Study On Optimization Technology Of Wellbore Structure Based On Safety Risk Assessment

With the demand for oil and gas resources increasing, the objectives of exploration and development for oil and gas have transferred from shallow formation to deep formation through drilling deep or ultra-deep well. While in the process of deep well drilling, down-hole accidents such as well lost, well kick, well collapse, well stick and so on occure frequently because of the complex pressure system, the frequent changes in lithology and the less information of adjacent wells. So the well structure designed by the traditional method can’t meet the needs of deep or ultra-deep well drilling, we should combine with safety and risk assessment to enhance the reasonability and security of well structure. So in response to the problem of the low reasonability and security of deep well structure and the frequent down-hole accidents in the process of deep well drilling, this paper firstly analyzed the potential risks and their influencing factors in the process of deep and ultra-deep well drilling, then analyzed the uncertainty of formation pressure based on normal distribution probability density and triangular distribution probability density and established security risk assessment model centering on the risk probability. Secondly, this paper also depicteded the curve of drilling fluid density with credibility for preventing down-hole accidents based on the analysis for the uncertainty of the formation pressure distribution information and put forward a set of top-down, outside-inside optimization design method for well structure based on the curve and pressure constraint criterion. The most advantage of the method is the introduction of drilling fluid density curve for anti-well stick and avoid the step of repeatedly verifying whether cause well stick or not in the traditional method and could design well structure continuously based on the pressure constraint criterion. Finally, this paper used the established security risk assessment model to prepare a potential risk prediction program for well structure of deep or ultra-deep well based on Visual Basic6.0and predicted the probability and location of down-hole accidents in the process of DaBei203well drilling and designed the well structure of DaBei203well by using the basic data of this well.The results of risk prediction showed that:the location and probability of down-hole accidents in the process of deep well drilling can be quantitatively calculated, and the security risk assessment model can well assess the security of deep well drilling and the reasonability of well structure. Design examples showed that:the optimization design method based on security risk assessment can flexibly and reasonably design deep well structure according to the abnormal pressure system and complex lithology, and the well structure designed are more safe and reasonable than the actual situation. All in all, utilizing the results of this study not only can predict the location and probability of down-hole accidents and provide theory data for the on-the-spot decisions, but also can design the deep well structure which can avoid down-hole accidents theoretically. This results have important positive meaning to reduce the down-hole accidents in the process of deep well drilling.