Successful Wellbore Strengthening Case Studies Through Holistic Geomechanics Understanding in Severely Depleted Reservoirs

Abstract Drilling through depleted reservoirs have always been challenging as it poses significant risk of lost circulation which always costs time and money to the operators which could lead to well control events when losses are uncontrollable. Besides these issues, it could also result in higher skin factor which could adversely affect production. Hence, it is always better to adopt a proactively approach to prevent losses by various means such as managed pressure drilling, formation strengthening or changes of well design. In drilling campaign A, fracture gradient (FG) of the depleted producing sands are so low that managed pressure drilling (MPD) and managed pressure cementing (MPC) are expected to be require which made some of the wells in the campaign become uneconomical. While in drilling campaign B, the given prognosed pore pressure and fracture gradient (PPFG) were so tight that successful drilling was in question and, the drilling schedule was revised to allow for operator to reassess the commercial viability of the project and develop various strategies to address potential considerable to severe losses and to deliver wells objectives. With the concern on high reservoir depletion, differential sticking and risk of severe losses, initial geomechanics model for the production section was developed based on available petrophysical data. Due to the criticality of the geomechanics model accuracy, additional geomechanics model validation was also performed. ECD simulation was conducted at various operations such as drilling, back-reaming and cementing to incorporate the wellbore strengthening methodology and enhanced fracture gradient (EFG) in order to drill through the depleted sections smoothly. The wellbore strengthening methodology comprises prediction of two main elements namely fracture width which is the size of the fracture that is expected to be initiated in the rock when drilling ECD is higher than formation fracture initiation gradient, and selection of the right size of strengthening materials and their concentration. During actual drilling operation, formation integrity test (FIT). was conducted across the depleted sections and additional downhole measurement data such as mini-frac test data were acquired to verify the geomechanics model. In addition to wellbore strengthening methodology, the strengthening materials performance tests were also done. Prior to drilling, various laboratory tests were conducted such as particle size distribution, fluid loss and permeability plugging tests. Shaker screen analysis was also performed to ensure that the wellbore strengthening materials are retained in the mud and not excessively discarded at surface. Two wells, Well A and Well B, employed above methodology to validate the accuracy of the geomechanics models and implemented customised formation strengthening strategy, particularly for depleted intervals. The size, type and concentration of the wellbore strengthening materials were carefully designed as part of the mud formulations. Their concentrations were monitored in the active mud and regular replenishment was done regularly to ensure their optimal concentration in the mud throughout drilling. In Well A, the drilling window is observed to have increased by 1.5 ppg in the depleted main reservoir sand after wellbore strengthening implementation. The value was confirmed by doing dynamic FIT. In Well B, the EFG is observed to have increased by 4.3 ppg from original FG value from geomechanics model. No losses were reported during drilling and cementing. These two wells showcase the robust drilling solutions for the wells with very tight drilling margin which enabled them to be constructed conventionally without the need of MPD or MPC and saved USD 8 millions for subsequent 5 wells. The methodology solution can also predict the enhanced fracture gradient (EFG) that can be achieved upon deploying the designed formation strengthening strategy based on reliable geomechanics model outputs.