An Integrated Approach for Reservoir Characterization of Condensate Banking Using Pressure Transient Analysis PTA: A Case Study Using Data from Five Gas Condensate Fields in the Sultanate of Oman
Authors: Cobanoglu Mustafa , Shukri Ibrahim 
Published: January 2020

In rich gas fields, condensate banking refers to the formation of condensate around the wellbore when the reservoir pressure drops below dew point. Condensate banking severely damages reservoir performance and results in loss of production capacity and ultimate recovery, especially for reservoirs with low permeability and high Condensate Gas Ratios. Understanding this process and quantifying its impact is therefore crucial to robust field development projects economics and effective well and reservoir management. The characterization of condensate banking is an integrated process where several reservoir parameters like fluid properties (PVT), geology (i.e. permeability), and rock properties (i.e. SCAL data) needs to be considered. Because of condensate banking, different flow regions with different characteristics are created within the reservoir where Pressure Transient Analysis (PTA) can identify these flow regions. This study attempts to characterize the condensate banking using PTA, shows examples of pitfalls in well test analysis of rich condensate fields and provides the methods proposed to identify condensation effect in PTA analysis.

In order to detect, quantify and characterize condensate banking, a workflow was developed using PTA along with other field data (PVT/SCAL/production). The process to define this workflow includes;

Conducting a review of critical parameters affecting the condensate banking process (PVT, SCAL, reservoir properties…. etc.),

Collecting and interpreting all PTA’s (51) from 5 large gas condensate fields to understand the condensation process characteristics in terms of PTA diagnostics (i.e. determine the mobility change in the reservoir)

Developing a novel integrated workflow and procedures for condensate banking characterization and quantification using Inflow Performance Curves (IPR).

The key conclusions of integrated approach are follows;

PVT (maximum condensate drop out), SCAL (Krg & Ng) and permeability plays a critical role for condensation effect

Several PTA interpretations demonstrated that the expected 3 mobility regions associated to condensation effect may not able to be seen due to near wellbore & reservoir effects (i.e. wellbore storage, fraccing) that has been masking the condensation response,

Even in case of hydraulically fracced well, condensation will still negatively affect the well performance.

None of 5 field cases has shown any indication of a "capillary number effect" (i.e. velocity dependent relative permeability)

IPR is proposed and used to quantify the impact of condensation drop out on production performance.

An integrated workflow to detect, quantify and characterize condensate banking using PTA analysis has been developed and implemented in five large PDO gas condensate fields.

This paper presents a novel systematic approach to achieve Condensate Banking Characterization Using PTA for gas condensate reservoirs as well as it provides several real cases to validate the workflow. Results can also contribute to a better reservoir management, quantification of the possible productivity losses of the well.