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Case Study of a Prolific Tight Gas Reservoir, Lajas Field, Neuquén, Argentina

Abstract

Since gas was initially tested from tight sandstones of the Lajas Formation at Aguada Toledo-Sierra Barrosa area in 2004, the Lajas Field has gone through discovery and delineation stages, which included 3D seismic acquisition, 11 new wells drilled, the acquisition of conventional and sidewall cores, complete log suites, and microseismic monitoring of hydraulic fractures. Field development started in 2013 and with more than 100 wells planned, it is becoming one of the most relevant tight gas fields of Argentina, with considerable volumes of gas proved in a productive area of 100 sq km. The reservoir is located in the Cupen Mahuida elongated structural anticline, which is an east-west oriented 4-way dip closure formed by oblique inversion as a fault propagation fold, originated by the Huincul arch activity. The structure strongly controls the gas trap and storage. The Lajas Fm is part of the middle Jurassic Cuyo group. It consists of 900m of sandstones and siltstones deposited in a prograding delta system, from delta front to fluvial facies. Net pay varies from 100 to 300m according to the structural position. Average porosity is 7%, permeability ranges from 0.1 to 0.001 md, and the pressure gradient is normal. In some zones mobile water has been observed. The gas composition is 90% methane and the water saturation is 50%. One of the main challenges for well stimulation is how to identify the water bearing intervals in order to design long fractures contained to avoid the mobile water. The reservoir stimulation has evolved from a few big single point fracs to an average of 10 limited entry fracs. EURs are calculated by conventional and Rate Transient Analysis for this type of reservoir. Relationships between petrophysics and EURs have been established. Production contribution from individual layers and their evolution in time are determined by PLT analysis. The field production has had a dramatic growth during the last 2 years due to an aggressive drilling plan, turning it into the 3rd largest gas field in YPF. The data and experience indicates that, as well as in Greater Green River Basin, these tight gas fields in Neuquen Basin seem to occur in conventional traps. An exception is the lower part of the reservoir which could be associated with a BCGA. There is a huge volume of tight gas resources not yet deeply investigated in this basin which could capitalize on the success and lessons learned from this project.