Calculation of Extending Limit of Horizontal Well Drilled in Offshore Hydrate Bearing Sediments

摘要

Numerous natural gas hydrate production technologies have been proposed and the corresponding production rate of each technology has also been simulated by researchers. It has been found that the long reach horizontal well performs better than vertical well according to the simulation results. However, how long a horizontal well can be drilled in hydrate bearing sediment (HBS) is unknown to us.The mud weight window is narrower compared with other oil and gas reservoirs and the build rate will be quite large if horizontal wells are carried out in hydrate formation. Thus, there are two main constraint factors of drilling long horizontal wells in hydrate sediments: the formation will fracture because of annulus frictional pressure Losses; and the axial compressive force of drill strings can’t be transferred effectively to bottom for the large build rate. This paper calculated the extending limit based on these two aspects.A case study is carried out based on the parameters of Shenhu Area, South China Sea. The target zone is 280m below sea floor and the fracture is as low as 1.15 g/cm3, quite difficult to drill horizontal wells. According to the calculation results, and the horizontal section limit can be 682.4m considering the fracture of formation; while, the horizontal section can only be drilled as far as 1111.9m considering the axial compressive force transferred. What’s more, the effects of mud flow rate, hookload and drillpipes are discussed.This paper built an approach to calculate the extending limit of horizontal wells drilled to offshore HBS, based on fracture pressure and mechanical aspects respectively, and discussed several ways to expand the extending limit by optimizing hydraulic parameters or drillpipes assembly. The results will be instructive to drill long reach horizontal wells in HBS and have great significance to the exploitation of offshore hydrate resources in the future.