Identification of Shale Gas Target by High Definition Oil Based Microresistivity Image Logs in Horizontal Longmaxi Shale Wells

Abstract

The identification of the shale gas target in the Longmaxi shale horizontal wells presents challenges due to the similar gamma ray readings of neighboring layers and the complex structural faults that seismic data cannot detect because of resolution limitations. This makes the correlations of the shale gas log evaluation and actual gas production between horizontal laterals difficult. The average thickness of the shale gas target in vertical offset wells and pilot wells is about 5 m. The well trajectories of the horizontal wells in the Longmaxi shale were planned from the gamma ray logs of vertical offset wells and seismic data, and the placement of the horizontal wells was performed by a gamma ray log measurement-while-drilling tool. Identifying the shale gas target layer and optimum stimulation staging design in two oil-based mud horizontal wells were the objectives in this case study. The lengths of the horizontal wells were 1821 m and 1300 m. The similar gamma ray readings were in the Lower Silurian shale gas target layer and the Upper Ordovicican Wufeng shale layer. Between the layers was the widespread shell limestone of the top of the Upper Wufeng Formation, from 0.2 m to 0.6 m thick in the field studied. The similar gamma ray readings indicated multiple possibilities for the shale gas target along the horizontal wellbores, among which were portions that were out of the shale gas target zone and portions that were inside the shale gas target zone. In addition, the structural faults that cut through the horizontal wells made discriminating among the multiple possibilities more complex. New-generation high-definition oil-based microresistivity image logs were run in the two oil-based mud horizontal wells. The objective was originally to identify natural fractures, which information was commonly used in perforation cluster design and stimulation staging. However, the high-definition oil-based microresistivity image logs provided more detailed structural information along the horizontal wellbores, including displacement faults and structural dips. With the help of 3D structural modeling techniques, the true stratigraphic drilling polarity and structural model of the horizontal wells revealed the position of the horizontal wellbores relative to the shale gas target layer. The portions inside and outside the shale gas target zone were identified from the structural model. The new-generation high-definition oil-based microresistivity image logging was a good solution for the identification of the shale gas target in the Longmaxi shale horizontal wells. It eliminates the multiple possibilities of the shale gas target from gamma ray logs along the horizontal wellbores. The more detailed structural information about fractures, faults, and the portions inside the shale gas target zone was used in optimum stimulation staging design. In addition, the oil-based microresistivity image logs were used to distinguish between open fracture and cemented fracture by inversion processing.