Joint inversion method of electromagnetic wave resistivity logging while drilling and its application

In view of the fact that the electromagnetic wave resistivity logging while drilling(LWD) is affected by low invasion of drilling fluid under the condition of a shaft with positive high pressure difference in offshore oil and gas fields, a joint inversion method of electromagnetic wave resistivity logging while drilling was proposed in this study. Then a selection scheme of initial inversion value was designed based on the separation degree of apparent resistivity curves, so as to obtain the original formation resistivity and drilling-fluid invasion depth through inversion. Numerical simulation results show that the accuracy of initial value selection can meet the requirements of engineering application, and the relative errors of inversion parameters such as true formation resistivity and drilling fluid invasion depth are less than 0.5%. The true formation resistivity obtained through respective inversion of time-lapse logging data is consistent, proving that this method is steady and reliable. A resistivity inversion application study was carried out according to LWD data in western South China Sea. In combination with inversion results, the statistics were performed on drilling-fluid invasion laws to obtain the statistical relationship between drilling-fluid invasion depth and various parameters such as formation porosity, permeability, drilling-fluid column pressure, drilling-fluid soaking time and formation resistivity variation, etc., which provides a reference for the prediction and evaluation of drilling-fluid invasion. This joint resistivity inversion method can be applied to LWD in other seawaters, and has great significance to reservoir evaluation and reserve calculation.