INVASION IN SPACE AND TIME

摘要

All wireline and logging while drilling tools are to some extend adversely affected by the borehole fluids and invasioa The severity of these effects depends on the depth of investigation of the tools, and at the moment in time that the measurements are made. Logging while drilling tools often measure while the invasion process is still in progress, and wireline logging tools after a steady state is reached. Environmental corrections charts and algorithms for individual tool have been available for a long time, but corrections are usually still based on simplistic invasion profiles, and seldom take the time factor into account. The sophistication of modern tools is apparently not matched by improvements in invasion correction programs.rnThis paper reviews the invasion process and discusses the effects of various mud types on most common logging tools. Logging suites are recommended for certain mud types that allow the log analyst to alleviate invasion and borehole effects. Several field cases are used to illustrate the invasion process. A MWD log with 7 repeat runs was analyzed with forward models, in which the user can specify the invasion parameters interactively. The results prove that the piston invasion model is usually adequate for deep resistivity calculations, but a more detailed invasion profile is required for porosity log corrections.rnIt is concluded that borehole and invasion effects can probably never be completely eliminated, but by combining the responses of various tools and forward modeling, the most likely solution can be found, and erroneous interpretations avoided.