Yuriy Antonov

Invaded zone evolution reconstructed from logging data

Summary Physical properties of the near-wellbore zone change during drilling and other jobs in the well. Formation fluids are displaced by drilling mud filtrate as a result of pressure overbalance. This leads to the change in petrophysical properties of the near-wellbore zone and creation of the invaded zone, which is accompanied by mudcake buildup on the borehole wall, slowing down the filtration. These processes are controlled by petrophysical properties of the formation, such as porosity, permeability, and fluid saturations. Therefore, the study of invaded zone evolution is very important and provides the means for petrophysical evaluation of the reservoir. Moreover, saline concentrations in mud and formation water are usually different. This difference, together with heterogeneous fluid distributions in the invaded zone, leads to significant changes in electrical and other formation properties in the near-wellbore zone, as measured by logging tools. This work presents a hydrodynamic model of invaded zone evolution. The model takes into account two-phase filtration, salt transport, mudcake buildup, and pore plugging in the formation. Various drilling modes and water-based mud properties are taken into account. The presence of horizontal layers and influence of their petrophysical parameters on invaded zone evolution are investigated as well. The developed methodology was applied to a specifically designed field experiment (including drilling, measurement-while-drilling (MWD) measurements, and wireline logging) at experimental test area in Oklahoma U.S.A. The results of this controlled experiment are consistent with the numerical modeling of invaded zone evolution during drilling and wireline log responses, as presented in this paper. Inversion of hydrodynamic modeling results, together with wireline logging data, provides the means to calculate formation porosity, permeability and fluid saturations. This paper presents the results of such calculations as well as their comparison with conventional logging interpretation data and core analysis measurements.