Successful Drilling to Completion of a Highly Stressed Depleted Tight Gas Sandstone Reservoir Based on Geomechanical Study
Authors: Naidu Nakireddi. A. , Hinaai Qasim , Ismaili Ibrahim , Smit Jeroen , Youssef Hamada , Hendra Suhendar , Kumar Rajeev 
Published: November 2016

In the quest to achieve higher production, horizontal wells are being drilled in PDO concession area located in the Sultanate of Oman. The goal is to link a number of tight sand gas accumulations to central production facilities. Troublesome formations, including a variety of layered silty-shales (Al Bashair, Miqrat), and a depleted gas bearing sandstone formation (Amin), pose a huge challenge for wells drilled at high inclinations in high stressed environment. Earlier exploration and appraisal wells encountered stuck pipe, wide breakouts and hole cleaning incidents while drilling through those formations. The high risk of wellbore instability in accessing the reservoirs with horizontal well drilling threatened the commercial viability of the project.

This paper describes how a geomechanical study was used to mitigate wellbore instability in the decision making process during drilling operations of the horizontal wells in one of the Clusters. The geomechanical study output helped to select specific mud weights and to plan casing points in order to drill the critical landing and horizontal sections through breakout prone Ghudun formation and the 450m-500m thick silty-shales (Al Bashair, Miqrat) overlying the depleted sandstone reservoir.

A Mechanical Earth Model was constructed to represent the state of stress and mechanical properties of the overburden and reservoirs. The model incorporated data from a number of sources including laboratory-measured mechanical properties, closure pressure and breakdown pressure from hydraulic fracturing jobs, leak-off data and drilling records from the earlier vertical and high angle wells. Examination of the stress profiles showed considerable horizontal stress anisotropy and breakouts in Miqrat formation. Rock mechanics test data suggested anisotropic elastic and rock strength properties as confirmed by sonic anisotropy. This suggested that anisotropic Young’s modulus and rock strength properties vary between 10% and 15%. These anisotropic properties were used in the wellbore stability analysis.

The geomechanical study helped in making a number of critical decisions in the well design including the decision not to drill the Miqrat and Amin formations in one section as was done previously in high angle wells. This will allow the drilling of the highly stressed Miqrat formation with mud weight in the range of 12.8 kPa/m to 13.1 kPa/m to mitigate breakouts, stuck pipe, and to drill depleted Amin reservoir with mudweight of 10.8 kPa/m. Lower mud weight in Amin reservoir would minimize reservoir damage and improve ECD management with proper hole cleaning. Risk of mud losses and differential sticking in depleted Amin formation were reduced with proper mud formulation and rheology. BHA design was optimized while landing inside Amin reservoir to mitigate stuck pipe due to mud overbalance in the silty-sandstone layers in deeper Lower Miqrat formation.

The Amin reservoir section could be drilled safely as per the planned well path. In horizontal sections, the CBL’s are generally very poor in spite of precautions and good cementing practices were followed. Coupled with good cementing practices and planning, 100% cement bond log was achieved. This resulted in higher than expected production from the well with very encouraging results overall.