A Feasibility Study of Producing Natural Gas from Subsea Hydrates with Horizontal Snake Wells

This study focuses on technical feasibility of producing natural gas from offshore gas hydrate deposits using a new technique called horizontal snake wells (HSW). This paper addresses engineering and well productivity issues for the new technique. Coiled tubing string is proposed to drill HSW in gas hydrate reservoirs for increasing well productivity and reducing wellbore collapse problems. A new analytical model was derived to predict of maximum achievable wellbore length (MAWL) based on the theory of buckling failure. Furui's equation for gas reservoirs was used for predicting the initial well productivity at the MAWL. Gas lift method is proposed to remove water for achieving a constant bottom hole pressure that is slightly below the hydrate dissociation pressure. Gas production forecast was made on the basis of material balance. Net present value (NPV) method is employed to perform economics analysis. A gas hydrate reservoir in the South China Sea was taken as an example to investigate the feasibility of producing natural gas from gas hydrate deposits using the HSW technique. The new analytical model gave the MAWL under extreme well friction conditions prior to buckling failure of coiled tubing. Furui's equation predicted commercial well productivity at the MAWL, depending on the loop separation of the HSW. If the bottom pressure is maintained with gas lift technique at a constant level slightly lower than the hydrate dissociation pressure to prevent wellbore collapse, material balance forecasted high NPV of HSW. It is concluded that the HSW is a feasible technique for economically producing natural gas from offshore gas hydrate deposits. This paper presents a novel method called HSW for producing natural gas from offshore gas hydrate deposits. It also demonstrates how to evaluate the productivity and thus economics of the HSW.