Corrections of High Frequency Induction Isoparametric Wireline Logging Tool (VIKIZ) data in high deviated wells filled with conductive muds

Gorbatenko A.A., Suhorukova K.V.

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

[e-mail]

High frequency induction isoparametric wireline logging tool (VIKIZ) is extensively used in Russian oil fields, especially in Western Siberia. It is successfully applied in wells with low deviation filled with resistive muds (1-3 Ohm.m). But nowadays conductive muds are also used both in vertical and deviated boreholes. In such cases VIKIZ logs are influenced to various factors which are not significant in resistive borehole fluids. If we are to achieve accurate interpretation results it is necessary to dispose of negative effects which mask formation response.

First of all, inversion programs are built for case when the tool is located on the axis of borehole. In real situation, while moving up tool usually lies on the wall of borehole, especially in deviated wells. In such case borehole effect is not compensated. In addition, eddy currents crosses regions with two different conductivities. It causes polarization of the border and this also influences to signal. These effects increase apparent conductivity for coil arrays with shallow depth of investigation. The shallowest array will be influenced the most and thee deepest almost won’t change its signal. As a result, interpreter can find invasion in formations where it actually absent. One can avoid this problem using simple correction by previously calculated data. Note that relative amplitude (A2/A1) is more stable to such effect that phase shift. But usually it also needs correction (Fig. 1).

Fig. 1. Tool decentralization effect for array DF05 (0.5 m, 14 MHz). Data calculated for borehole diameter D=0.124 m. RoS – mud resistivity; RoP – formation resistivity.

Fig. 2. Real caliper (left) and VIKIZ (right) logs. Caved borehole causes significant noises.

Another problem is rugose and caved borehole. Since VIKIZ employs high frequencies (from 0.875 to 14 MHz) conductive masses near borehole cause overshots which can mask thin layers and impede finding borders of layers and calculating true formation resistivity. In caved well it can be registered both single overshots and noise along all the log. As illustrated at fig. 2, borehole rugosity causes signal fluctuations which mask borders and formation response, but in smooth part of well fluctuations disappear. Sometimes this noise has constant period and amplitude. It also can be caused by special borehole shape formed while drilling. Investigations have shown that although curves logged at caved borehole have high-amplitude overshot, their average level depends on invasion and formation resistivity. It is also noted that aplitude of overshot depends on contrast between mud and formation resistivity and depends linearly on cavern depth. It means that such noises can be removed by frequency domain filter or another type of filters.

Deviated well can be also seen as case of dipping beds. There are three direct effects of dip. First of all the apparent thickness of a resistive layer is lengthened as the tool takes a longer distance through it. This can be easily corrected, if desired, by changing the depth index to one that is perpendicular to the bedding planes. Second, the eddy current lines, which circulate in a loop that is concentric with the tool, now cross regions of two different conductivities. This is a type of shoulder effect, reducing the resistivity near the boundary. On a multi-array induction log, the deep logs are much more affected than the shallow logs, giving the impression that there is invasion. Finally spikes is observed, because electrical charge at the boundaries.

After corrections which help us overcome effects described above we can make inversion and gat accurate formation electrical parameters.

References:

1. Darwin V. Ellis, Julian M. Singer. (2007) Well Logging for Earth scientists, Multi-Array and Triaxial Induction Devices, 179, Springer, USA, 692pp

2. Эпов М.И. Шурина Э.П. Нечаев О.В. Прямое трехмерное моделирование векторного поля для задач электромагнитного каротажа// Геол. и геофиз. - 2007. - Т. 48. - № 9. - С. 989-995

3. Суродина И.В., Эпов М.И. Влияние биополимерных буровых растворов на диаграммы высокочастотного электромагнитного каротажа// Геология и геофизика, 2012 г. (в печати)

4. Игнатов В.С., Сухорукова К.В. Влияние эксцентриситета зонда на сигналы высокочастотного электромагнитного каротажа / НТВ «Каротажник». – Тверь: Изд. АИС, 2009. – Вып.182. – С. 101–110.

Дата добавления: 2015-10-29