Summary

The Aguarague field is located in the Devonian basin, northwest of the province of Salta, Argentina. Gas production is obtained from clastics of the Huamampampa formation through an intense network of natural macro- and microfractures. The reservoir is in a north/south-trending symmetrical anticline.

Five high-deliverability gas wells watered out between 1991 and 1994; as a result, the field gas production decreased substantially. This triggered a study in 1995 to try to find a means of extending the life of the reservoir. The study indicated that without any modifications, gas production could be expected to continue, at most, until 1999.

This paper describes how a detailed fracture characterization led to a dewatering project using gas lift, which significantly reduced the gas decline of the reservoir. The project also brought back to gas production some of the wells that had previously watered out. The 1995 forecast is compared with the actual production performance to date.

Introduction

Argentina deep drilling (approximately 4500 m) and discovery of hydrocarbons in NOA basin, Serranias de Aguarague, Baja de Oran, and Ipaguazu and in the Devonic and Carbonic sub-basins started in 1951 with the discovery of Campo Duran, a gas condensate reservoir, in the Serrania de Ipaguazu. Madrejones field, north of Campo Duran, was discovered in 1952. The field also produced gas and condensate. A shallower well was drilled in Pena Colorada in 1968. The well produced small oil quantities from the top of the Los Monos formation. Ramos field was discovered in the Serrania de San Antonio in 1977. The field produced gas and condensate from the Huamampampa formation.

The Aguarague field, the object of this study, was discovered in 1979 with the drilling of well Cu.x-1. The field went on production in 1979 with well Cu.x-2, which produced initially at approximately 500 000 m3/d from the Huamam-pampa formation. To date, 16 wells have been drilled, of which 13 reached the Huamampampa formation. The field reached a production of more than 4 million m3/d in 1985. However, five high-deliverability gas wells watered out between 1991 and 1994; as a result, the field gas production decreased substantially.

Fig. 1 shows a structural map on top of the Huamampampa formation. Water was advancing from the north in such a way that wells Cu.x-2 and Cu.x-1 had watered out by May 1991, well Cu.x-3 by May 1993, well Alo.x-1 by June 1994, and well Sa.Ag.-4 by January 1995. Water was also advancing from the south; well Tr.x-1 watered out in January 1986, well Tr.x-206 in August 1991, and well Tr.x-199 in September 1992. Without any modifications, all the wells were going to water out by 1999.

Location and Geology

The Sierra de Aguarague field is located in the Devonian basin, northwest of the province of Salta (Fig. 2), west of National Route No. 34 near the city of Tartagal.

Sierra de Aguarague is a structure that forms part of the Sub- Andean system. The area contains a series of anticlines trending in a north/south direction. The flanks are very steep, with a regional dip toward the south. The Aguarague structure starts in Bolivia, approximately 200 km from the border with Argentina, penetrates Argentina, and ends approximately 120 km to the south of the border, near Rio Bermejo.

The Huamampampa formation, the object of this study, is a low-porosity, low-matrix-permeability, and highly fractured quartzitic sandstone of the Devonian Age with an average thickness of approximately 400 m.

Fig. 3 shows a stratigraphic section of Aguarague. The Huamampampa formation was deposited in a marine environment in a platform with a very small dip under shallow conditions. This formation most likely comprises a regressive sequence. Conversely, Los Monos shales were deposited in a slightly deeper environment under a transgressive sequence. This formation is essentially pelagic.

The Aguarague anticline is considered a fault-bend fold1 system. A lower plane in the base of the Paleozoic and an upper plane in Los Monos formation define the geometry. There is a ramp connecting both planes that cuts the lower Paleozoic and the Huamampampa formation.2Fig. 4 shows a cross section of the Aguarague anticline. The western flank dips approximately 22°, as indicated by the dipmeter of well Cu.x-2. The eastern flank is steeper, dipping between 44 and 52° based on data from well Q.Ga.x-1.

The source rock is composed of Devonic shales. There is a large contact area between the source rock and the west flank of the anticline. Gas migrated through the west flanks to the present location in the reservoir.2

It is likely that the fractures were formed when gas had already been generated in Los Monos shales. The fracturing produced a zone of dilatancy in the Huamampampa, Icla, and Santa Rosa formations. The vacuum created within the fractures helped migration of gas from the shales toward those fractures.

Fracture Classification

Natural fractures at Aguarague are of tectonic origin. They are predominantly the result of folding, although fault-related fractures associated with fault-bend folding movements are probably present as well. The largest degree of fracturing is of a tensional type and occurs along the axis of the anticline.

The fractures with the largest pervasive permeability at Aguarague correspond to Type 2 fractures using Stearns' classification. 3–5 Based on our experience with other anticlines of the Sub-Andean system, it is likely that fractures of Type 1 are also present in the west flank of the anticline. Type 1 fractures would explain the migration of gas through the west flanks, as postulated by Lesta and Kozlowski.2 The population of the fault-related fractures is not large but, when present, can provide very large permeabilities and significant problems from the point of view of water production.

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