Issues of Horizontal Well Log Interpretation: an example Longmaxi-Wufeng Shale in Fuling Gas Field of Eastern Sichuan Basin

Abstract

The wireline logs in horizontal wells provide a great opportunity to estimate the lateral variation of reservoir properties. This is a big benefits compared to the log data from vertical wells. But the interpretation of horizontal well log data also brings many new issues. One major issue arises from the data acquiring of log data in horizontal wells. Due to the difference of detection depth and detection direction of various logging tools, the measured zones are not the same. This is not a significant problem for vertical wells, but it has caused some abnormal phenomenon that were observed in horizontal well log data. In this research, we summarized these abnormal phenomenon, then explained the reasons, and finally proposed the methods to deal with it. Another issue is the instability of horizontal wells in mudrocks reduced the quality of logging data in multiple parts of the horizontal section. To generate a reliable assessment of shale reservoir, this researched proposed a methods to detect the low-quality data by caliper logs. The third issues is related to identify the formation tops in the horizontal section of horizontal wells. Repeat of formation tops become common in horizontal wells, especially in the areas where the structure is complex. This makes it a challenge to pick the formation tops in horizontal wells for shale reservoirs. Longmaxi-Wufeng Shale in Fuling Gas Field is a high-quality, organic-rich mudrock. Shale gas production from this formation has been significantly increased in the last three years. In Fuling Gas Field, horizontal wells have been predominantly drilled with a few vertical wells, which is typical for shale reservoirs. In this research, we use it as an example to discuss the issues of petrophysical analysis of horizontal wells. First, we carried out the log correlation primarily based on gamma ray and the set of porosity logs, to estimate the spatial distribution of Longmaxi-Wufeng Shale. Then, log normalization and quality control were completed to improve the reliability of the interpretation results. Uranium log with the differences among density, neutron and acoustic logs were combined together to interpret TOC content, which was verified by core-measured TOC content. Finally, the volume percentage of clay minerals were estimated by CGR (gamma ray from thorium and potassium only) log and neutron log in aid of XRD data and ECS logs, as an intermediate step to proximately estimate the brittleness of organic shale.

AAPG Datapages/Search and Discovery Article #90335 © 2018 AAPG 47th Annual AAPG-SPE Eastern Section Joint Meeting, Pittsburgh, Pennsylvania, October 7-11, 2018