A numerical method for potential implementation of underbalanced drilling in high pore pressure reservoirs

The growing number of depleted reservoirs around the world and the increasing necessity to recover hydrocarbons more efficiently has been forcing the oil and gas industry to continuously improve its drilling technology. ‎ Due to vast application of overbalanced drilling (OBD) and the facilities that are more accessible logistically, the benefits that Underbalanced drilling (UBD) can bring for the reservoir production are often ignored. Especially when it comes to high pressure reservoir, more often drillers are prone to use OBD than UBD due to less cost and challenges. A reliable MEM as an indicator of Wellbore ability to withstand mechanical problems in either high pressure or normal pressure reservoir, can be used to investigate UBD feasibility study. Once the feasibility of UBD approved, an alternative option for drilling such reservoir can be highlighted. UBD is a drilling process in which the mud weight of drilling fluid is intentionally less than the formation fluid density of the being drilled. This underbalanced pressure condition allows the reservoir fluids to enter the wellbore during drilling, thus preventing fluid loss and related causes of formation damage. In this study, a numerical simulation using Finite-difference model (FLAC 3D code) is applied to evaluate the feasibility of UBD in high pore pressure reservoirs in real field. To achieve this objective, the minimum possible mud weight that ensures wellbore stability is estimated by considering an elastoplastic model. The growth of the plastic area around the wellbore is studied and utilized as an indicator of wellbore instability. Moreover, a detailed study is conducted in the zones with no safe mud windows to determine the optimum mud weight.