Renovation of Old Well Logs Run in Boreholes Drilled with Oil-Base Mud

Abstract

The problem of correcting for oil-base mud filtrate invasion has been resolved using modern well logging technology of tools and interpretation techniques. However, many well logs from old wells remain uncorrected. Old interpretation assumed no oil base mud filtrate invasion. The consequences may vary between unnecessarily perforating a water-bearing zone to even worse by completely by-passing a hydrocarbon formation. Lau et al. (1989) developed a correction for oil-base mud effects on neutron and density logs, however the standard formation evaluation techniques from the Dual Induction Resistivity Log, DIL, relies on knowledge of the resistivity of the invaded zone, Rxo. Since no electrode-type tool can work in oil base mud to measure Rxo, a synthetically derived Rxo from the Electromagnetic Propagation Time (EPT) or the Thermal Neutron Decay time (TDT) logs is used. In the absence of these unconventional EPT or TDT logs, interpretation is performed assuming no oil mud invasion and the deep induction resistivity, RID, is reading the true formation resistivity, Rt. However, it has been proven that oil mud filtrate will invade the formation sometimes to a diameter greater than 120 inches. This invasion will greatly affect Rt masking the hydrocarbon potential of the reservoir to the extent that a water zone may appear as hydrocarbon-bearing. Without proper consideration to the oil-base environment surrounding the logging tools, essential petrophysical parameters such as true formation porosity and resistivity cannot be accurately measured. Techniques and concepts such as crossplotting log j versus log Rt or log Rxo, BVW, shale zonation index, fracture partitioning coefficient, etc. may not be all conducted. Evaluation of properties such as Sw, Sxo, Smo, m, n, jma, jf, FII, etc. will not be reliable. Consequently, zonation of a heterogeneous reservoir into its hydraulic units cannot be accomplished. In this study, a new practical and cost effective technique is introduced to correctly evaluate formations with deep oil-base mud filtrate invasion of old well logs in the absence of EPT, TDT, logs, i.e. without a prior knowledge of Rxo or Rw. This will allow characterization of old reservoirs drilled with oil-base muds using the available old conventional well logs without the need for running new expensive well logs.