Determination Of Safe Window Of Drilling Fluid Density For Deepwater Drilling

With the developing of world oil industry, the area of exploration is stepping into deep water gradually. South China Sea is abundant of oil & gas resources, whose foreground is fully tremendous. Due to the peculiarity of geology and offshore environment, several special drilling problems are faced when drilling in deepwater, especially, the challenge of narrow drilling fluid density window. Therefore, to predict three formation pressure profiles accurately and design drilling fluid density window appropriately has significant meaning of maintaining wellbore stability, decreasing the possibility of lost circulation and borehole collapse.Theoretical analysis and laboratory experiment are employed in this paper. The shale compaction simulated experiment device is developed based on a review of abnormal pore pressure and argillutite compaction mechanisms. The process of clays compaction and abnormal pore pressure establishment are simulated with this device. Interrelationship between velocity and effective stress, porosity and bulk density changing of experimental sample in the process of loading and unloading also have been studied in this paper. Laboratory investigations indicate that unloading can be detected by using a crossplot of velocity-density data, and the revised Bowers method can be used to estimate not only the abnormal formation pressure caused by undercompaction, but also can estimate such factors caused by pore fluid expansion. Compared to traditional pore pressure estimation methods, the new method considers in a more comprehensive, and estimation results are more accurate and scientific.Secondly, the borehole stress state is analyzed on a basis of rock mechanics and isotropic mechanical models for stress distribution around extended reach well. A linear elastic and isotropic mechanics model for deepwater well stability analysis is developed based on the characteristics of deepwater drilling environment. The determining method of equivalent drilling fluid density for conventional and dual gradient drilling technology is also presented in accordance with Coulomb-Mohr strength criterion and the maximum tensile stress failure criteria.Finally, the calculation software for determining safe window of drilling fluid density in deepwater is compiled with Visual Basic language. By using the software, the influence of water depth on in-situ stress state, collapse and fracturing pressure is analyzed in different conditions. Study shows that the wellbore stability of ERW in deepwater is not only a function of strength properties of formation rock and stress state of borehole wall, but also affected by factors such as the well trajectory (inclination, azimuth), water depth and in-situ stress orientation etc. The overburden stress and horizontal stress will be changed with the water depth increasing, and then, the fracture pressure will also be changed. As a result, the margin between pore pressure and. fracture pressure gradient is becoming narrower and narrower. Compared to conventional drilling method, the borehole pressure of dual gradient drilling method can match the narrow pore pressure and fracture pressure window well. So, casing level can be reduced and casing program will be optimized by DGD in deepwater drilling.