Four-Dimensional Stress Evolution and Optimization of Child Well Fracturing Parameters in Multi Sub-Layers Shale Gas Reservoir

The research is aimed at the first infilled multi-well pad in deep shale gas reservoir of Sichuan Basin China. Since parent wells are not able to improve producing reserves of the pay zone, the deployment of infill wells is imperative. Therefore, the effect on infilled well fracturing caused by parent well's pressure depletion need to be investigated. This paper calculates the four-dimensional in-situ stress evolution by a comprehensive numerical model. And then several fracturing parameters of infilled well are optimized from two aspects: fracture geometry and stress disturbance range. The results show that: Stress disturbance range in horizontal is about 1.6 times of fracture length, in vertical is approximately equal to fracture height. For the infilled well, W-type well pattern and cluster spacing 15m are more suitable, the best fracturing time is about 24 months after the production of the parent well. 1. INTRODUCTION Based on the practical experience in the development of medium-shallow shale gas, the well spacing less than 300m is generally adopted for deep shale gas in China to improve producing reserves. However, due to the limited comprehension of the geomechanical characteristics, the horizontal and vertical stress disturbance range of parent wells are not able to cover the inter-well pay zone. At the same time, the hydraulic fracture height failed to achieve the expectation and also failed to cover the entire pay zone in vertical direction. Based on this, infill wells must be deployed above the parent wells on the top of main pay zone to achieve the purpose of increasing the reserve control range of the well group in vertical and horizontal. As the production of shale gas wells decreases by more than 50% in the first three years, there is a pressure drop of up to 20MPa in the reservoir around the parent well. The depletion of pore pressure affects the in-situ stress, causing the in-situ stress of the reservoir to continuously change over time (i. e. four-dimensional in-situ stress), leading to interference with the parent well during fracturing of the filling well, and even causing fracturing hits. Resulting in a sudden decline in the production of the old well. This is a new problem restricting the sustainable and efficient development of shale gas in China.