Modification of the CVD Test Approach and Its Application For Predicting Gas Deliverability From Gas - Condensate Reservoir of Talang Akar Formation in Indonesia
Authors: Salam Eteng A. , Mucharam Leksono , Abdassah Doddy , Parhusip Budiman , Panjaitan Pahala R. 
Published: December 2007

Abstract
A compositional model is implemented in this study to model a gas-condensate reservoir behavior. The model is basically developed by introducing a two-phase flow model to the Constant Volume Depletion (CVD) approach for predicting gas production performance from a gas-condensate reservoir. This paper also presents a calculation procedure that used in this model to determine the performance of gas condensate reservoir such as reservoir pressure, condensate saturation, AOF, gas and condensate recovery, and gas deliverability. For handling gas and condensate production performance of the reservoir, the Flow Ratio concept is also coupled to the CVD model by which the amount of gas and condensate production can then be determined for each time step. Using the model developed, therefore, both gas and condensate flowrate from a gas-condensate reservoir can be predicted, and so for gas deliverability prediction. Prediction results of several field case study are also presented in this paper.

Introduction
The Constant Volume Depletion (CVD) test model has been widely used in modeling gas condensate performance prediction from a gas reservoir. This approach is basically developed to simulate a gas condensation process of a gas reservoir by assuming constant volume during production takes place. This technique is able to identify the condensation process and to determine the amount of condense formed during the pressure depletion scheme takes place in the reservoir. In the original concept of CVD model, the condensate phase occurred in the reservoir is assumed immobile, but the gas and condensate phases in both reservoir and surface might change as the production takes place. This concept is clearly presented in Figure 1. Of course, this concept is valid when the condensate phase saturation occurred in reservoir is less then its critical saturation. When the condensate phase in reservoir is greater than the critical saturation, then two-phase flow model must be included to the model. In reality, the condensate either from water or/and hydrocarbon may flow together with gas to surface.

Modification on CVD Model
The model developed is basically a modification of CVD material balance for handling two-phase flow problem in gas-condensate reservoir when condensate phase is mobile or having condensate saturation greater than its critical saturation. The gas and condensate flow from formation to wellbore, in this case, is controlled by their relative permeability behavior in the reservoir system. However, when the relative permeability data is not available, this model also includes the relative permeability model to simulate the behavior of two-phase flow performance in the reservoir.
Using commonly CVD calculation procedure, the increment pressure drop in reservoir is usually assigned arbitrarily to generate gas expansion and gas production through wellbore. The pressure drop and the productivity index, in this case, are considered as two restriction parameters which determine the volume rate of each fluid phase, gas and condensate, from the reservoir to wellbore/s. In additions we also introduce the Flow Ratio (FR) concept to the model for determining the flowrate of each phase from the reservoir when pressure depletion is taking place.
The gas productivity of a gas production well is usually determined using an isochronal test or a modified isochronal test which is conducted from a new gas well.
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