Theory And Technology On Drilling Fluids For Wellbore Strenthening

Drilling fluid for wellbore strengthening is one of the most important drilling fluid techniques to extend the narrow Mud Weight Window (MWW). The wellbore pressure containment can be improved, and then the upper limit of the MWW can also be enhanced by wellbore strengthening. The technique makes it safe to drill in the narrow or negative MWW conditions, so it is the important requirement of the oil&gas exploration and development.This thesis took the characteristics of the leakage formations as the cut-in point, researched the basic theory of stopping lost circulation and wellbore strengthening systematically, carried out the experiment and numerical simulations for wellbore strengthening, and then researched the theory and method for optimizing the drilling fluid for stopping lost circulation.First, according to the characteristics, the leakage formation can be categorized into weak formation and fractured formation, and the engineering phenomena of these two types were studied, then the internal and external causes affecting the wellbore pressure containment were analyzed. The substance of wellbore strengthening was revealed, including enhance the fracture resistance of weak formation and rebuilding the fracture pressure of fractured formation.Aided by the linear elastic Rock Mechanics and Fracture Mechanics, the mechanisms of enhancing the wellbore pressure containment of weak formation were studied. Based on the fracturing mechanism of weak formation, the computational method for fracture pressure considering microcracks was proposed. The factors affecting the fracture pressure of weak formation were analyzed, and the theory called anti-split preventing induced fracture. This theory believes that the fracture pressure can be increased significantly by improved the sealing properties of drilling fluid, and it can pay a role in preventing lost circulation.The mechanism of rebuilding the fracture pressure for fractured formation was investigated. The mechanism includes preventing the induced fracture from propagating, increasing the fracture reopen pressure and the fracture pressure of the fractured formation. It holds:it is the first job to prevent the induced fracture from propagating, and improve the pressure distribution in the fracture is an effective method for rebuilding the fracture pressure. The lost circulation material (LCM) should plug at certain place near the fracture mouth, which is the best plugging way, and the fluid pressure at the fracture tip must be smaller than the minimum horizontal stress, and the smaller, the more beneficial to resist the propagation of fracture. Inducing and maintaining the induced stress field is the key factor increasing the fracture reopen pressure, the Lost Circulation Material (LCM) must act as proppant agent to maintain the induced stress field; the borehole wall will generate new fractures when the wellbore pressure is high enough, furthermore, the multifissure state must be accounted when analyzing the lost mechanism and design the LCM.Laboratory simulation experiments for weak formation were carried out with fracturing experimental equipment, then the fracture pressure, the fracture propagating pressure and the reopening pressure under different injecting fluid system were investigated. The results have showed that the pressure containment of weak formation can be increased significantly by improving the filtration and sealing properties of the drilling fluids, which confirms that the "anti-split" theory is correct.Laboratory experiments for fractured formation were carried out with the novel experimental equipment, the effect of fracture shape, the type, the granularity and the concentration of LCM on the plugging effect were investigated. It holds:the fracture in the formation should be simulated using the equipment with fracture of enough length or depth, and the plugging location in rough fracture wall was shallower than the smooth wall. Rigid particle is the ideal material to determine the plugging location. Whether the fracture can be plugged or not depends on the granularity of LCM, and the loss volume of drilling fluid depends on the proportion and scale of the LCM, which means that the small particles can decrease the loss volume.Based on the theory and method of particle flow numerical simulation, the particle flow model was set up with the PFC3D, and the mesoscopical mechanism of bridging and plugging was studied. The bridge particles of irregular shape were simulated with the Clump logic of the PFC. It holds:the particle of irregular shape can plug the fracture width which is wider than itself granularity and the plugging manner was "Single Particle Bridging"; the plugging location and plugging rate relating to the D50and D90of LCM. The concentration of LCM has little effect on the plugging location and should have optimum value for different condition.Based on the laboratory and numerical simulation results, the design theory and methods of drilling fluids for enhancing wellbore pressure containment were proposed. Gray correlation method was used to analyze the sensitivity of the factors affecting the plugging effect. The results have showed that the gradation of LCM was the main factor while the concentration of LCM was the secondary factor for fracture plugging.The idea that the casing shoe resistance and the crushing strength of the LCM must be considered when design the density of the drilling fluids is presented. It holds that the surface pressure should be junior to the crushing strength of LCM, in order to prevent the formed bridge plug from crushing.The research results in this thesis will improve the scientific cognition of the theory and technology of drilling fluids for wellbore strengthening, and they has important theoretical and practical significance for preventing and stopping lost circulation of drilling engineering.