Numerical Simulation Of Loss Of Drilling And Completion Fluid In Natural Fracture Network System

Lost circulation is one of serious accidents while drilling in fractured formations, which will not only cost drilling time, lose expensive drilling fluid and cause the formation damage, but also can cause other terrible accidents such as well collapsed, blowout and so on. Nowadays, with the oil and gas resources exploration and development improving and conventional oil and gas resources becoming fewer and fewer, the drilling and completion engineering have been gradually moving towards deep and extra-deep formation, deep-water and pressure depletion formation, in which lost circulation becomes more outstanding. Lost circulation has severely restricted the safe and efficient drilling and exploitation of oil and gas reservoir.Based on the tight conglomerate reservoir of Jiulongshan structure as the research object, this paper established a prediction model of lost circulation pressure, a mathematical model of drill-in fluid percolation and a mathematical model of solute transport in natural fracture network, by which the lost circulation behavior in natural fracture network can be descripted quantitatively in the aspects of occurred conditions, process and results. The mechanism and dynamic characteristics of drill-in fluid loss in natural fracture network were confirmed. Sensitivity analysis of rheological parameters and dynamical retrieval of fracture width around the borehole were carried out. Besides, this paper also confirmed the distribution range of damage zone caused by drill-in fluid loss and evaluated the extent of formation damage quantitatively. These researches can provide some therotical foundations for optimization design of drill-in fluid and plugging removal process of acidization.This paper established a prediction model of lost circulation pressure in fractured formation. Based on the comprehensive studies on the mechanism of lost circulation in fractured formation, drill-in fluid loss can be classified into three types:natural fracture loss, fracture propagation loss and fracturing loss. The lost circulation pressure models of above three models were established and the prediction model of nature fractured formation was also validated based on the lost circulation datas of research area.This paper confirmed the dynamic characteristics and rules of drill-in fluid loss in natural fracture network and conducted the dynamical retrieval of fracture width around the borehole, in which drill-in fluid was viewed as Herschel-Bulkely formula. Based on the flow balancing theory, a mathematical model of drill-in fluids loss in the fracture network was established. The dynamic characteristics of drill-in fluid loss in natural fracture network was obtained by the numerical solution, which showed that the relation curve of drill-in fluid loss rate and time presented the increasing trend drastically before reducing sharply. How rheological parameters of drill-in fluid influenced lost circulation was analyzed. Drill-in fluid loss rate decreased with the increase of circulation pattern index, consistency coefficient and yield point. The dynamical retrieval of fracture width around the borehole was also conducted. The dynamical widths of six fractures connected with the borehole in the distance of0.5m from the borehole were about844.Oμm, which was beyond the sealing capacity of the exiting drill-in fluid.This paper determined the distribution range of damage zone and evaluated the extent of formation damage due to drill-in fluid loss in natural fracture network. The mathematical model of solute transport in natural fractured network was established based on double continuous medium method. The distribution of dimensionless concentration in time and space was determined by finite difference method, and then the distribution range of damage zone was also confirmed. When the cumulative time of lost circulation reached100h. the radius of damage zone was about17.0m. The relative error between the numerical result and the result of pressure built-up test analysis was less than10%, which proves the reliability of the model. The distribution profiles of water saturation and gas phase permeability in the damage zone were obtained from the concentration profile by Wylli method and the equivalent skin factor of damage zone was also confirmed. The equivalent skin factor of damage zone was about7.5. The relative error between the numerical result and the result of pressure built-up test analysis was less than10%, which proves the reliability of the model.