Pore pressure evaluation of formation testing while drilling under supercharged conditions

Abstract

Formation Testing While Drilling (FTWD) to evaluate pore pressure has significantly progressed in geo-energy exploitation. However, with the increasing number of unconventional reservoirs, this technology is facing some challenges owing to the common occurrence of supercharging in the vicinity of boreholes. The pressure response of FTWD under supercharged conditions is different from that of non-supercharged; thus, conventional methods — such as Pressure DrawDown (PDD), Pressure BuildUp (PBU), and Formation Rate Analysis (FRA) — cannot be used to determine the original pore pressure. In this paper, a new mathematical model of the FTWD pressure response under supercharged conditions is proposed by considering the time-dependent filter cake, supercharging, skin, geometry, and storage effects. The calculated results indicated that when the formation permeability was lower than 5 mD, the supercharging effect on the pressure response of FTWD cannot be neglected. The pressure response curve of FTWD under supercharged conditions only includes three segments: the fast pressure drawdown stage, pressure buildup stage, and recovered pressure decreasing stage. It is also found that the pressure response in the later stage is independent of the withdraw withdrawal pumping modes and the pressure response curves are nearly the same. Based on the pressure response characteristics, a novel method to evaluate the original pore pressure was proposed. Case studies show that the proposed method can obtain the correct virgin pore pressure after a few iterations and the relative error is lower than 0.2%. Compared with conventional methods, this method is more accurate and saves time; thus, it can be used to evaluate the original pore pressure of FTWD under supercharged conditions.