| The development of oil and gas resources will generate a large amount of waste water.Especially with the increasing of the proportion of unconventional energy development,Water consumption and waste water production are much higher than conventional wells and increase risk of environmental pollution.Reinjection is one of the most important methods to dispose waste water associated with oil and natural gas production.Many abandoned wells or newly drilling wells were used as reinjection well and were fractured to enhance the injection ability.Due to the existence of cracks or fractures in the formation,the wastewater reinjection process also poses a pollution risk to the formation environment.Therefore,the establishment of dynamic analysis models with complex fractures has important guiding significance for the design of wastewater reinjection project.This paper takes the complex fractures and boundary as the research object,and divides the wastewater reinjection process into three subsystems:flow in the wellbore,flow in the fracture and flow in the formation.First,fluid mechanics,seepage mechanics and source function theory are used to establish each fluid flow equation in each subsystem.For complex fracture systems,the semi-analytical theory was used to discrete and coupled units between and within the fractures.For complex outer boundaries,boundary integral equations based on boundary element theory and boundary discrete methods based on constant elements are obtained.Then the flow equations of the three subsystems are coupled in the Laplace space to form a linear equation system that reflects the relationship between the discrete units.The optimization of the matrix structure and the iterative algorithm improves the efficiency of the model.Finally,the Guass elimination method and Stehfest numerical inversion are used to solve the linear equations.Based on the above-mentioned theoretical methods,the water injection process of vertical fracture well and horizontal fracture well under different fracture distribution conditions is simulated respectively,and the characteristics of water injection well flow stages and water injection in complex fracture structures such as fishbone fractures and asymmetric fractures are analyzed.In addition,according to the actual production conditions of the mine,a variable working condition and a fracture growth model are established.The method proposed in this paper can quickly and conveniently establish fracturing well models to deal with seepage problems under various complex fracture conditions.The calculation results are accurate.The results of simulation of fractured wells show that:The scale of the primary fracture and the permeability of the formation are the main factors affecting the flow stage characteristics and water injection capacity of the injection well;When there are secondary fractures,the bottom-hole pressure derivative curve will have a significant downward trend in the later stage of the wellbore storage stage.Affected by the number of branch fractures and the diversion ability,some flow stages of vertical and horizontal fractured wells are not obvious;Changes of water injection pressure can reflect the characteristics of the boundary.The fracture system in the formation can increase the water injection capacity of the well in the early stage.But the final water injection volume depends on the effective volume of the formation.When the water injection system or formation fracture conditions change,the pressure derivative curve at the bottom of the well will abruptly change,and gradually approach the derivative curve of the whole process of production under the current working conditions.The establishment of a dynamic analysis model for sewage reinjection wells provides a theoretical basis for analyzing the actual formation boundary and fracture system distribution patterns and the adjustment of the working system of the injection wells.There are 116 figures,13 tables and 124 references in this dissertation. |
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