Optimal Process for Design of Fracturing Treatments in a Naturally-Fractured Volcaniclastic Reservoir: A Case History in the Cupen Mahuida Field, Neuquen, Argentina

Abstract

The declining reserves in conventional gas reservoirs imply that the oil and gas companies must consider the production of tight no-conventional gas reservoir. The Cupén Mahuida Field is a naturally fractured reservoir composed of series of volcanic and volcaniclastic layers developed into a synrift stage at the Triassic in the Neuquén Basin. To obtain economical production all interesting layers had to be fractured. Therefore, we can assimilate that field to a tight gas, no-conventional reservoir. Due to the huge difference in production between the different layers fractured in the existing wells, and due to the differences in behavior of the pressure during the treatments, it was decided to conduct a detailed analysis of each case. This included well construction, perforating strategy, geology, petrophysics, images log, minifracs and fracture treatments. The results of the study showed two directions of work: First, necessity to modify the operations schedule; second, we needed to improve the definition of what are the layers candidates to stimulation. So operationally, we decided to change the casing size, the perforation scheme and we included as normal practice the use of proppant slugs to reduce the tortuosity effects we may see. The selection of candidate was improved by a better understanding of the formation thru an evaluation of outcrops that permitted to characterize the volcano clastic events and to correlate them with images, nuclear magnetic resonance logs and pressure measurements. With that information and the pressure response in the minifrac analysis we define the optimum size of treatment and the final proppant concentration. Introduction This case study describes a multi disciplinary evaluation of fracture treatments in a naturally fractured volcanoclastic reservoir. It corresponds to the Precuyano formation in the Cupén Mahuida field situated at around 80 Km of Neuquén city, center of Argentina (fig. 1). All the evaluated wells are vertical. Fig. 1: Location Map The net pay is composed of several layers (3 to 7) of porous volcanic rock situated between 3100 and 3700 m and is overpresurized. The net pay height of each layer may vary between 5 and 30 m. Based on the first petrophysical interpretation it was considered that the net pay could be assimilated to a non-fractured clastic rock, and the stimulations were first designed on that basis. The first wells drilled were exploratory and completed with 7" casing. Now the development wells are completed with 5" casing. Those changes allowed to improve the perforation strategy and to reduce the fracture entry problems. An overview of the first results showed that the response of the reservoir was the one of a naturally fractured rock, and that we could not draw a direct relation between the production results of the individual layers and the log information as porosity, height and pressure. So, each individual fracture was evaluated and its production in time, looking at more detailed reservoir characteristics, as natural fractures density, rock deposition system, to be able to define what layers are best candidates and how to design the frac operation for the different type of candidates.

Analysis of Cuttings’ Minerals for Multistage Hydraulic Fracturing Optimization in Volcanic Rocks

“…Since the discovery of the world’s first volcanic reservoir in the United States in 1887, the extreme properties of volcanic reservoirs have attracted countless researchers. The Interior Coastal Plain reservoir and Thrall field in the United States; Bacuranao and Motembo fields in Cuba; , Furbero Gabbro reservoir in Mexico; Minami Nagaoka gas field in Japan; Akita, Niigata, and Neuquén basins in Argentina’s Austral; − and Browse basin in Australia are early discovered volcanic reservoirs that have been adequately studied by scientists around the world.…”