On Drilling Method Optimization and Prediction of a Feasible Range between Drilling Fluid Density and Discharge in Deep-Water Drilling at Shallow Hydrate Formations

Abstract

Abstract: During deep-water drilling for horizontal wells at the shallow natural gas hydrate formation, the feasible range of drilling fluid density and discharge can be rather narrow in a hydrate reservoir because of its weak cementation properties. Upon decomposition of hydrate cuttings in annulus, complex multi-phase multi-component flow s in a wellbore are challenges to both the prediction and control of drilling in safety and bottom hole pressure. In China, riserless and riser drilling patterns were respectively selected for 1 st and 2 nd  rounds of pilot production of hydrates. The influence of different drilling patterns on the feasible range of drilling fluid density and discharge still needs to be further investigated. Specific to the above problems, impacts of hydrate phase transition were taken into account to establish bottom-hole temperature field and pressure field models in both patterns of riser and riserless drilling. In addition to analyzing how different drilling parameters affect bottom-hole pressure in two drilling patterns, the feasible ranges of drilling fluid density and discharge were also figured out in such conditions. It is demonstrated by relevant results that bottom-hole pressure during riserless drilling at the hydrate formation is far less sensitive to the drilling fluid density compared with riser drilling; and the drilling fluid discharge takes second place. The feasible range between drilling fluid density and discharge is approximately 2.4 times greater in riserless drilling than that in riser drilling as far as the hydrate formation is concerned. Moreover, pump pressure required by riserless drilling is lower than that of riser drilling at the hydrate formation; and their difference can be further expanded as drilling fluid discharge and density increase.

Keywords: Keywords: Hydrate reservoir, Horizontal well, pressure field, drilling pattern, feasible range