Wellbore Flow Rules in Marine Natural Gas Hydrate Reservoir Drilling

Abstract

As a new unconventional clean energy, natural gas hydrate has attracted attention in recent years. In marine natural gas hydrate reservoir drilling, hydrate cuttings will return upward in annulus. In this process, hydrate cuttings will decompose under higher temperature and lower pressure in upper wellbore. Because of the decomposition gas, liquid–solid two-phase flow in annulus turns into gas–liquid–solid complex multiphase flow. This is different from that in conventional hydrocarbon reservoir drilling. Based on this, temperature and pressure models in wellbore are established. Hydrate decomposition process is divided into destruction of hydrate particles crystal surface structure and desorption of methane gas molecules at decomposition interface. And a hydrate kinetics decomposition model is derived considering fugacity of methane, decomposition rate constant, and superficial area. Then, complex multiphase flow coupling models are established considering the coupling relationships among varying temperatures, pressures, pipe flow, and hydrate decomposition in annulus. Finally, influences on temperature and pressure in annulus are analyzed through numerical simulation method. Moreover, multiphase flow coupled with hydrate decomposition in annulus has been calculated through finite-difference method under different operation parameters. The result shows that with higher delivery rate, temperature of cyclical drilling mud will be higher at bottom hole and lower at wellhead, leading to hydrate decomposition critical point moving down. And gas holdup and solid concentration will both decrease. Also, with increscent density of drilling mud, the critical point and end position of hydrate decomposition will both move up for higher pressure in annulus. And gas holdup and solid concentration will decrease. Besides, with increscent rate of penetration, hydrate decomposition critical point will move down to descendent pressure in annulus. And gas holdup and solid concentration will increase, and flow patterns will transform more acutely in annulus of upper wellbore. This research shows that rate of penetration should not be excessively high when drilling in marine natural gas reservoirs, and enhancing delivery rate and density of drilling mud can ensure safety control. This research provides important theories for wellbore flow rules in marine natural gas hydrate reservoir drilling. In addition, it can be used for analysis of wellbore flow safety.

Copyright 2017, Shaanxi Petroleum Society.

This paper was prepared for presentation at the 2017 International Field Exploration and Development Conference in Chengdu, China, 21–22 September 2017.

This paper was selected for presentation by the IFEDC&IPPTC Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the IFEDC&IPPTC Committee and are subject to correction by the author(s). The material does not necessarily reflect any position of the IFEDC&IPPTC Committee, its members. Papers presented at the Conference are subject to publication review by Professional Committee of Petroleum Engineering of Shaanxi Petroleum Society. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of Shaanxi Petroleum Society is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of IFEDC&IPPTC. Contact email: paper@ifedc.org or paper@ipptc.org.