Role of Geomechanics in Appraising a Deep Tight Gas Reservoir: Amin Formation, Oman
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 142788, ’Role of Geomechanics in the Appraisal of a Deep Tight Gas Reservoir: A Case History From the Amin Formation in the Sultanate of Oman,’ by Satya Perumalla, SPE, Daniel Moos, Colleen Barton, SPE, and Thomas Finkbeiner, SPE, Baker Hughes; and Sultan Al-Mahruqy, SPE, Markus Weissenback, William Walton, SPE, and Hisham Al-Siyabi, Petroleum Development Oman, prepared for the 2011 SPE Middle East Unconventional Gas Conference and Exhibition, Muscat, Oman, 31 January- 2 February. The paper has not been peer reviewed.

Overbalance leads to near-wellbore porosity and permeability damage in the rock matrix and fractures. Also, poroelastic effects caused by invasion contribute to difficulties in initiating and propagating hydraulic fractures. Damage to natural fractures intersecting the well can prevent their detection, leading to missed productive intervals. Alternatively, underbalanced drilling (UBD) can avoid these effects and, thereby, indicate the gas potential of these reservoirs better. However, not all reservoirs are suitable for UBD because there can be a greater risk of wellbore mechanical instability. Therefore, performing geomechanical analysis before drilling can help evaluate the feasibility of UBD operations. 

Introduction

In North Oman, Petroleum Development Oman is exploring for gas in the Cambro-Ordovician Haima-supergroup Amin reservoir at depths of 4000+ m true vertical depth subsea. This formation is a tight sandstone, with porosity ranging from &lt;3 to 10% and (ambient) permeabilities ranging from 0.001 to 1 md. Drilling exploration wells in this region is challenging because of the tight and abrasive nature of the formations and the large in-situ-stress magnitudes—high horizontal compression—that prevail in North Oman. Proving the existence and mobility of hydrocarbon gas, especially at commercial rates, may depend on wells intersecting a natural-fracture system that is permeable enough and is connected to a sufficiently large volume of reservoir rock and on the near-wellbore region of the reservoir not experiencing irreversible damage by drilling overbalanced.

In light of these challenges, the decision was made, for an upcoming exploration well targeting the Amin formation, to drill a deviated well to maximize potential from the natural-fracture network and to drill in underbalanced mode to avoid invasion and to characterize any produced reservoir fluids. To understand the risks better before and during the drilling campaign, a geomechanics study was commissioned to support well planning and reservoir characterization in a phased approach.

Exploration-Well Planning Stage

It was important, while planning the vertical exploration well, to consider the structural uncertainties with seismic imaging of the Amin reservoir and to keep the well as far away as possible from any mapped major faults. To explore for open fractures in the Amin formation with a highly deviated sidetrack, the surface location of the vertical pilot hole was placed such that the planned sidetrack trajectory within the Amin reservoir could be drilled through the zone perceived as the most fractured on the basis of seismic-attribute interpretation.