Mexico and is the second-largest producing field in the world. It is comprised of five fields, with the main pay zones consisting of highly fractured, vuggy carbonate formations from Jurassic, Cretaceous and Lower Paleocene geological ages. Matrix acidizing has always been the main stimulation process used to improve production from these carbonate reservoirs and this is especially the case now that this mature complex has reached its production peak. As with all acidizing programs, a critical factor for success of the treatments is distribution of the acid between all productive zones. Since most producing wells are not homogeneous and contain layers of varying permeability, even distribution of the acid is a difficult task. In addition, the water saturation of the various zones has a major effect on the acid distribution. Since acid is an aqueous fluid, it will tend to predominantly enter the zones with the highest water saturation, in many cases resulting in increased water production. This brings with it the multitude of problems associated with high water production. This paper will present the results of approximately 55 high permeability wells ranging from 1,000 to 6,000 md, which have been acidized using a novel acid diverter based on associative polymer technology (APT). This polymer inherently reduces the formation permeability to water with little or no effect on the permeability to hydrocarbon. Data from production logs from several of the treated wells will be presented, which show excellent oil production distribution along the perforated intervals. In addition, production logs will also be shown for wells acidized with other diverters, such as foams and in-situ crosslinked acid, which showed poorer results. Introduction Cantarell Field The Cantarell field is the second-largest oilfield in the world behind the Ghawar field in Saudi Arabia. It is located offshore in the Gulf of Mexico, 47 miles northeast of Ciudad del Carmen, Campeche. The main hydrocarbon zones in Cantarell are highly fractured carbonate formations from the Jurassic, Cretaceous and Lower Paleocene geological ages (Fig. 1). The field is made up of a number of subfields or fault blocks, with the main fields being Akal, Chac, Kutz, and Nohoch. Production started in 1979 and reached a peak of 1.1 million B/D in 1981 from 40 oil wells. By 1994 the production was down to 890,000 B/D. One year later it was producing 1 million B/D due to the addition of new platforms and wells and a nitrogen injection program. This program was capable of injecting a billion ft3/day of nitrogen to maintain reservoir pressure. By 1996 the field was producing 2.1 million B/D.
In the Mexico marine region, gas breakthrough is common in naturally fractured carbonate oil reservoirs. Increasing the gas production reduces crude oil production, and eventually the wells become uneconomic and are shut-in in spite of the remaining recoverable reserves. A typical example is the Akal field, a large fractured 300-to 1000-m thick carbonate reservoir whose permeability varies between 0.3 and 5 darcy. The gas-oil contact zone moves by as much as 8 m/month as the natural gas and nitrogen gas from gas injection moves through the natural fractures and invades the oil zone. This condition results in production decline, reservoir pressure decrease, and oil remaining in the matrix.Efforts to selectively shutoff the gas have been unsuccessful due to the low-reservoir pressure and high-permeability contrast. When pumping water-based fluids, the increased hydrostatic pressure causes the treating fluid to travel down the natural fractures and away from the gas cap. This condition led to abandoning the gas-invaded intervals and recompleting lower in the reservoir, leaving some recoverable reserves.To selectively shutoff gas entry in fractured reservoirs, a stable foam-delayed crosslinked fluid was proposed for use by service company. The fluid with a high-foam quality (FQ) and low density rides over the crude and into the natural fractures/fissures, communicating with the gas cap. Once set, the fluid creates an impermeable seal with high-extrusion resistance.The stable foamed fluid has been successfully used to selectively shutoff unwanted gas production in wells that have been, in some cases, shut-in for several years due to excessively high gas/oil ratios (GOR). Following the treatment, the oil production was restored to the same level as prior to the gas breakthrough.The success of the initial campaign demonstrated that it is possible to restore the production levels of shut-in wells and recover otherwise lost reserves. This result has a very significant positive impact on the economics of operating the field. The current plan is to extend the use of the technique to other fields.
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