Establishment of the productivity prediction method of Class III gas hydrate developed by depressurization and horizontal well based on production performance and inflow relationship

Productivity prediction is one of the important issues for the economic and effective development of gas hydrate deposit. Therefore, this paper proposes a method to establish productivity prediction for the Class III gas hydrate developed by depressurization and horizontal wells. The method are divided into four steps, including numerical simulation, production performance analysis, revision of formulas and quantitative calculation of key factors. The prediction method is based on inflow performance relationship (IPR) of gas hydrate. Considering the shape of IPR curves, Fetkovich's formula and Vogel's formula are selected as the basic formulas respectively for the increasing part and decreasing part of gas hydrate production. To revise Fetkovich's formula, the deposit pressure is replaced by comprehensive deposit pressure, which is determined by energy coefficient B₀. Through the analysis and evaluation of influencing factors, B0 is described by a equation related to gas hydrate saturation. To revise the Vogel's formula, the lowest production pressure is set as the four phase point pressure, leading to the change of production pressure difference and maximum gas production rate. The key coefficient C is calculated by a equation related to gas hydrate saturation and well spacing. Compared with simulation results, the results of proposed formulas show good accuracy in the prediction of production rate of gas hydrate. The results of test point predicted by revised Fetkovich's formula almost coincide with simulation results. The average error between results of revised Vogel's formula and simulation results is 4.24%. So the proposed method can be applied to predict the productivity of gas hydrate for its good accuracy, fast speed and simple calculation.