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Xie, Xingli (Research Institute of Petroleum Exploration and Development) | Luo, Kai (Research Institute of Petroleum Exploration and Development) | Song, Wenjie (Research Institute of Petroleum Exploration and Development) | Li, Baozhu (Research Institute of Petroleum Exploration and Development) | Li, Shi (Research Institute of Petroleum Exploration and Development) | Zheng, Xitan (Research Institute of Petroleum Exploration and Development)
As the well flowing bottomhole pressure falls below the dew point, liquid dropout occurs in gas condensate reservoirs. Gas condensate well behavior is unique in a sense that it is characterized by severe loss of well deliverability. Thus well productivity and gas re-injection are two among the critical issues in development of many gas condensate reservoirs, particularly for the high condensate yield cases.
Generally retrograde condensate is viewed immobile at reservoir conditions so that the standard dry gas flow equations based on flow-after-flow, isochronal and modified isochronal testing, etc., have frequently been used to analyze gas condensate well productivity. However, due to the combined effects of the bi-phase flow of gas and condensate near the wellbore and reduction in well deliverability caused by liquid buildup, this approach seems to be inadequate and usually leads to misleading results, suffering doubt and criticism up till now. Therefore, there is a need to develop a rigorous equation to express the thorough flow behavior of gas and condensate in porous media.
Based on a detailed description of flow behavior in gas condensate pools, a definite solution problem is formulated. Subsequently the analytic solution is derived under pseudo-steady-state conditions. The derivation procedures are detailed. Finally a rigorous overall deliverability equation desired is obtained by incorporating the effect of non-darcy flow in the vicinity of well bore. Then the parametric calculation methods are discussed briefly. A calculation example using the new equation is compared with the result calculated by the standard dry gas productivity equation. The comparison shows that the standard binomial productivity equation usually used in the dry gas condition over-predicts the production potential of gas condensate well.
Additionally, in order to investigate the quantitative efficiency of condensate revaporized by lean gas injection, the laboratory experiments of gas injection have been made in a PVT cell, and above and below the dew point in a long core apparatus. Comparison of the tests shows more recovery of condensate in longcore above the dewpoint is obtained than below the dewpoint, justifying the popular argument with regard to the condensate recovery that the full pressure maintenance is superior to the partial pressure maintenance. However, the full pressure maintenance in the high-pressure reservoirs may not be the best choice due to the expensive investments on equipment. A comprehensive cost evaluation is suggested, but this is out of the scope of the present study.
This work was originally completed to provide the optimal development strategies for a rich gas condensate field, which is being developed in West China. The original study comprised geological characterization, numerous well test sensitivities, PVT characterization, gas cycling simulation and laboratory tests, as well as simulating the effects of condensate dropout on well productivity. The ultimate objective of this work was to make a nomination for a gas injection plant. However, The presented study is only confined to the effects of condensate dropout on well productivity as well as the feasibility of gas re-injection below the dewpoint.
Development of retrograde gas condensate reservoirs necessarily requires accurate well productivity predictions for the purpose of an investment evaluation on gas processing facilities. When the well flowing bottomhore pressure (FBHP) drops below the dewpoint, Liquid condensate usually drops out and a condensate ring forms around the wellbore. The high condensate saturation in the ring usually reduces effective permeability to gas, resulting in severe decline of gas production. This is the phenomenon of "condensate-blockage" called by Muskat1,2. Therefore, the effect of liquid condensate dropout on PVT, absolute and gas relative permeability should be considered in studying gas-condensate well performance.
Shi, Juntai (China University of Petroleum (Beijing)) | Li, Xiangfang (China University of Petroleum (Beijing)) | Shi, Depei (China National Logging Corporation) | Xu, Hanbing (Research Institute of Petroleum Exploration & Development) | Li, Baozhen (China University of Petroleum (Beijing)) | Zhou, Jianrui (China University of Petroleum (Beijing))
Abstract In the process of well testing on gas condensate well, if the condensate liquid retrogrades from gas phase, which will adhere to the surface of the rock, the key reservoir parameters would vary, such as the permeability and the skin factor. The changes of reservoir parameters could affect the deliverability of a gas condensate well, by changing the coefficients of A and B in the deliverability equation. These changes make it inapplicable to use the deliverability equation for a conventional gas well to interpret the deliverability of a gas condensate well. In view of the problem above in the process of well testing on gas condensate well, the model in this paper was built up in view of different pressure region, and could reflect the deliverability equation under the specific testing condition, so it could interpret the deliverability of a gas condensate well under different production stages. A new method to model and construct a deliverability equation with variable coefficients or many independent deliverability equations, which can represent the performance of well, was put forward in this paper. Through applying the deliverability test method put forward in this study to a case, results obtained from this method keep consistent with the real testing data, so the deliverability of a gas condensate well obtained by using this method in different process of production can be correctly evaluated. The created deliverability equation could reflect not only the characteristics of gas reservoir testing and dynamic performance, but also four-dimensional changes of rock properties and flowing parameters of reservoir fluid, so the well performance in the future could be more accurately predicted.
