Experimental Investigation Into Revaporization of Retrograde Condensate

Abstract

Abstract A high condesate saturation ring usually forms in a gas condensate reservoir where pressure decreases below the dew point, especially around the wellbore. Gas injection/cycling is a main recovery process of choice for preventing loss of valuable condensate liquids. In this process the reservoir is often maintained above the dew point. Although full pressure maintenance development seems ideal in terms of preventing liquid loss, for economic reasons, this process may not be profitable. In recent years, many gas condensate reservoirs discovered have very high dew point pressures, say above 50MPa, in this case the full pressure maintenance will result in much expensive cost of the injection equipment and a large risk of safety problem. Thus one thought usually arises that whether the reservoir may be cycled at lower pressure after undergoing previous depletion below the dew point. To address this issue, a series of experiments in this paper are performed and compared to quantitatively investigate the revaporization efficiency of retrograde condensate by lean gas injection. The studies include three main parts:gas injection below the dew point in the PVT cell;gas injection above andbelow the dew point in long core equipment. Instead of the simple synthetic fluid and the packed core systems used in the previous literature, the actual gas condensate fluid and about 1m long-core are employed here. The gas condensate used is rich and has high wax content. As a comparative basis, the phase behavior is first detailed at length, including constant mass expansion and constant volume depletion. The measurements in the PVT cell show that lean gas injection leads to increase in the dewpoint of the rich gas condensate. An interesting finding from the first part is that lean gas can effectively revaporize not only the intermediate but also heavy hydrocarbon (i.e., C20+), which is contrary to the common belief that the heavy hydrocarbon cannot be revaporized by lean gas. Comparison of the second and the third parts illustrates that more condensate recovery of the former is obtained than the latter. This value is consistent with the conventional idea that full pressure maintenance is superior to the partial pressure maintenance according to the condensate recovery. However, when the expensive investments of the high pressure equipment are taken into account, the full pressure maintenance may not be the best choice. Therefore, it is recommended that an economic analysis be done to evaluate its feasibility. This is beyond the scope of the present study. To our knowledge, this paper is the first report of experimental studies on the revaporization of condensate using actual reservoir fluid/core system in long core equipment. 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 revaporize the retrograde liquid.