Petroleum Development Oman is actively developing tight gas reservoirs which include the Paleozoic Amin Formation in Northern Oman.
Natural fractures in this reservoir can be critically stressed and permeable at in-situ stress conditions, however, production may not be sufficient for commerciality. Experience from tight gas reservoirs in Oman and around the world shows that production is often only commercially feasible when the formations are hydraulically fractured.
The low porosities (3-10%) and permeabilities (1-0.001 mD) of this Amin formation, are reflected in the mechanical properties of the formation, with high compressive strengths (100 – 250 MPa). These high strength values limit not only fracture initiation, but also the identification of suitable intervals favorable for fracturing, which has become a critical factor for a successful stimulation program.
High in-situ stresses and large variability in pore pressure and rock properties are also observed at the Amin level. The pressure distribution in this area is believed to be affected by compartmentalization and faulting, which is accompanied by a significant variation in stress orientation across the major fault systems of Northern Oman, which may influence well planning strategy.
Geomechanical characterization of Amin using open-hole logs and conventional petrophysical techniques are seen to be inadequate for characterising this reservoir for hydraulic fracturing zone selection and design. Understanding the uncertainties in the models resulting from data inputs is the key in identifying and justifying potential intervals for fracturing.
This paper discusses how core based laboratory tests combined with log data were used to improve the geomechanical model to identify intervals favourable for fracturing, while providing a geomechanical framework for explaining fracturing successes and failures in offset wells. This work is continuing, in parallel with other completion optimization efforts, in order to increase the success rate of hydraulic fracturing in the Amin and to help establish a successful development strategy.