Time-Dependant Responses of High-Definition Induction Log and Case Studies

The process of drilling mud filtrate invading into a reservoir is time dependant. It causes dynamic invasion profiles of formation parameters such as water saturation, salinity, and formation resistivity. Thus, the responses of a high-definition induction log (HDIL) tool are time dependent. The logging time should be considered as an important parameter during logging interpretation for the purposes of determining true formation resistivity, estimating initial water saturation, and evaluating a reservoir. The time-dependent HDIL responses are helpful for log analysts to understand the invasion process physically. Field examples were illustrated for the application of present method. 1. Introduction Resistivity log is an effective method to estimate the critical properties of a reservoir in petroleum exploration. Induction log is an important tool to measure the formation resistivity. In addition to traditional dual-induction log tools [1], new array type induction devices [2], such as array induction tool (AIT) and high-definition induction log (HDIL), can provide more messages. The AIT device was designed with 10-, 20-, 30-, 60-, and 90-inch (25, 51, 76, 152, and 229？cm) depths of investigation, and the HDIL tool provides six depths of investigation, 10, 20, 30, 60, 90, and 120？inches (25, 51, 76, 152, 229, and 305？cm), respectively. The HDIL instrument is a typical multiarray induction logging tool that measures the formation resistivity simultaneously with six arrays. However, the resistivity measurement is affected by invasion process strongly. When a reservoir was opened, drilling mud filtrate poured into permeable and porous formations; thus, an invaded zone was formed. The behaviors of the invaded zone are very different from the original formation. The physical understanding of an invasion process is essential for logging interpretation and reservoir evaluation. Conventional invasion model is the static step-invasion profile [3] that presumes that the resistivity varies sharply at the boundary between invaded zone and uncontaminated formation. In fact, the displacement of moveable native fluids in a reservoir by mud filtrate is a percolation process. The formation- and fluid-related parameters do not vary in a step-invasion style. In addition, a realistic invasion process is time dependant. At the beginning of bit penetration, the rate of invasion is rapid. With the lapse of time, mud cake is built at the wall of borehole. With the building of mud cake and the extension of invading geometry area, the invasion rate decreased. The dynamic invasion