Invasion in the Time Domain from LWD Resistivity: An Untapped Wealth of Information

摘要

Petrophysical evaluation is normally carried out using depth-indexed logs. This is sometimes supplemented with time data from stations (such as formation pressure measurements and NMR stations). The depth-based data may be complemented by a short repeat section designed to confirm the results (depth data acquired at a different time). This depth-based approach is applied to both logging while drilling (LWD) and wireline data sets. With wireline logging and interpretation, environmental conditions are relatively stable over the period of logging. The same cannot be said for acquiring data with the drilling assembly, i.e., LWD. The LWD data is acquired in a very dynamic environment (both physically and in terms of fluid movement). Variable rates of penetration lead to variable times after bit. However, the drilling process is not constant with depth. The BHA is moved up and down over many parts of the formation multiple times. In addition, the BHA is held stationary at different points (e.g., for connections). The result is a complex set of invasion profiles. Invasion modeling is shown to demonstrate the impact of the dynamic conditions during drilling on the invasion / resistivity responses. LWD tools acquire data with a time index and record data over most of the time the BHA is in the borehole (all when batteries are present). Depth-indexed data is extracted from the larger time-indexed acquisition data set. The time-indexed data is rarely requested or presented for evaluation. However, these time-indexed measurements contain great potential for understanding the reservoir. The opportunity exists to use stationary LWD data as a direct dynamic measurement of the formation and movement of fluids. These dynamic measurements can provide information on the quality of the mudcake at the time of measurement. There are also conditions where flow rates and, by inference, formation permeability can be derived from these time-based measurements (i.e., effectively injection logs while drilling). The examples shown illustrate some of the untapped potential that exists with every LWD data acquisition run. Significant opportunities exist with this time-based data, from enhanced quality measurements to the interpretation of dynamic downhole data.