Evaluation of formation mobility inversion methods for formation testing while drilling

Formation mobility is a key parameter for characterizing reservoir permeability and predicting productivity. Formation mobility can be obtained in real-time by formation testing while drilling (FTWD), which is vital for improving the efficiency of oil and gas development and reducing production costs. In terms of FTWD, adopting an appropriate formation mobility inversion method is the key to accurately obtaining the mobility of the formation. The objective of this research is to evaluate the applicability and accuracy of formation mobility inversion methods in various coupled formations. Therefore, the mobility inversion methods for FTWD were reviewed in detail. Then, the process of finite element simulation of FTWD in various coupled formations, including uncoupled, hydro-mechanical coupled, thermal-hydro coupled, and thermal-hydro-mechanical coupled, was introduced. Based on the pressure response of FTWD generated by numerical simulation, four case studies of formation mobility inversion were performed using three different inversion methods, namely pressure drawdown (PDD), area integration (AI), and formation rate analysis (FRA). Finally, an evaluation was performed to assess the applicability of the inversion methods to different coupling formations, and the suggested method for each formation was provided. The results showed that the interpreted formation mobility based on the pressure response of FTWD is the apparent mobility near the wellbore, which can be influenced by the multi-physical coupling effect. The PDD method has a good applicability to high-mobility formations, a general applicability to low-mobility formations, and a poor applicability to tight formations. Both the AI and FRA methods can be used to invert formation mobility under different coupling conditions, and the accuracy of the FRA method is generally superior to that of the AI method. If the formation is in a coupled thermal-hydro-mechanical state, it is recommended to use the AI method, PDD method, and FRA method to invert the mobility for high-mobility, low-mobility, and tight formations, respectively. The results of this paper are beneficial for the accurate interpretation of formation mobility, which can promote the development of oil and gas resources.