A wellbore stability analysis of a horizontal well drilled through unconsolidated sandstones

Wellbore stability studies are essential to guarantee the success during drilling oil and gas wells. This case study shows a Post Morten analysis of a horizontal well successfully drilled along a depleted reservoir, but with a narrow mud weight window, after a lost circulation during the cementation of the previous casing. The main goal is to show that even having a well-established general methodology for wellbore stability analysis, specific challenges can require different solutions to calibrate the mud weight window with field evidences. Challenges faced during drilling this wellbore were: (i) the wellbore was drilled after a lost circulation during cementing the previous phase. (ii) A narrow mud weight window conditioned by unconsolidated sands with poor resistance parameters into the reservoir; (iii) the well path implies a critical combination of stresses around wellbore wall; (iv) a depleted reservoir. The in-situ stress prediction of horizontal stresses involved different criteria and showed a strong difference between the overburden and the reservoir. Minifrac tests performed in plenty of wells along the field presents a considerable variation along the reservoir, showing in-situ stress variation along the field. The analysis imposed a series of iterations by a carefully guided calibration of the input parameters and failure criteria. The analysis allows to calibrate and explain field evidences, including the losses during cementing