Effect of Gas Recycling on the Enhancement of Condensate Recovery in Toual Field Algeria, A Case Study
Authors: Belaifa Elhadi , Tiab Djebbar , Dehane Adel , Jokhio Sarfraz 
Published: December 2007

This paper provides an insight into the mechanism of gas injection process in reducing gas-well productivity losses due to condensate blockage in the near wellbore region. A technique is proposed to predict performance of gas-condensate reservoirs where cycling is employed.

Toual gas condensate field with its two reservoirs, TAGS & TAGI, was used as a case study in this paper. Comparison between the primary depletion and gas recycling was studied and the economic feasibility of the project was evaluated.

An equation of state (EOS) with 6 pseudo components was used to characterize the complexity of the hydrocarbon column. The model used consists of a grid of 2600 and 1875 cells (25´15´07) and (25´15´05) for TAGS & TAGI respectively. After a successful history match, MER (maximum efficiency rate) of 9% and 8% were used to optimise the primary depletion of TAGS & TAGI reservoirs respectively.

After running many cases, it was found that the injection of 100% produced gas for 20 years is the optimum case for the development of the Toual Field. It was observed that the condensate recovery increased as the duration and the volume of the injection gas increased. Prediction runs indicated that the injection of 100% of gas, starting in 2005 will result in a total recovery of 973.38 MSTCM and 777.53 MSTCM (22.2%, 30.5%) for both the reservoirs respectively over a life span of 20 years.

Introduction

A gas condensate reservoir may be produced in one of the following two ways:Producing the reservoir by natural depletion andby injecting all or part of the dry gas produced back into the reservoir, thereby maintaining the reservoir pressure above the dew point, the main cause of the liquid drop out and reduced deliverability.

As the dew point pressure is reached, natural depletion usually leads to the accumulation of an increasing liquid saturation in the pores. The percentage of heavy components in the production decreases with the time that the dew point is reached until the retrograde pressure is attained. At this point the percentage increases but only very gradually. Thus the condensate production is a decreasing function of time.

This paper represents the results of the simulation study of both reservoirs TAGS and TAGI of Toual field and proposes the following principle phases for the field:A revision of all geological and petrophysical data also all previous interpretations.Development of geological reservoir model.Simulation and matching of physical properties of gas using Peng-Robinson equation of state (PR-EOS).Determination of volume in place at initial conditions.Simulation and history matching of production of the reservoir using a 3D compositional simulator.Revision of performance of the reservoir with different scenarios of development.

Evaluation of the Reservoir Fluid Properties

1- TAGS Gas Zone

The reservoir pressure draw-down at time of the sampling was 17 Kg/cm2. This draw-down is relatively small and it is likely that the separator samples were representative of the reservoir fluids. Although the laboratory study appeared to be a full depletion study, the retrograde liquid data presented in the present evaluation were obtained entirely from a simulated depletion. The maximum volume of retrograde liquid will be about 4.3 percent of the hydrocarbon pore volume. The laboratory relationship between compressibility factor and pressure was very poor. These data were given little weight in this evaluation study.

2- TAGI Gas Zone

The reservoir pressure drawdown at time of the sampling was only 4.8 Kg/cm2. It is probable that the separator samples are representative because of the favourable down hole conditions. The dew point of the recombined reservoir fluid was 377.0 Kg/cm2 absolute at 105 °C. It is probable that the reservoir fluid was originally saturated at the initial conditions of 378.6 Kg/cm2 absolute and 108 °C at the reference level of - 3099 m, and the data in this evaluation study have been adjusted to this pressure.