Numerical Model of Countercurrent Spontaneous Imbibition in Underbalanced Drilling: Formation Damage Investigation

摘要

Although mud filtrate invasion is mostly concerned in overbalanced drilling (OBD), it is shown that it also occurs during underbalanced drilling (UBD) operations. UBD is a drilling operation in which the hydrostatic head of a mud column is maintained at a pressure less than that of fluid in the porous medium. Formation damage due to mud invasion in OBD could be eliminated in UBD. However, this benefit of UBD can be missed when pressure suddenly turns to overbalance and/or spontaneous imbibition. During UBD it is difficult to maintain mud pressure less than formation pressure especially for short periods of time for operational reasons such as bit trips, and it will cause major damage due to nonexistence of internal and external mud cake. Also, spontaneous imbibition, which occurs in low-permeable reservoirs, causes drilling fluid invasion. In this case, the hydrostatic head of drilling mud is less than formation pore (oil phase) pressure, but it may be higher than water-phase pressure in the formation because of capillary pressure. This causes the flow of water into the formation, which causes formation damage. The aim of this study is to present a model for spontaneous imbibition of water into the water-wet formation during underbalanced drilling and to investigate the effect of drilling fluid, reservoir rock, reservoir fluid, and drilling operation parameters on the extension of damage in nonfractured formations. The numerical solution of governing equations and sensitivity analysis on different parameters are presented. The results show that in the case of strong capillary pressure, (1) the damage due to spontaneous imbibition is considerable and (2) the most effective criteria are temporary overbalanced and static drilling fluid situations where both increase the extend and amount of drilling filtrate invasion. Also, it was concluded that a low UBD pressure difference causes damage, whereas the very high UBD pressure difference is inefficient.