New Methodology of Effective Hydraulic Fracturing in High-Thickness Formation
Authors: Sanchez Raul Horacio , Agut Regis Germain , Coulon David , Sentinelli Roberto 
Published: December 2007

The Aguada Pichana field is located in the center of the Neuquén Basin in the province of Neuquén, being at present, one of the main gas producers in Argentina. The completion programs of Aguada Pichana wells imply the stimulation of Middle Mulichinco Formation (primary target) through hydraulic fractures.

Mulichinco Formation is 30 to 80 meters thick and has a variable permeability throughout the pay zone. The gas drainage from the best permeability zones causes a differential depletion in reservoir pore pressure, affecting by consequence the mechanical properties of the rock in its whole thickness. This petrophysical and mechanical behavior of the reservoir, added to the possibility of finding free water in the lowest part, makes it difficult to reach the best results by means of a unique fracture.

Within the optimization process that is followed in the development of this field, the implementation of a strategy of selective stimulation, through the pumping of two hydraulic fractures directed to reach different challenges, provides the best option for obtaining better results. In order to stimulate the base of the zone, the first stage of fracture includes an aggressive design of high conductivity with the aggregate, in some cases, of Relative Permeability Modifier additive (RPM) in the frac fluid for water control. In the top of the zone, the second stage is characterized for being a fracture of greater length, diminishing the convection effects.

This work summarizes the designs, operational planning and results of the new methodology of implemented hydraulic fractures.

Introduction

Aguada Pichana field is located in central part of the Neuquen Basin. The field produces gas and condensates from the sandstones of Mulichinco formation (Valanginian to Hauterivian). The Mulichinco is characterised by a relatively thick sedimentary column, ranging from 150 to 250 meters, with 30 to 80 meters of pay zone, from South to North. In Aguada Pichana, the reservoir level being produced is composed of sandstones deposited in a fluvio-tidal to littoral environment, characterized by low to very low permeabilities, partly belonging to the Tight Gas Reservoirs category and thus requiring specific stimulations through hydraulic fractures.

Aguada Pichana field conditions have changed through its productive life. The Middle Mulichinco formation is producing gas from sandstones at 1650m depth with an average permeability varying from 0.1 to 5 mD. Reservoir pressure has fallen from the original 2500 psi to 900 psi in the most productive areas.

The following section is a detail of possible reasons explaining the low production post-frac flows obtained in wells that were completed along the 2004–2005 completion campaign.

Review of Previous Treatments

In Aguada Pichana field, the Standard well-completion program included the stimulation of Middle Mulichinco Formation through a one-stage hydraulic fracture.

In general, although the net productive interval to stimulate was greater, only 10–15 meters were perforated, in order to stimulate Mulichinco zone in a single stage. Simulations with 3D models were done in order to predict downward fracture growth into the underlying sands containing movable water. After fracture stimulations, the perforated interval showed to be in the most convenient area to start up fracturing. The design volume was fit to limit the vertical growth of fracture, without growing down into possibly water bearing levels.