Abstract

Re-injection of produced gas is a common strategy to maintain pressure and to meet environmental regulations in gas condensate reservoirs and also to store gas until the market available. Although the best idea to prevent liquid loss is the full pressure maintenance development above the dew point pressure, the partial pressure maintenance process below dew point may be desirable for economic reasons, as the dew point pressures in some gas condensate reservoirs discovered in recent years is very high. In this case one major concern is whether gas cycling above the dewpoint is more profitable than below the dewpoint. Therefore, a knowledge of accurate condensate recoveries under two different development schemes is vital for making the accurate economic assessment and making final decisions.

To the end, a series of experiments have been done to investigate the condensate recovery based on the above two different development strategies and quantitatively determine the revaporization efficiency of retrograde condensate by lean gas injection. First the gas injection below the dewpoint was performed in a long-core apparatus. In order to calculate the quantitative amount of the revaporized condensate, other two parallel tests of gas injection above the dew point in a long core system and gas injection below the dew point in the PVT cell were conducted as a comparative basis. The actual gas condensate fluid and a long core system are employed here. The gas condensate used is rich and highly waxy. As a complete study, the routine phase behavior is first detailed, including constant mass expansion and constant volume depletion. An interesting finding in PVT cell shows that lean gas can not only effectively revaporize the intermediate but also C20 plus components. Comparison of the tests in long-core system illustrates that more condensate above the dewpoint is recovered than below the dewpoint. This is consistent with the conventional idea that the full pressure maintenance is superior to the partial pressure maintenance according to the condensate recovery. It should be noted that all the condensate recoveries reported here in three runs don't include the additional withdrawal of condensate in the slowdown process to abandonment pressure.

Introduction

Historically, there are mainly three schemes of withdrawal of gas condensate from a reservoir1: natural pressure depletion to abandonment pressure, full pressure maintenance by gas cycling and partial pressure maintenance by means of gas cycling after previous depletion. The natural depletion is the simplest development scheme due to the requirements of low initial capital investments, high initial revenue and the least engineering design. Field experiences usually show that, however, gas condensate well productivity decreases rapidly when the reservoir is producing below the dew point, resulting in a high condesate saturation ring near the wellbore. In order to alleviate the impairment of condensate accumulation about the wellbore, gas cycling is frequently applied to prevent the condensate liquid loss and to revaporize the retrograde liquid.

Simply from the reservoir engineering standpoint, the full pressure maintenance may be the best way to maximize the condensate recovery. However, under the high pressure con-ditions this will result in initial expensive investment on the injection equipment and face larger risk of safety problem. Therefore questions have long been frequently raised concerning whether gas cycling can be used at a certain pressure level below the dewpoint without severely affecting the amount of condensate left in the reservoir. The processes within the reservoir include the displacement of wet gas by the injection gas, the revaporization of condensate upon contact by dry gas into the equilibrium gas and the production with the flowing gaseous phase. At this point the detailed economic analyses are usually recommended to evaluate the feasibility.

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