A new geosteering approach significantly improves effectiveness and efficiency of geosteering in shale gas fields such as the Silurian Longmaxi gas shale in the south Sichuan basin, China. This formation presents more challenges than in classical sand/shale formations even when logging while drilling (LWD) imaging is available because of the cyclic gamma ray (GR) response of the targeted shale formation, subseismic microfaults with unknown displacements, and sudden structure changes from subseismic microstructures.
The new approach introduces more geosteering elements including correlation in the true stratigraphic thickness (TST) domain with considerations of the cyclic nature of shale gas zones; conducting multi-scenario modeling to verify the most likely scenario fitting the overall structural trend and the cyclic features; using an 3D geological model as the overall guidance of structural variations; and using a key bed detection strategy when the above methods can't precisely determine structural position.
The sweet section of Longmaxi shale has five sub-zones with respectively regressive or transgressive sequences, and the bottom zone with extremely high GR readings. The cyclic nature often resulted in inconclusive structure correlation based on changes of GR readings only. Field experiences showed numbers of subseismic microfaults with unknown fault displacement, and sudden changes of structural dips due to sub-seismic microstructures can be encountered, which can make the standard geosteering method insufficient to judge the actual position of the horizontal lateral. The new approach especially the multi-scenario modeling method in TST domain and the downward detection geosteering strategy to track the bottom high GR marker shale zone can clarify in-situ geology and minimize potential out-of-zone risk. The new approach has ensured all drilled horizontal wells was implemented 100% in the sweet section during the pilot and factory drilling phases. This paper demonstrates several examples to highlight its capability to handle complex situations for effective geosteering in contrast to the standard approach. The field practices also show this new approach can help to reduce unnecessary changes of drilling directions to ensure a smoother wellbore for subsequent operations. Our experience suggests iteratively updating the 3D geological model with improving accuracy must be treated as part of the overall geosteering strategy. Also a real-time decision making scheme throughout all involved segmens must be ensured for seamless operation.