An Integrated Approach to Ensure Horizontal Wells 100% in the Right Positions of the Sweet Section to Achieve Optimal Stimulation: A Shale Gas Field Study in the Sichuan Basin, China
Authors: Xing Liang , Lizhi Wang , Jiehui Zhang , Chenggang Xian , Gaocheng Wang , Xiao Liu , Chunduan Zhao 
Published: November 2015

It is crucial to place a horizontal shale gas well in the optimized position of the sweet section for effective stimulation. However well placement in the Silurian Longmaxi gas shale formation in Sichuan Basin presents significant challenges. This paper presents an integrated approach to ensure horizontal wells 100% placed in the expected positions of the sweet section.

This integrated approach firstly constructed a 3D geological model as reference of the overall trend of structural variations and dynamically updated it per drilling progress. This model contains interpreted micro-structures, micro-faults and natural fracture corridors to reduce structural and lateral reservoir uncertainty. Secondly, the structural correlation in the True Stratigraphic Thickness (TST) domain method enhanced the capability and reliability in the presence of multiple cyclic GR sequences of the shale formation. The innovative real-time multi-scenario forward modeling in the TST domain can identify the most likely scenario to make decision on the probable bit position and its penetrating direction. Finally, a risk mitigation strategy was applied in case of extreme.

The Longmaxi formation contains numerous sub-seismic micro-structures with suddenly local structure changes and micro-faults with unknown displacements. Its sweet section has five sub-layers (I1-I5) with respectively regressive or transgressive GR sequences. But the optimal horizontal lateral position for hydraulic fracture initiating and ultimately effective stimulation is within the sub-layers I-3 to I-4 with about 10 meters of thickness only. Not only requires it accurately landing and placing the lateral, also as smooth wellbore as possible with minimized dog-legs for consequent casing and cementing operations because of the fairly deep depth and high dip angles of the Longmaxi. Hence the standard geosteering method, which updates a 2D cross-section model for structural correlation using changes in GR values and formation dips, can often be insufficient.

The integrated approach were implemented after one well had significant parts of lateral drilled out of the sweet section. It ensured all drilled wells were accurately placed 100% along the optimal position. It proves that efficient well placement constrained by Oil Based Mud environment can be achieved by cost-effective geosteering tools through this integrated approach. The most difficulty situation was that one horizontal well can encounter multiple sub-seismic micro-structures and / or micro-faults with frequent local structural changes. Examples highlight the capability of this approach to handle complex situations in comparing with the standard method. Helped by proactive well placement, drilling efficiency, and casing and cementing operations were improved by evolving understanding of in-situ geology and stresses. High production achieved boosts the economics to develop this shale gas field, also confirms the success of this integrated approach.