A criterion to estimate the minimum and maximum underbalanced pressures of drilling mud in Iranian carbonate reservoirs

Underbalanced drilling is in fact the best drilling method for oil reservoirs that their pressure has been reduced. This study aimed at obtaining a criterion for estimating the minimum and maximum underbalanced presUnderbalanced drilling is an advanced technology and a drilling operation in which the drilling fluid pressure inside the well is less than the pore pressure of sures of the drilling mud for enhancing oil recovery and preventing damages to the formations in the Iranian carbonate reservoirs. To this end, data on 13 wells in various Iranian oil fields were first collected. The oil wells were simulated at different drilling mud pressures with the help of the finite difference method (FDM)-FLAC2D. The elastoplastic normalized yielded zone area (NYZA) method was used for stability analysis and estimation of the minimum and maximum underbalanced pressures of the drilling mud. The NYZA diagram was plotted separately for each well. Considering the drilling mud pressures on the NYZA diagram in the range of NYZA=1 to the pore pressure, a minimum and maximum drilling mud pressure of NYZA=1 and NYZA=0.2 was calculated, respectively. Eventually, a correlation was proposed as a fraction of the pore pressure to estimate the minimum and maximum underbalanced pressures. This correlation is expressed as a factor or fraction of the pore pressure. The minimum and maximum underbalanced pressures of the drilling mud to enhance oil recovery and prevent damage to the formation can be estimated solely by inserting the pore pressure of the formation in this correlation.