Abstract
Ordos basin in central China is well known for its rich accumulation of natural resources, including Triassic tight oil and Permian tight gas. A recent exploration breakthrough shows that Ordovician shale in the same basin is also promising. The purpose of this study is to capture the engineering details of two horizontal exploration wells exploration in Wulalike formation, which mark the first production of marine shale gas in Ordos basin.
The Ordovician Wulalike formation in the Ordos basin was previously seen as source rock. During early exploration in the 2010s, the formation was found to be gas bearing. However, the Wulalike shale formation shows very different features compared to the Triassic lacustrine shale in the same basin and the Silurian marine shale from Sichuan. The abundance of natural fissures, the low reservoir pressure, and the tendency to produce water are unique challenges and concerns for the Wulalike shale formation.
Based on the pilot well evaluations, two horizontal wells were drilled and completed in the Wulalike formation in different locations in the western Ordos basin in 2019–2020. Both wells were well-landed in the target zone and were completed with multistage large-scale fracturing treatments. Following the well completions, flowback and production tests lasted for 3 to 5 months. Production tests showed that well 1 reached an economically acceptable gas rate in natural flow for a long-term period, producing 20,000 to 60,000 std m3/d, and well 2 produced good gas in the early period but was soon overwhelmed by massive water production. Both wells were evaluated with production logging tools. In well 1, fiber-optic distributed acoustic sensing (DAS) and distributed temperature sensing (DTS) were used, and in well 2, a production logging tool (PLT) was used.
The positive gas production from both wells marks the first production of marine shale gas in the Ordos basin. The understanding of the geology and reservoir, the use of unconventional fracturing and completion practices, the assistance of energized fluid, and post-treatment artificial lift are the technologies that helped achieve this success. Further study is needed on the complexity of the natural fissures to lower the risk of unwanted water production from the Wulalike rocks.
The first successful production from the Wulalike is very critical for the exploration of the Ordovician section in the Ordos basin because it helps to confirm a favorable exploration and appraisal area of 2000 to 3000 km2, which has the potential to turn into a huge reserve. This case study provides value from a technical standpoint, as very few success stories have been reported from low-pressure shale gas previously in China or worldwide.
Volume fracturing and drainage technologies for low-pressure marine shale gas reservoirs in the Ordos Basin
