Anomalies in Resistivity Logs Caused by Borehole Environment

Ko Ko Kyi, Retired Principal Petrophysicist

Resistivity logs are critical input data for petrophysical evaluation as they are used for identification of possible hydrocarbon bearing intervals, as well as determination of water saturation in reservoirs of interest. Therefore, it is important to ensure that resistivity logs are properly acquired, using appropriate type of resistivity logging tools, suitable for the borehole environment, such as hole size, type of mud, mud salinity etc., in which these tools are deployed. Generally, resistivity logging tools are divided into two main types, namely induction tools and laterolog tools. Induction resistivity tools are used in non-conductive or fresh mud and the laterolog resistivity tools are used in conductive or high salinity muds. In addition to the drilling mud, hole size also has an effect on the resistivity logging tools. Wireline logging tools are generally 3 ½ to 4 inches in diameter and are not suitable for very big holes as the mud around the tool will have significant influence on the tool response.

Even Logging While Drilling (LWD) tools have limitations on the size of the hole in which they can be deployed effectively. A very saline mud inside a large borehole can have serious effects on the response of an induction logging tool. Borehole size, as well as eccentricity of the LWD resistivity tool can have deleterious effects on resistivity logs. LWD resistivity tools work on similar principle as the wireline induction resistivity tool and therefore borehole size, high salinity mud, eccentricity can cause anomalous readings of the LWD resistivity tool. The following examples illustrate the anomalies in resistivity logs caused by high salinity mud in a large borehole on wireline resistivity logs; eccentricity and borehole enlargement effects on LWD resistivity tools.

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Figure 1: The Logging While Drilling (LWD) 2MHz Phase Shift Resistivity RPCEHM (red colour curve) and Gamma Ray (GR) logs have anomalous readings due to eccentering and borehole effects inside 17 ½ inch hole, which have an hour-glass shape due to drilling with mud motor and a bent sub. The mud motor drills an in-gauge hole in sliding mode, but drills an enlarged hole when in rotating mode, creating an hour-glass shape hole. GR and resistivity logs (especially the 2MHz Phase Shift, in red colour) are affected by the shape of the well bore. The variations in hole size will affect the GR log due to correction for mud weight and volume around the tool. Resistivity is affected because LWD propagation tools work best when centered in the hole. If there is a big contrast between mud and formation resistivities, the electromagnetic wave will see this contrast on one side before the other, which will distort the wave into a non- spherical one causing interference of the 2MHz readings, resulting in an anomalous response on the resistivity log.

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Figure 2: The MWD (Measurements While Drilling) GR/Resistivity logs Figure 3: The wireline Array Induction Resistivity curves are behaving appear normal in this 17 ½ inch vertical hole section, drilled with a high erratically. This is due to the effect of high salinity mud inside a big (17 ½ salinity mud (82,000 ppmNaCl equivalent). inch) hole. A laterolog type of tool would have been a better choice in this case. Due to remoteness of the well location, the only tool available for the surface hole logging was an induction tool. Same well as in Fig. 2.

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Figure 4: The anomalous LWD resistivity log readings are caused by tool eccentricity Figure 5: The wireline Array Induction Resistivity logs are less affected by the inside an enlarged borehole, due to the big difference in resistivity of the SBM enlarged hole interval, although the shallowest resistivity log RT06 sees more (Synthetic Oil Base Mud) and the resistivity of water bearing formation. The hole SBM effect in the bottom part of the water bearing sand. (Same well as in Fig. 5) enlargement can be seen on the ultrasonic caliper log (blue colour log in Track-1).

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