Optimum Mud Overbalance and ROP Limits for Managing Wellbore Stability in Horizontal Wells in a Carbonate Gas Reservoir

Abstract

Saudi Aramco has been drilling horizontal and multilateral wells to develop gas fields. Due to production-induced decreasing reservoir pressures along with tight nature of the reservoir rock, development activities have focused more in placing wells towards the minimum horizontal stress direction and complete with multistage hydraulic fracturing aimed to improve lateral reservoir contact enabling higher production at sustained rates thereby increasing recovery with less number of wells to be drilled. Horizontal wells drilled in the minimum horizontal stress direction are more challenging due to complex geological conditions and compressional in-situ stress conditions. The data shows that some wells were drilled without major difficulty while some other wells encountered more problems leading to stuck pipe events. A detailed study was conducted to identify the nature of these problems and ascertain major controlling factors for this variable drilling experience to make future operations safer and more efficient through recommendations based on a diagnostic analysis of the observed problems in existing wells. Analysis of data suggests that excessive borehole breakouts and faster rate of penetration (ROP) are the key contributing factors to the observed drilling challenges. In addition, differential sticking has been found a potential risk across high porosity and/or depleted zones. As a result, optimum mud weight based on predrill geomechanics model was recommended to manage the hole stability. In addition, a safe limit of ROP as a function of hole azimuth was identified to manage efficient hole cleaning and avoid stuck pipe due to pack-off. The recommendations made based on this analysis enabled successful drilling and timely completion of several horizontal wells across the field.