Numerical Study of Thermal Recovery on Class-3 Gas Hydrate Reservoir

Abstract

Gas hydrates are solid ice-like clathrate compounds in which molecules of gas (primarily methane) are trapped within the crystal structure of molecules of water Gas hydrates form and exist when the water pressure is high and the temperature is low Consequently gas hydrates can be found in seabeds permafrost and deep oceanic sediment A Class-3 gas hydrate reservoir has no free gas or water layer below only a single hydrate-bearing layer with an upper and lower burden Once the pressure or temperature is outside the gas hydrate equilibrium zone the gas hydrate dissociates There are three methods for inducing gas hydrate dissociation: depressurization thermal recovery and inhibition Currently depressurization is the most promising recovery technique; its driving force depends upon propagating a pressure disturbance However in a Class-3 gas hydrate reservoir with high gas hydrate saturation relatively low effective permeability caused by a low initial mobile phase that can cause poor depressurization The purpose of this study is to apply the thermal method in the Class-3 gas hydrate reservoir The present study therefore used three different heat sources (steam injection warm water injection and a heater) and then compared the performances of these different mechanisms of heat transfer dissociation behaviors and production The STARS numerical thermal and advanced processes reservoir simulator was used (Computer Modelling Group Ltd ) in this study STARS couples thermal conduction multiphase fluid flow rock mechanics and geochemistry and has been validated for gas hydrate simulation by the National Energy Technology Laboratory (NETL) USA Initial hydrate saturation was set at 70% and 800 meters of dual horizontal wells were designed in the center The upper well was the producer and the lower well was the injector The three different heat sources used were the Heater Method steam injection (SI) at 300 °C and warm water injection (WWI) at 85 °C Injection rates of enthalpy were equal SI and WWI had higher gas production rates in the early stage because of the advection of hot fluids (more efficient heat transfer mechanism than radiation) but both had problems when hot fluids from the injector were recovered in the producer Once this breakthrough occurred the driving force weakened The Heater Method yielded a continuously stable gas production rate and the dissociation front was more like the layer-up The recovery factors were 32 22% for the Heater Method 14 2% for the SI method and 26 04% for the WWI method In a case study of Four-Way-Closure-Ridge three pairs of horizontal wells were used Using the Heater Method and with a 40% pressure decline the recovery factor was 38 66% (1 522 billion standard cubic meters [BSCM])

Date of Award	2017 Sept 1
Original language	English
Supervisor	Bieng-Zih Hsieh (Supervisor)