Fluid dynamics in collectors by using VIKIZ technique

Mikhaylov I.V.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia

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There is a relationship between logging time and borehole environment electrophysical parameters. So, a single logging tool measurement reflects some stage of fluids distribution only. Therefore, interpretation of early measurements can considerably differ from such, if logging was done in durational time interval after drilling [1]. Making use of VIKIZ technique, which has a unique geological section resolving power [2], allows tracing collector’s electrophysical parameters evolution. It is feasible by considering VIKIZ logs, obtained in a different time spans after a drilling process.

Productive reservoir (Fedorovskoe oilfield, Western Siberia) cross-section by the directional hole with a horizontal completion is represented in Figure. The well trajectory and the spontaneous potential curve are represented in the left well log track. In turn, the VIKIZ logging traces, obtained in the different time spans after drilling, are figured in the following log tracks. These VIKIZ traces were gotten as step-by-step wellbore increasing. Logging dates are 17.11, 19.11, 21.11 and 23.11, respectively.

The time-lapse VIKIZ logs directly reflect the fluid dynamics in the vicinity of borehole. Furthermore, the respective time-lapse sounding curves (the right part of the Figure) add visualization to the logging results. The sampling places for creating sounding curves are shown with the help of arrows.

The picket 2015 m along the well trajectory is characterized as gas-bearing. The replacement of gas by fresh mud filtrate resulted in the collector resistance changing. The nonsimultaneous VIKIZ sounding curves indicate that the invaded zone resistivity considerably increases in the course of time. Moreover, there is the apparent resistivity minimum in the VIKIZ intermediate sondes sensitivity area. It reflects the low resistivity annulus presence, which directly denotes mobile gas occurrence. This area is formed between the invaded zone and unmodified reservoir’s part. In the course of time, the low resistivity annulus front penetrates deeply into the reservoir, considerably understating its apparent resistivity.

The picket 2220 m is more water-saturated than oil-saturated. On the sounding curves, corresponding to the early measurements (17.11 and 19.11), the apparent resistivity minimums are situated in the VIKIZ intermediate sondes sensitivity area. Those minimums are the indicators of the formed low resistivity annuluses, which reflect movable oil presence. The subsequent measurement results (21.11 and 23.11) are expressed on the sounding curves as resistive invasion without low resistivity zone signs, falsely denoting collector’s water saturation.

Fig. The VIKIZ logs obtained in the different time spans after drilling (the middle part) and plots of resistivity as functions of radial distance from the borehole (the right part). Fedorovskoe oilfield, Western Siberia.

The picket 2356 m is from the oil-saturated interval. The low resistivity annulus, which is formed here, reveals itself on the all measurement results. Respective sounding curves show the low resistivity annulus dynamics as well as the invaded zone and virgin formation dynamics. The invaded zone resistivity increases many times in the course of time due to the formation water replacement. In turn, the low resistivity annulus, gradually widening into the virgin formation, understate the VIKIZ long-spaced curves apparent resistivity. This fact neglecting leads to incorrect log data interpretation.

Thus, one of the significant factors influencing the interpretation results is measurement time. Time-lapse VIKIZ measurements make it possible to trace near-wellbore electrophysical parameters evolution. It provides more reliable reservoir’s properties estimation.

The author expresses thanks to Professor Yuriy N. Antonov for his guidance and supporting in researches.

References:

1. Antonov Yu. N. VIKIZ logs interpretation (1999). NTV “Karotazhnik”, Tver: Publishing House AIS, № 64, pp. 6-43.

2. VIKIZ Method for Logging Oil and Gas Boreholes (2002). Edited by Epov M. I. and Antonov Yu. N. Novosibirsk: Branch “Geo” of the Publishing House of SB RAS, 112 pp.

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