Abstract
Abstract Based on the comprehensive statistic and analysis on some representative geological and physical data, the classification criteria on net pay for shale gas reservoir of the Wufeng-Longmaxi formation in Shunan area, Sichuan Basin, are proposed, which include porosity (φ), gas saturation (Sg), density of rock (DEN), brittleness index (BI), and gas content (Vt). When the porosity, gas saturation, brittleness index, and gas content are larger than 3%, 30%, 40%, and 1 m3/t, respectively, and the density of rock is lower than 2.7 g/cm3, then this formation can be seen as the net pay. The application of two key parameters, gas content and brittleness index, could reflect the reservoir resource basis and fracability, respectively. The gas content has a positive correlation with porosity and total organic volume, and the brittleness index has a positive correlation with siliceous and carbonate content. According to the range of these two parameters, the net pay can be classified into three types. For type I, the gas content and brittleness index are larger than 4 m3/t and 50%, respectively. For type II, either the gas content or the brittleness index is lower than 4 m3/t and 50%. For type III, the gas content should be larger than 1 m3/t and lower than 4 m3/t, and the brittleness index is between 40% and 50%. The field application case indicates that the Wufeng formation and low member of the Longmaxi formation have good quality and mainly consist of type I and II formations. In addition, it is found that there is a positive correlation between the penetration ratio for type I formation and the testing production and estimate ultimate recovery (EUR). While this ratio is larger than 50%, the testing production rate and EUR will be over 15×104 m3/d and 8000×104 m3 with a probability of 92%, which meet the requirement of exploitation with reasonable economic benefits.
Highlights
With the increase of exploration extent for oil and gas reservoirs in China, it is difficult to find the large amount of conventional oil and gas reservoirs, and more unconventional oil and gas reservoirs are needed
In the shale gas reservoir for the Qiongzhusi formation, there are 27 exploratory wells in total; only four wells meet the requirement of commercial gas production rate, which demonstrates that this formation has low exploitation potential
In the Ordovician Wufeng-Silurian Longmaxi formation, at present, the gas production rate produced from this formation has reached 10 billion cubic meters per year, which is the main target formation for marine shale gas reservoir in Southern China and has great exploration potential [3]
Summary
With the increase of exploration extent for oil and gas reservoirs in China, it is difficult to find the large amount of conventional oil and gas reservoirs, and more unconventional oil and gas reservoirs are needed. In the Ordovician Wufeng-Silurian Longmaxi formation, at present, the gas production rate produced from this formation has reached 10 billion cubic meters per year, which is the main target formation for marine shale gas reservoir in Southern China and has great exploration potential [3]. The high-quality shale gas reservoir for the Ordovician Wufeng-Silurian Longmaxi formation in the Sichuan Basin is the main producing layer for the development on marine shale gas in Southern China. The sedimentary thickness of the Wufeng-Longmaxi formation in the Sichuan Basin is about 250~350 m; in order to improve the economic benefits further during the development of shale gas reservoir, it is crucial to study the internal formation characteristics and determine the criterion for classification evaluation on the target area. The relationship between the quality of formation drilled by horizontal wells and the production rate is clarified, which directs the optimization on the drilling technology for shale gas horizontal wells and the improvement on the cumulative production rate for single horizontal well
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