ABSTRACT

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.

INTRODUCTION

Natural gas hydrate (NGH) is a kind of solid crystalline compound where natural gas molecules and water molecules are trapped (Motghare and Musale 2017). In the past several decades, interests in NGH of organizations and companies all around the world have been souring, for its large-scale reserves and its possibility to become an alternative energy. It has been generally accepted that the reserve of natural gas contained in NGH is larger than the known conventional gas reserves (Makogon 1982, Collett, Bahk et al. 2013). For NGH is formed under the condition of high pressure and low temperature, it is mostly found in shallow layer of deepwater and permafrost environment, as shown in Fig. 1. Russia's Mesoyaha gas field is the world's first and the only gas hydrate reservoir for commercial exploitation(Makogon 1966, Makogon and Omelchenko 2013). However, it's estimated that more than 99% of NGH deposits are distributed in offshore sediments (Economides, Wang et al. 2012). Many countries including America, South Korea, Japan, India, China have launched their key national projects to exploit and development NGH, in which most of the wells drilled are for logging or coring operations. Up to now, three production test campaigns of oceanic NGH have been implemented. Japan have implemented two production tests in Nankai Trough in 2013 and 2017 respectively (Fujii, Suzuki et al. 2015, Tamaki, Fujii et al. 2017). China have succeeded in production test in Shenhu Area, Southern China Sea, which lasted for 60 days(Wu and Wang 2018). However, the production wells drilled in the production tests were all vertical wells and there are still no cost-effective ways to extract gas from offshore NGH reservoirs.

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