Research On Formation Pressure Prediction Of Underbalanced Drilling

Underbalanced drilling technique is a hot point and development direction of drilling techniques after horizontal well drilling. It is a strategy measure to developing the underbalanced drilling technique for discovering and protecting reservoir.It is very important to predict formation pressure during oil exploring and drilling process. It can supply important pressure technique data for drilling parameter design and well structure, and have important meaning for formation protection, enhancing drilling success rate and decreasing drilling cost.Many formation pressure prediction methods were developed throughout the world, for it is takes effect on deciding the well structure, drilling fluid density, well head equipments and drilling operation measures. It is very difficult to predict exactly by using only any one method, the results change a lot in different area and different geologic depositional environment, so the compositive methods should be taken for increasing the predicting accuracy. In the paper a compositive method is applied according the drilling conditions, the prediction before drilling and while drilling integrate together to make the result more accuracy. Seismic data method is applied before drilling; the standard drilling velocity method and rock strength method are applied while drilling.Based on the seismic compressional wave data, the directly prediction method is used to computer formation pressure according the relative model of seismic wave velocity with formation pressure; after that, using the standard drilling velocity method and rock strength method, the formation pressure can be predicted through the realtime geologic data while drilling.The method is proved on site that it overcomes the dependence of computer methods already existing to different area and different conditions; it can predict the formation pressure under the bit accurately.The influence factors on underbalanced value also be studied deeply at the same time and the design principle of negative pressure difference is given. Through the research on the wellbore rock firm degree, the different characters of negative pressure difference design for vertical and horizontal wells are discussed; and the max production pressure difference of underbalanced drilling is given.