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.
Carbonate reservoirs in the southern region of Mexico are deep, hot, and naturally fractured. The intensity of natural fractures varies greatly. Wells in these formations are typically completed with multiple perforated intervals, with different producibility and pressure. The oil production is dependent on the presence of natural fractures, which, with time, are invaded by the aquifer, leaving the remaining reserves in the low-permeability formation matrix. The challenge when stimulating these wells is not only to selectively divert the treating fluid away from the natural fractures, but also to reduce water production from the natural fractures and fissures after the treatment. To divert treating fluid from the lower-pressured natural fractures requires a viscous fluid with fibers or solid particles, whereas to limit water production after the treatment requires a disproportionate permeability modifier (DPM). However, the adsorption of conventional cationic low-viscosity DPM polymers is often limited in carbonates. This led to the development of a viscous zwitterionic DPM (ZDPM) fluid to both divert stimulation fluids from the natural fractures and reduce water production after the treatment. The ZDPM fluid comprises two polymers and a viscoelastic surfactant at a concentration below the critical micelle concentration. The adsorption and DPM properties of the polymers are further enhanced by adjusting the pH and salinity of the fluid. More than 150 treatments have been performed in mixed carbonate formations with natural fractures and fissures using the ZDPM fluid. The oil production increased by as much as 220% while at the same time the water cut was reduced by up to 23%. This is supported by core flow testing in which the ZDPM fluid reduces the effective permeability to water by up to 80%, whereas in oil-wet cores, the fluid water-wets the core and effectively stimulates oil production, in some cases by more than 25%. The use of the ZDPM fluid has greatly improved the economics and feasibility of treating wet producers in naturally fractured dolomitic limestone reservoirs. This paper summarizes the laboratory evaluation conducted to validate the use of viscous ZDPM fluid and the successful application in the field to divert stimulation fluids from the natural fractures and reduce water production after the treatment.
Tip-Screen-Out (TSO) stimulations of high permeability reservoirs have been performed in North and South America (i.e., USA, Venezuela, Trinidad, Colombia), in Africa (i.e., Nigeria, Gabon, Congo, Ivory Coast, Cameroon), in Europe (i.e., North Sea, Italy), in the Middle East (i.e., Saudi Arabia) and in Asia (i.e., Indonesia). As reservoirs in these regions mature, fluid loss control and fluid-inflicted formation damage become increasingly important and conflicting issues. On one hand, the use of classic fluid loss additives and/or fluids with good fluid loss control characteristics (i.e., polymer-based fluids) is typically associated with well production impairment because of residues left in the formation. On the other hand, fluids that are considered nondamaging, such as viscoelastic surfactant (VES) fluid systems, typically exhibit limited fluid loss control capabilities, making fluid efficiencies low and proppant placement inadequate. Nondamaging fluids are needed that can deliver adequate fluid loss control under severe reservoir conditions (i.e., upon concomitant occurrence of high permeabilities and low reservoir pressures).Bachaquero field located in Lake Maracaibo, Venezuela, and managed by Petróleos de Venezuela (PDVSA) is a critical example of these depleted, high-permeability reservoirs. Permeabilities in this field range from 100 mD to 1 D. The average pressure gradient is 0.2 psi/ft. VES fluids typically have been used to hydraulically fracture reservoirs in this field. However, fluid efficiencies observed in recent jobs were as low as 2% and proper proppant placement under these conditions was no longer possible.In our study, we consider relevant technical aspects and case histories in a systematic study that is aimed at assessing the usefulness of a new-generation VES fluid tailored for high-permeability reservoirs to address low fluid efficiencies in the Bachaquero field. The study encompassed adjustments in fluid formulation to ensure compatibility with local production water and crude oils, specific engineering and quality assurance/quality control guidelines, and field trials in 10 wells, followed by quantification of the enhancements achieved in fluid efficiency and well production.Field tests showed that fluid efficiencies increased by up to 325%, as compared to conventional VES fluids previously used in this field. Production expectations were exceeded in all tested wells by an average of 65%. The study concluded that the new-generation VES fluid is suitable for use in the Lake Maracaibo area and it has been adopted as the fluid of choice for Bachaquero field. What we learned from this successful campaign is valuable when assessing stimulation strategies for mature high-permeability reservoirs worldwide.
An excessive water cut or high gas/oil ratio in a production interval presents a major concern in sustaining oil production, often requiring fast and efficient workover solutions to enhance the oil recovery process. Wells in the Cantarell field, a mature depleted field in the Bay of Campeche in the Gulf of Mexico, are facing drastic decreases in their production and, depending on producing zone, an increase in either water cut or gas/oil ratio. Other developed fields in Mexico's Region Marina, such as the Ku-Maloob-Zaap, have constantly increased their hydrocarbon production through the years with an incipient increase in water and gas increments.The high water cut and gas increments have had a strong impact on the production strategy, opening the opportunity for application of non-conventional, innovative, and engineered solutions to isolate or abandon production intervals invaded by gas or water and continue production from upper or deeper zones. The pay zones consist of naturally fractured, vugular carbonates with permeability as high as 5 Darcies, from Paleocene, Cretaceous, and Jurassic formations. Their characteristics present the following challenges that need to be overcome to succesfully achieve the required isolation: Loss of fluid circulation, which is necessary for well control and also negatively affects the fluid placement in the workover jobs. High gas presence, gas cap driven, which is a major concern on surface. Lack of primary zonal isolation, resulting in migration of water or gas to the productive intervals. This paper summarizes the non-conventional technologies and techniques applied to isolate the water and gas producing intervals and their synergistic performance: reticulated gel, lost circulation fiber tecnologies and gastight slurries integrated in an engineered solution. Results from field cases demonstrate the design, execution, and evaluation of these applications.
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