Abstract This paper presents a methodology for determination of gas-oil relative permeability curves using well performance data from retrograde condensate wells. Computations are based on PVT, producing gas-oil ratio, flowing bottom-hole pressure and material balance. The methodology presented was successfully applied to Britannia wells across the field, and findings were verified with numerical simulation to show proper modeling of condensate banking. Production performance of Britannia wells is monitored through back-pressure curves, constructed by plotting single-phase pseudo-pressure difference versus flow rate on log-log scale, using individual well separator test data. As condensate accumulates near the wellbore, reducing productivity, well performance deteriorates (points move left) from the established pseudo steady state (pss) deliverability line. Once productivity deterioration becomes negligible, the well performance response shows a linear trend. The empirical deliverability equation, delta pseudo-pressure versus flow rate log-log, yields a linear relationship with a slope of (1/n). Using this relationship, n is determined from the observed well deliverability line, shifted left due to condensate blockage. The coefficient C is then calculated at the pss deliverability line by assuming that the coefficient n remained unchanged. With accurate gas-oil relative permeability data included in the two-phase pseudo-pressure integral, plotting delta two-phase pseudo-pressure versus flow rate for separator test points should develop an alignment with the pss deliverability line. An objective function is defined as the difference between the delta pseudo-pressure, determined using the empirical deliverability equation, and the measured delta two-phase pseudo-pressure for separator test flow rates. The process outlined in this paper aims to find a gas-oil relative permeability curve that produces the best alignment, that is, the minimum objective function. In complex fields, e.g. Britannia, multiple gas-oil relative permeability curves might be needed to model condensate banking. In the absence of relative permeability data, the presented approach could provide valuable information on relative permeabilities.
Abstract Underground flow in gas condensate reservoirs usually consists of single-phase gas flow at initial reservoir conditions (reservoir pressure Pr larger or equal than dew point pressure Pd). However, with the depletion of the gas condensate reservoir, when its flowing bottom-hole pressure of a producer falls below the dew point pressure of the corresponding reservoir fluids, a relatively high liquid saturation will build up near the wellbore, which is named as "condensate blocking". The formation of this "condensate blocking" in the near-well region will lead to the reduction in gas relative permeability and result in loss of well deliverability. This deliverability loss is also indicated by the field production data from many gas condensate wells. In this paper, this loss is investigated by conducting a series of fine-grid numerical compositional simulations with the reservoir data. The well deliverability index (PI) at initial reservoir pressure, slightly above dew point pressure for a single-phase gas flow was found to drop rapidly due to condensate banking. The gas PI at a reservoir pressure equal to half of dew point pressure is only about 20% of the value at a reservoir pressure, slightly above dew point pressure for low permeability cases (permeability less than 100md). We also found that PVT properties of reservoir fluids, absolute permeability, and production pressure drawdown of the producer all have significant influence on the well gas deliverability. Introduction Gas condensate reservoirs are becoming increasingly important with more and more new condensate fields discovered worldwide as exploration drilling encounters greater depth, higher pressure and higher temperature conditions. Defining well deliverability is an important issue for reasonable development of gas condensate reservoirs and through which we gain the most profit from a dedicated reservoir. However, accurate forecasts of deliverability are still a challenge because the need to understand and account for the complex processes that occur in the near-well region, more work need to be done. For gas condensate reservoirs, when the flowing bottom-hole pressure falls below its dew point pressure of the reservoir fluid, condensate liquid will start to drop out from gas and a region of high liquid saturation may build-up near the producer wellbore. This liquid saturation build-up leads to a reduction in gas relative permeabilities and thus a loss in gas well deliverability. This effect is referred to as "condensate blockage" by Muskat and "condensate blocking" by other researches such as Fevang, Henderson and Whitson. We will use term "condensate blocking" in this paper. When we try to define well deliverability, it is essential to take into account of this "condensate blocking" effect. Due to this "condensate blocking" near wellbore, well deliverability in the gas condensate reservoir often decreases with pressure drops from initial value. Field production data from gas condensate producers also have shown that their deliverability is curtailed when flowing bottom-hole pressure falls below the dew point pressure of the fluid in-place. It is accepted that this reduction is caused by the net effect of the various phenomena occurring in the near wellbore region. This net effect is a balance between those effects that reduce well deliverability (relative gas permeability reduction), and those effects that increase well deliverability (Capillary desaturation, viscous stripping). Experiments show that capillary desaturation and viscous stripping of condensate may also occur in the near wellbore region and may alleviate the deliverability reduction in gas condensate reservoir due to condensate blocking.
The pdf file of this paper is in Russian. To purchase the paper in English, order SPE-161972-MS.
Deliverability of low permeability gas-condensate wells decreases when the flowing pressure falls below the saturation pressure. To some extent the deliverability can be mitigated by high matrix gas velocity. Condensate is stripped off next to the well reducing the fluid saturation and improving the well inflow properties. Non-Darcy turbulence flow in turn decreases the well capacity. With well flowing data only, it is difficult to differentiate between the two effects that act in the opposite direction. Gas condensate relative permeability measurements were designed to distinguish between the Velocity Stripping and Non-Darcy flow effects. The experiments were modeled with the Eclipse compositional simulator and the velocity dependent parameters were adjusted to match the experimental data. With the core experiment derived parameters it was possible to uniquely match the well test results of a gas condensate well. The test interpretation is in accordance with the lab results. In the case investigated the Velocity Stripping effect is very significant and fully compensates the adverse Non Darcy flow effect. A sensitivity analysis has shown that both effects are important for an un-fractured well, but insignificant for a fractured well.