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.

Heterogeneity of properties along horizontal shale gas wells has a significant impact on the quality of completion as well as on production from each stage. This heterogeneity can exist in the quality of the reservoir, such as changes in kerogen content and maturity, free porosity, and water saturation, and it can also be seen in factors which directly affect the course of induced fracturing in the well. These factors can be faults, natural fractures, stress regime changes, etc.Historically, it has been difficult to map these properties because some of the measurements required logging in openhole environments using wireline tools. Recently, logging-while-drilling and through-casing-logging measurement techniques have been developed to achieve similar results in a safer, more controlled environment.

The Upper Urho Sandy Conglomerate Reservoir in Xinjiang oilfield features complex lithology, low porosity and low permeability, and a majority of secondary pores with poor connectivity. Due to those characteristics, the log responses are complicated and it is difficult to determine the oil saturation with conventional resistivity method. In this paper, the reservoir heterogeneity was studied based on image logs and nuclear magnetic data, and oil saturation was calculated using array dielectric data. An integrated reservoir evaluation and fluid identification approach was established and applied in five wells located in different fault blocks. The testing results from four of them have proved the effectiveness of the proposed method. This is the first time to use array dielectric tool to identify fluid type in conglomerate reservoir with volcanic matters, and it has been introduced to similar oil reservoirs in other oilfields, which also obtained good application effect.

Unconventional reservoirs oil and gas resources have great potential for development, especially in North America, which has been successfully achieved commercial production. Shale oil is one of the unconventional resources. Most of the shale oil reservoirs have complex lithology, poor petrophysical characteristics, complex pore structure, and so on, especially for lacustrine shale oil formation. This paper describes an approach and workflow to characterize the Chang7-3 member shale oil reservoir in the Ordos basin, China by integrating the high tech digital rock physics core analysis data with other special core analysis data to calibrate the reservoir petrophysical properties. The special unconventional core analysis method taken for this project are Tight Reservoir Analysis technology (TRA), Thin Section scanning (TS scanning), Mercury Injection Capillary Pressure test (MICP), N2 and CO2 Gas sorption test, XRD and Nuclear Magnetic Resonance analysis (NMR), and the new logging technology employed are gamma ray spectroscopy logging (LithoScanner*), nuclear magnetic resonance logging (CMR*), dielectric logging (ADT*). The new core analysis and logging technology not only depict the characters of the shale oil reservoir from microscopic to macroscopic scale, but also guarantee to establish the accurate method for reservoir identification and evaluation. The data analysis from above led to the development of evaluation models for organic matter quality and reservoir quality. Analysis of the production data revealed that the hydrocarbon abundance of the Chang7-3 member lacustrine shale oil reservoir is controlled by both organic matter quality and reservoir quality. A production forecast chart of Chang7-3 Member lacustrine shale oil reservoir was constructed based on the organic matter quality and reservoir quality. The application of the developed methodology and workflow achieved very good results and is supported by the test data from multiple wells drilled in the study area.

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