For the last eight years acid fracturing stimulation has been performed in the Veracruz basin to increase gas production in naturally fractured carbonate formations. Several techniques have been proved to enhance the results of these jobs, including the diverting technique and the application of nondamaging viscoelastic fluids technology specifically a self- diverting viscoelastic based acid system and a polymer-free viscoelastic surfactant gel for fracture initiation and propagation.
This paper presents the results of successful acid fracturing treatments recently performed using nondamaging viscoelastic surfactant-based fluids to increase gas production from naturally fractured carbonate formations.
Most of the wells drilled and completed in the carbonate formations of cretaceous age in Veracruz basin do not flow naturally because of low matrix permeability and drilling/completion fluids damage. Therefore, acid fracturing treatments are necessary to produce these wells at economical rates.
The objective of these treatments is to increase gas production by creating a deeply etched fracture that will bypass the damaged zone and connect to the network of natural fractures and/or fissures. The etched fracture length affects well performance in low-permeability reservoirs; increased fracture length should result in greater production response. The use of nondamaging viscoelastic fluids is necessary to obtain longer effective etched fractures. Combining this with the properties of a viscoelastic diverting acid will significantly increase fracture conductivity as well as the effective stimulation of all perforated intervals.
The fluids were combined by using a pumping technique that consists of alternating stages of nonreactive fluids, reactive fluids and diverter fluid to reduce wormhole creation at the nearwellbore area and increase etched fracture length.
Four field cases are presented in this paper. In three, multiple intervals were treated, resulting in longer etched fractures as deduced from after-treatment production response and radioactive tracer logs. In the fourth case, not all the intervals were effectively stimulated because diversion was not used.
Increased demand for gas in Mexico has prompted Petroleos Mexicanos started to concentrate on gas exploitation to satisfy internal consumption, developing mainly the Burgos and Veracruz basins. This increase in the use of gas is owing to a rapidly developing household market and to the substitution of gas for heavy fuel in industrial use. More natural gas power plants are being installed in the country, and industries are more committed to using cleaner fuels in their processes. Increasingly stringent environmental standards are motivating this demand.
The Veracruz basin, with an area of 18,000 km2 is in east central Mexico in the state of Veracruz. It is limited to the west by a laramide thrusted foldbelt, to the north by recent volcanic rocks, and to the east by the continental slope. To the south, it continues as the Isthmus Saline basin. The Veracruz basin tertiary rocks are producers of dry gas. Sour wet gas, condensates, and oil are produced from the Mesozoic limestones of the deformed belt. Dry gas production in Veracruz basin started in 1958. Current production is 230 MMscf/D.1
The wells treated with these fracturing fluids belong to the Matapionche and Mecayucan fields, in which the productive zones are the Orizaba, Guzmantla, and Brecha San Felipe carbonate formations. The reservoir is composed of wackstone, mudstone and packstone. This region contains high-sulfur oil and associated gas in structural traps of Cretaceous age. Reservoir heights vary from 90 to 300 ft. Petrophysical characteristics include high storage capacity with low permeability, which is associated with the fracture and/or fissures network.
Since its introduction, the acid fracturing technique applied in Veracruz reservoirs has been optimized to satisfy gas market demand. The first acid fracturing treatments were pumped using conventional diversion techniques such as ball sealers and gelled oil-base fluids as pad for leakoff control and fracture initiation. Diversion was not successful in most of the jobs; also the cost and safety risks of using diesel for the large amount of treatment fluids became a big issue.