Influence of Permeable Wellbore on Formation Testing While Drilling and Mobility Inversion

Formation pressure and mobility represent two fundamental parameters that are essential for the development of oil and gas resources. These parameters can be obtained in real time through the process of formation testing while drilling (FTWD). It is highly probable that the drilling fluid will invade the formation during the FTWD process. Nevertheless, the prevailing theory regarding FTWD assumes that the wellbore is impermeable, thereby rendering its potential impact on FTWD and mobility inversion unclear. Therefore, to clarify the influence of the permeable wellbore on FTWD and mobility inversion, a mathematical model of FTWD seepage was first proposed by involving the permeable wellbore. Secondly, the finite element method was used to solve this model, and this model was verified by using the analytical models. The pressure response curves and isobaric surface near the FTWD probe were then compared for both the permeable and impermeable wellbores, and the influence of the permeable wellbore on the pressure response curves of FTWD was analyzed. Finally, the method of integral area was used to invert mobility, and the compressive influence of different factors on both the pressure response curves and mobility inversion was discussed for both the permeable and impermeable wellbores. The results indicated that the permeable wellbore has a significant impact on the pressure response curves and isobaric surface near the probe due to the limited pressure sweeping range around the probe and the invasion of drilling fluid. In the case of a permeable wellbore, the invasion of the drilling fluid into the formation can cause a supercharge effect around the well. This effect can cause an initial increase followed by a decrease in the pressure buildup phase. The pressure buildup always exceeds the original formation pressure, which can lead to an overestimation of the measured formation pressure compared to the original. Meanwhile, the permeable wellbore can also lead to an overestimation of the inversion mobility, but the impermeable wellbore has much less influence on the mobility inversion. To improve inversion accuracy, it is recommended to increase the rubber packer radius, lengthen the suction period, reduce the storage volume of the pipeline, and decrease the overbalanced pressure. However, these measures cannot mitigate the impact of the supercharge effect on formation pressure testing. This paper provides theoretical guidelines for the use of FTWD tools and data interpretation.