A simulation of mud invasion and characteristics of array laterolog responses in a low-permeability gas reservoir: a case study

Mud invasion is a complex problem in reservoir evaluation. The original distribution of formation fluid and resistivity near the wellbore changes when mud filtrate invades the permeability formations, and the electric logging response always shows distortion. In this study, the influencing factors in mud invasion analysis are investigated for a low-permeability gas reservoir based on a numerical simulation method. These factors include overbalance pressure, formation permeability, initial water saturation, and relative permeability, and we acquire the law of mud invasion and the main controlling factors of the invasion. Based on the simulation results, the array laterolog responses of different invasion situations are calculated by a three-dimensional finite element method, and we hold the opinion that the characteristics of these responses and the separation differences of the curves at different investigation depths are affected by the depth of the mud invasion. Furthermore, combined with the logging data and invasion simulation results, the calculation relation of the invasion depth is established using physical property parameters and array laterolog responses. This method effectively hides the influencing factors of invasion time and has good applications regarding the logging data of different invasion moments. In a low-invasion well case, the calculated result is in accordance with the actual situation and verifies the reliability of the method. The research shows that the combination of mud invasion characteristics and array laterolog responses can be applied to the prediction and evaluation of mud invasion depth in the study area. Furthermore, it can provide a service for fluid property evaluation and resistivity correction, as well as improve the reliability of reservoir evaluation by electrical logging.