Abstract
Mud invasion affects most data acquired by logging tools which in turn affects formation evaluation. Historically, correction programs have been designed to compensate for this effect. With the introduction of Logging While Drilling (LWD) it has became possible to gather data before invasion masks reservoir properties.
Complete logging suites run in a number of wells on-shore Abu Dhabi allowed quantification of invasion effects using logging while drilling, logging while pulling out and wire-line logs. The comparisons, in both horizontal and vertical holes, indicated considerable differences in resistivity and nuclear measurements between runs. The variations depend on time of exposure, porosity, differential pressure and permeability. In horizontal wells where exposure time is longer, resistivity values were greatly reduced by invasion, to the extent of masking true formation resistivity which could not be corrected through known algorithms.
These phenomena may explain differences between log interpretation and test results. In some cases, dry oil was tested in formations that calculated high water saturation, up to 70%, based on logs affected by invasion. It became apparent that the invasion profile had different characteristics in gas zones, providing a new tool for gas detection. In addition, monitoring invasion processes is a useful method for fracture detection by overlaying while-drilling and after-drilling resistivity curves. Fractured zones detected by this method were confirmed by bore-hole imaging techniques. This study proposes a complete framework for evaluating invasion effects leading to improved reservoir description.