Abstract

Studies presented in [1] show that during gas production from the fields located in the permafrost regions intensive hydrate formation occurs in the bottom-hole zone. The process begins from the moment of bringing well into production, and approximately in 100 days all free water transforms into a hydrate. As a result, well productivity within this period reduces to about 20%. One of the ways of well productivity recovery is the use of different types of heaters. The paper presents theevaluation of the above method efficiency. To study the dynamics of hydrate formation and dissociation the modified mathematical model proposed in [1] has been used. During the computational experiment the effect of heating intensity, gas well production rate and permeability of gas-bearing reservoir on dynamics of the formation and dissociation of natural gas hydrates has been studied.

The results of calculations for the intensive heating show that regardless of the distance from a well, rate of hydrate saturation growth and its absolute magnitude significantly depend on permeability and production rate and that hydrate formation occurs in a comparatively narrow zone near wells where all pore water transforms into a hydrate. But this hydrate-free zone is stable during successive gas extraction for a long time. If the heater temperature only slightly exceeds the equilibrium temperature of hydrate formation but its active period is much longer, then its influence is more localized than in the first variant. Moreover, here the size of the hydrate-free zone depends both on gas flow rate and bed permeability.

Introduction

Studies presented in papers [1,2] have shown that during gas production from the permafrost zone fields the intensive hydrate formation occurs in the bottom hole zone of a well. The above process initiates from the moment a well is brought into production, and approximately in 100 days all free water transfers into a hydrate. As a result, well productivity decreases by about 20% during this period. At some distance from the bottom hole zone hydrate saturation non-monotonically depends both on time and radial coordinate. Moreover, this tendency is more pronounced during gas extraction at constant bottom-hole pressure. Similar behavior is observed for water saturation, and the latter becomes higher in the distant zone of the bed as compared with its initial value. The above effect can be explained by the fact that at a large distance from a well, hydrate saturation lowers with time that brings about the excess free water, which remains stationary due to the character of phase permeability dependence on water saturation. When gas is extracted at a constant production rate the curves of the change in the bottom hole pressure have two typical rectilinear sections, the first of which is characterized by a comparatively small pressure drop and, accordingly, slight decrease of well productivity. Then pressure drops abruptly that indicates the completion of hydrate formation and considerable decrease in well productivity.

One way to recover well productivity is the use of different types of heaters. The paper presents the estimation of the efficiency of the above method.

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