| Since 1997,the United States has industrialized the use of liquid nitrogen fracturing fluid for oil and gas production.Since 2015,the use of liquid nitrogen fracturing fluid and jet assisted drilling in oil and gas production projects in China has been widely developed and popularized.Based on RFPA2 D thermal software system,seven kinds of high diameter ratio shale samples with height diameter ratio of 0.3,0.5,1.0,1.5,2.0,2.5 and 3.0 are selected in this paper.Combined with the material parameters of standard normal shale,the influence of frost heave cracks on the process and failure effect of shale samples with different height diameter ratio of single layer and multi layer is studied.Combined with the engineering construction cost control measures,the influence of different heat exchange cracks is studied The frost heave failure effect of single-layer and multi-layer shale samples with different ratio of height to diameter was studied.The significance and purpose of this study is to give full play to the low-temperature fracturing effect of liquid nitrogen on shale in the construction of high-cost shale oil and gas production project,so as to reduce the use of liquid nitrogen and reduce the cost,give full play to the destructive effect of liquid nitrogen to improve the shale oil and gas reservoir space,and enhance the permeability of fracture cracks.The main contents of this paper are summarized as follows:(1)study the relationship between strain and stress of shale samples with different height diameter ratio,based on this analysis,the law of acoustic emission events of shale samples with different height diameter ratio is analyzed,combined with the frost heave failure process and final failure form of shale samples with different height diameter ratio,the law of frost heave cracks of shale samples with different height diameter ratio is analyzed and summarized.The results show that: in the range of height to diameter ratio 0.3-0.5,the unit failure ratio is the smallest in the process of cold impact frost heave failure,because more strain energy is needed for the strain of the shale specimen in the range of height to diameter ratio;in the range of height to diameter ratio 1.0-2.0,the shale specimen model has been effectively damaged by cold impact,because the page of height to diameter ratio in the range is effective The shale specimen with small ratio of height to diameter of energy required for strain of rock specimen is obviously reduced,and the temperature stress produced by appropriate ratio of height to diameter is fully worked.For shale samples with height to diameter ratio greater than 2.5,due to the intense longitudinal temperature stress,the rapidly damaged main fracture insulates the heat transfer and leaves a large number of complete structures,so the failure of shale samples with height to diameter ratio in this range is not perfect.(2)according to the actual case,two kinds of shale physical parameters within the reasonable parameter range are selected to build the shale bedding model.The research shows that the bedding structure can significantly increase the frost heaving damage of the shale specimen with the height to diameter ratio range of 0.3-1.5,and the bedding structure can cause the minimum principal stress with higher peak value,and because the cracks on the weak structural plane of the structural plane sprout more quickly,after frost heaving The main fracture is more thorough than that of single-layer heterogeneous shale specimen.The influence of bedding direction on shale specimen with height to diameter ratio range of 0.3-1.5 shows that the direction of main fracture propagation is parallel to weak structural plane.The bedding structure will not significantly increase the frost heave failure of the shale samples with the height diameter ratio range greater than 2.0,and the bedding direction has a very obvious impact on the shale samples with the height diameter ratio range greater than 2.0,which is specifically reflected in the fact that more temperature gradients are formed in the shale samples with the bedding structure due to frequent heat exchange differences,which greatly reduces the elastic modulus of all the material units of the whole shale.(3)analyze the influence of the change of heat transfer coefficient and thermal conductivity on the stress of shale specimen,and establish a numerical model to analyze its influence on shale with different height diameter ratio.It is found that the failure process of shale specimen with height diameter ratio of 0.3-0.5 can be significantly improved by increasing the heat transfer coefficient of reasonable value,but the increase of heat transfer coefficient can significantly change the increased state of failure It can be divided into several parts: increasing the reasonable heat transfer coefficient can significantly improve the failure process of shale specimen with the height diameter ratio of 1.0-1.5.After increasing the heat transfer coefficient,the cold impact can destroy the whole structure of shale specimen model in this range from internal frost crack to lose the overall strength.When the heat transfer coefficient is greatly increased,the failure range of cold shock is increased,but it is very limited;increasing the heat transfer coefficient has no obvious influence on the failure process of the shale specimen with the height diameter ratio of 2.0-3.0,but has obvious influence on the failure form and fracture initiation range.With the increase of the heat transfer coefficient,the main fracture occurs in the shale specimen model with the height diameter ratio of 2.0-3.0 There is obvious displacement.With the increase of the heat transfer coefficient,the upward displacement of the initial crack initiation coordinate under the higher temperature stress caused by the higher temperature gradient is more obvious. |
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