Abstract

Gas recycling experiments were conducted on a gas condensate sample in a fractured model. Experimental Model was then numerically simulated with compositional simulator. The objectives were to investigate effects of gas recycling on condensate recovery from a naturally fractured reservoir under complete pressure maintenance process.

In laboratory scale we could accurately analyse behavior of in-place and injected gas in fracture and matrix blocks and understand how the production mechanisms affect ultimate recovery in this reservoir model. In reservoir scale, due to high heterogeneity and lack of required data such an accurate investigation is impossible.

In the simulated model after history matching of cumulative gas and condensate production and composition variation history of produced gas, some sensitivity analysis were performed. Simulation results show that gas recycling increases liquid recovery more than 2.5 times for the reservoir if the fracture model behaves as dual permeability fractured reservoir. If model behavior supposed to be dual porosity, condensate recovery decreases dramatically due to early break through of injected gas via fractures. Cyclic injection-production and decreasing the production rate are the factors directing the process to higher ultimate recoveries. Nitrogen injection effect has also been established showing less recovery than dry gas injection case.

Finally, lacks of data for a complete compositional simulation of such models are mentioned. If the data be prepared prior to performing the experiment, results could be up-scaled to the reservoir with higher degree of assurance.

Introduction

Gas recycling in gas condensate reservoirs has been recommended for several years as an optimum production scenario of increasing condensate recovery. For fractured media, the process is more complex due to early break through of injected gas in fracture network, and activation of production mechanisms like diffusion and gravity drainage of condensates.

Experimental gas recycling is most often used to predict recycling performance of the reservoir. With a complete rock and fluid analysis before starting the experiment, laboratory results can be up-scaled to design the recycling procedure in the reservoir. By preparing most similarity between laboratory model and reservoir conditions, probable errors would be minimized during up-scaling.

Laboratory gas recycling experiments are time and cost consuming; also in reservoir scale we face high heterogeneity and large scales and couldn't perform accurate investigation on recycling process. Numerical simulation of laboratory models prepares accurate investigation of recycling performance to select best conditions of pressure maintenance process for a reservoir. Most important factors are the injection and production gas rates, composition of gas to be injected, and when is the best time to start and finish the recycling to achieve most economic conditions.

Gas condensate behavior is highly sensitive to the heavy fraction composition of the fluid; so the most probable errors occur in fluid sampling and phase behavior modeling. Reducing deviations as will be discussed would yield in accurate up- scaling of the experiment to the reservoir.

Experimental setup and procedure

Six sandstone core plugs stacked vertically in a core-holder. Horizontal and vertical spaces were considered between cores and core holder as horizontal and vertical fractures (Fig.1). Average porosity of matrix was 21% and average permeability of matrix and fracture were measured to be 409 md and 550 darcy respectively. (See properties of rock samples in table 1.)

Gas and liquid samples were taken from surface separators and recombined with liquid gas ratio of 20 bbl/MMScf to represent the reservoir fluid. Compositions of reservoir gas and dry gas are shown in tables 2, 3 respectively.

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