Numerical And Experimental Investigations Of Drilling Fluid Losses In Fractured Formations

Lost circulation is severe while drilling fractured formations, which will not only cost drilling time, lose expensive drilling fluid, cause the reservoir damage, but also can cause the 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 engineering has been gradually moving towards deep and extra-deep formation, deep-water, pressure depletion formation and the complex formation. Mud losses has restricted the safe, rapid, efficient drilling and become the bottleneck of gas exploration and development. Lost circulation has become the biggest barrier for drilling safely, quickly, and efficiently.After years of research, the leak resistant and sealing technology for pores and micro fractures has already formed. However, there are still many challenges on controlling the mud losses in fractured formations, the theory of the drilling fluid losses is relatively weak and far from the level of modeling, quantitative and scientific. Therefore, it is crucial to understand the characteristics and regularity of drilling fluids losses and modeling drilling fluid losses in the fractured formations.This thesis chooses carbonate formations in northeastern of Sichuan and Tahe oilfield as the main research objects, statistically analyzed the data of more than150wells and loss information of483times and confirmed the characteristics and distribution of drilling fluid losses in the carbonate formations. Based on the comprehensive studies on the loss mechanism, mud losses are classified into three types:fracturing loss, fracture propagation loss and Pressure difference loss. Then a lost circulation pressure model of carbonate formations were established, which provides a basic theory for preventing the drilling fluid losses and changing the loss pressure.This paper presented the flow equations of non-Newtonian (Herschel-Bulkley) fluids in the1D-linear and2D-plane fracture, confirmed the non-linear function relationship between flow and pressure gradient. In order to save the numerical time, this paper simplified the motion equation and analyzed the error quantitatively. Results show that the error decreases with the increase of fracture aperture, the pressure gradient and flow behavior index and increased with the increase of the yield stress. The pressure gradient and fracture aperture are usually large when lost circulation occurs. Therefore, the error of simplified model is very small, which can satisfy the simulation requirement.This paper designed the high temperature and pressure mud losses dynamic evaluation apparatus, which concludes five sections, such as drilling fluids supply system, drilling fluids circulation system, fracture module, control system and data acquisition system. This apparatus can model lm length,1-10mm aperture and50mm height fractures, which satisfies the requirement of big-fracture losses evaluation. The results show that the error of1D loss model didn’t exceed12%, which demonstrated the effective of this model.This paper proposed the drilling losses models in the1D-linear and2D-plane fracture which considered the non-Newtonian fluids, the exponential (and linear) fracture deformation law, the fluid leak-off, the slightly compressibility of drilling fluids, and this model was solved by the finite-difference method. Then, the drilling fluids losses behavior in the infinite and finite fractures was simulated. The characteristics of drilling fluids losses curve and the factors, such as pressure sensitivity, overbalance pressure, fracture orientation and size, rheological properties of drilling fluids and fluid leak-off through the fracture wall which influence mud loss were analyzed.Based on the fractal theory, this paper probed the method of generating the rough fracture, compiled the computer procedure of generating2D rough fracture, established the model of non-Newtonian fluids (Herschel-Bulkley) losses in the rough fracture, investigated the effect of fractal dimension, standard deviation and grid size on mud losses, and demonstrated how the roughness can influence non-Newtonian fluids losses. The results show that the roughness has more influence on the non-Newtonian fluids than the Newtonian fluids. What’s more, the roughness can prevent the drilling fluids losses when the fracture surface touch rate is greater than zero.Based on the Monte-Carlo stochastic simulation theory, this paper constructed the2D and3D discrete fracture network (DFN) models and compiled the procedure of generating2D and3D discrete fracture network, SFNM and SFNM3D. Integrated with the drilling fluids losses in the single fracture, this paper established the model of drilling fluids losses in the fracture network, and the methods of finite difference and finite element are used to solve this model, laid emphasis on studying the characteristics and laws of drilling fluids losses in the2D discrete fracture networks, and carried out the preliminary.(?)imulation of drilling fluids losses in the3D discrete fracture networks. The results show that the curve of drilling fluids losses in the fracture networks had the fluctuation phenomenon and the fluctuation frequency and range was largely related to the construction of fracture networks.According to the theory of rock fracture mechanics and non-Newtonian fluids losses dynamics in the nature fracture, this paper constructed the pseudo3D model of drilling fluids losses in the induced fracture mud loss, and the process of fracture propagation and the characteristics and laws of mud loss rate was investigated. Results show that both the mud loss rate curve and the pressure in the fracture had the periodical fluctuation phenomenon when the fracture was spreading, which was similar to the curve of hydraulic fracture and proved the effective of the model.