Abstract
In oilfield development, the pore pressure difference between adjacent areas leads to cracks and slipping in the weak structural surface layer, which triggers the shear failure of the casing. The formation slip involves a large range of formation, and its amount is not proportional to the size of the slipping rock mass, which conventional physical models cannot simulate. In this study, based on the abnormal similarity theory, we derived the similarity coefficients of mechanical parameters with different horizontal and vertical proportions. Furthermore, an experimental device for simulating the formation crack and slip under interregional formation pressure difference was developed. Through the experiments, we obtained slip conditions under different pressure differences between adjacent areas and different oil layers and fault surface depths. The study shows that the pore pressure difference between adjacent areas is the driving force of the formation slips. The slip zone is located in the middle of two abnormal pressure zones, and the distance between the adjacent areas can affect the slip range. The deep burial of the oil layer and shallow depth of the weak structural surface can trigger a more significant formation slip. The experimental method proposed in this paper provides an experimental device and method for understanding the formation of cracks and slips on weak structural surfaces. The experimental results provide a theoretical basis for the prevention of shear-type casing damage caused by formation slip.
Highlights
When an oil field is developed, the average formation pressure difference between adjacent blocks gradually occurs, which leads to a formation pressure difference between adjacent areas
For formations with weak upper horizontal structural surfaces, the pressure difference between the adjacent areas can lead to the formation of cracks and slips at the weak horizontal structural surfaces, leading to shear-type casing damage
Taking the Daqing Oilfield in China as an example, at the bottom of the Nen’er member in the upper oil layers, there is a mud-shale standard layer, which can be used as a standard for identifying other layers because of its obvious logging characteristics
Summary
When an oil field is developed, the average formation pressure difference between adjacent blocks gradually occurs, which leads to a formation pressure difference between adjacent areas. For formations with weak upper horizontal structural surfaces, the pressure difference between the adjacent areas can lead to the formation of cracks and slips at the weak horizontal structural surfaces, leading to shear-type casing damage. With the long-term overpressure development of oil fields, casing damage is increasingly frequent, and even regional casing damage is formed in the standard mud-shale formations of the Daqing Oilfield [12,13,14]. Because the mechanism of slip casing damage on the mud-shale crack surface is still not clear, and the mechanical model of formation slip and experimental methods have not been formulated, only field experience or conservative methods can be generally used for prevention measures at present. Establishing a laboratory experimental device that simulates the sliding casing damage on a horizontal crack surface and reveals the mechanism of regional casing damage is critical. Based on the abnormal similarity theory, the corresponding relationship between the experimental model and the prototype formation slip was established, and the formation slip at different positions under different conditions was measured and compared with the theoretical calculation results, and the factors influencing the formation slip were analyzed
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