Research On Technology Of Enhancing Wellbore Resistance Capacity By Plugging In Fracture Formation

The formation with fracture and pore have low resistance capacity and the narrow safety window of the drilling fluid density, which will make the complex situation of mud loss, overflow, blowout, and borehole collapse happen during the drilling. So how to enhance the resistance capacity of the formation is the purpose of this paper.This paper firstly analyzed the cause of low resistance capacity and found the key factor was the development of the leaking-fracture and micro-fracture in the formation. It come to the conclusion that when the wideness of fracture was more than 150 micro-meter, the leakage began to happen and the rate of mud loss was affected by the depth and wideness of fracture, operating pressure, the delivery volume, the viscosity of the drilling fluid and so on. It also come to the conclusion that whether the unleaking-fracture leakoff or not was dicided by the minimum principal stress.The theory of enhancing resistance capacity was advanced in this study. Firstly, the leak-fracture must be plugged effectively to stop loss and prevent the fracture being enlarged. Secondly, the loss control materials must eliminate timely and effectively the induction forse of the unleak-fracture and prevent the unleak-fracture enlarged and leakoff.Wherefore, this paper puts forward the technology of plugging fracture immediately. Its theory was that the fracture was bridged and filled up by the blockingagent and forming a layer in the fracture with low permeability and high intensity at short time and very small loss. The drilling fluid filtered out through the borehole wall not being completely supplied so as to make the induction forse in the fracture vanish with the result that the fracture was never enlarged any more.The blinding layer could resisted the drilling fluid pressure, the borehole pressure and the closure stress of the rock in fracture surface. According to the mechanism,the rigid particle, a kind of special inert intensive irregular granular material,was optimized suitable for the technology of plugging fracture immediately.The performance of rigid particles in enhancing the resistance capacity of formation was investigated by laboratory tests. It was found that a layer of low permeability could be form in model fracture using the rigid particlse with low loss and very short time, and the layer was able to bear a pressure of 5MPa or even higher. Therefore experiments showed that the rigid particles could exactly meet the requirement of plugging fracture immediately and improved the resistance capacity of formation. It was also confirmed that the rigid particles'resistance capacity had little influence by temperature, dissolved by acid, and not react with formation fluids or drilling fluids, and its have little influence on drilling mud and the well logging.On the basis of the mechanism research and experiments in laboratory, it is put forward that the plugging while drilling technology and the sealing loss on stop drilling technology, and it was tested for many times in DiNa area of Tarim oilfield and Kelamayi oilfield.The results showed that compared to adjacent wells, the loss volume was reduced by 73.6% and the number of leakage was reduced by 67% happening in the same layer in experimental wells where the technology of drilling and plugging fracture immediately was applied. With the sealing loss of stopping drilling, the equivalent circulating density of experimental wells could be improved by 0.1/cm3 or higher.It had formed the plugging fracture technology while drilling using rigid particles added to all the recycling process of mud, the drilling-in slugging technology of preventing and controlling loss, the technology of controlling loss after stopping drilling and the technology of controlling loss before well being cemented.In all, A feasible and effective technology, called enhance resistance capacity immediately, which formed by research,experiment and so on, provided us a theory and tool to achieve a major purpose of the safe, rapid and efficient drilling at the formation of low resistance capacity.