A Methodology of Underbalanced Drilling Design in Depleted Reservoir

Abstract When overbalanced drilling is done in depleted reservoir, lost circulation occurs and cause formation damaged. Underbalanced drilling (UBD) is proposed to be done to prevent lost circulation and increases production due to minimizes formation damaged. Besides, as formation being drilled underbalanced, it is possible to produce hydrocarbons from permeable zones, therefore earlier benefit could be acquired by company as producing phase began. When a well is drilled underbalanced, formation fluids flow into the wellbore from any permeable formation in the open-hole section. It is possible to use the volumes of produced hydrocarbons, from a well drilled underbalanced, to provide an indication of the productivity of pay zones that have been penetrated (GRI, 1997). The study in this paper is started from reservoir simulation to estimate the current reserve and plan for re-entry and new drilling well. Furthermore, drilling fluid selection and pressure window are analyzed based on reservoir characteristics and engineering approach. The last, the total hydrocarbons gained during underbalanced drilling in reservoir section will be predicted by Vogel equation. Introduction Drilling underbalanced into formations means to maintain a lower annular borehole pressure than the formation pore pressure while drilling a well (Nellayappan et al., 2008). The hydrostatic head of the drilling fluid is intentionally designed to be lower than the pressure of the formation being drilled. There are several benefits related to underbalanced drilling, which are:Underbalanced drilling can improve a penetration rate due to reduced "chip hold down" and decreases hydrostatic pressure at the bit face. Reduction in "chip hold down" refers to easier removal of solid cuttings, decreases hydrostatic pressure at the bit face, and reduces stress in the rock being drilled, thus allowing the formation to fail more easily. The bit life will be improved due to minimized contact with the formation while penetration depth is increased.In depleted reservoirs, lost circulation often occurs due to overbalanced drilling. With underbalanced drilling, there is no fluid flow into a formation, so lost circulation is reduced.With no flow into formation, formation damage is minimized, so productivity will increase, and the need for simulation lowered. Reduced formation skin will increase recoverable hydrocarbons so that the productivity will be superior to conventionally drilled wells. As there is no skin, the drainage area will increase and resulting in lower abandonment pressure; thus raising the recovery factor significantly more than overbalanced drilling would allow (Dahl and Vos, 1999).