Geomechanical Impact of High Tectonics and Overpressure in Fracture Design — a Tight Gas Reservoir Case in Argentina

Abstract

Abstract

Lajas formation is a tight gas reservoir with extremely low permeabilities ranging from 0.1-0.5 micro darcies). It is formed mainly by intercalated shales and siltstones that can only be produced commercially after hydraulic fracturing. In this field the formation is overpressured (approaching 0.7 psi/ft) and subjected to a significtative tectonic load that varies from one location to another in the area, the same as lateral variations in pore pressure. These two factors make the minimum horizontal stress to be very close to the vertical stress in some locations and even at specific depths. Hydraulic fracturing has been troublesome, showing for example cases of vertical fracture contention inside thick reservoir packages that in other locations showed perfect vertical growing. The phenomenon is associated to the closeness of the fracture gradient to the vertical stress. Furthermore, some stiff reservoir intervals behave as fracture barriers inside this high pressure environment, stopping the fracture development, hence the fracture efficiency itself.

An integrated geomechanical, petrophysical and fracturing analysis was performed in order to understand the fracture geometry development and optimize the fracture treatment. The analysis required as input data a full set of advanced logs including magnetic resonance, mineralogy and advanced acoustic tools. Raw data from these logs was processed to obtain the best petrophysical and mechanical properties characterization for reservoir evaluation, geomechanical TIV modeling and fracture geometry simulation.

The analysis showed there is no a recipe for perforating location and fracture treatment in Lajas formation, since each location required different completion (perforating clusters) and stimulation strategies (stages, pumping schedules, materials and techniques) depending on stress and pressure environment and gas saturation and porosity. The last is especially true in this intercalated tight gas formation where gas saturation from magnetic resonance and total porosity corrected by gas and mineralogy are required for better formation evaluation while TIV geomechanical modeling helped to model a more realistic and physically consistent stress field.

This work summarizes the variables that determined the completion strategy and hydraulic fracturing success in the high pressure / high stress environment of Lajas formation.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016