Logging evaluation of the engineering quality of the Paleogene Funing Formation oil shales in the Subei Basin

Abstract

Shale oil reservoirs, characterized by no productivity, are developed by horizontal drilling and volume fracturing, and it is very important to optimize shale fracable intervals based on engineering quality logging evaluation. The Paleogene Funing Formation shale in the Subei Basin is taken as a typical example in this study. A sonic scanner is used to calculate the elastic parameters, including Poisson's ratio and Young's modulus. Dynamic and static parameters are converted through core analysis data. The brittleness index and in situ stress are calculated according to Poisson's ratio and Young's modulus. In addition, a one-dimensional rock mechanics model was constructed with the shear slowness in a single well to calculate three components of in situ stress. Finally, considering the difference in the brittleness index and horizontal stress differences between different layers, brittleness index (BI) and (BI/(σH-σh)) were selected to describe the engineering quality. According to oil test data, the larger the brittleness index and the smaller the horizontal stress are, the higher the capacity after fracturing. A cross plot of the brittleness index and BI/(σH-σh) is established to divide the reservoir types. Consequently, there are two types, including I high productivity(engineering quality characterization parameters >2.2) and Ⅱ medium-low productivity (engineering quality characterization parameters < 2.2), in the Paleogene Funing Formation in the Subei Basin. High productivity produces oil after fracturing, which suggests that the classification results of sweet spots depending on the engineering quality characterization parameters are better. Logging evaluation of the engineering quality of shale oil reservoirs can provide a theoretical basis and technical guidance for optimizing favorable fracability and high productivity layers and provide scientific guidance for drilling and fracturing layer optimization of shale reservoirs.

Keywords

• shale oil
• engineering quality
• brittleness index
• in situ stress difference
• logging evaluation
• funing formation
• subei basin