Validating of the Reservoir Connectivity and Compartmentalization Through the Use of the CO2 Compositional Gradient, Mass Transportation Simulation, and Asphaltene Analysis
Authors: Daungkaew Saifon , Mullins Oliver C. , Johan Zailily Johfizah , Lehne Eric , Zuo Julian Youxiang , Sinnappu Suresh , Pfeiffer Thomas , Giok Lin Tan , Tunku Abdul Muthalib Tunku Indra , Hong Teh Yat , Rameli Hazwani , Ludwig John Thomas 
Published: May 2016

More than 40% of the world's conventional gas reserves are in reservoirs that contain significant amounts of H2S and CO2. The presence of these gases results in a number of challenges for the Field Development Plan (FDP). For a field with multiple fault blocks with unknown fault transmissivity, a key challenge is to understand the field connectivity and compartmentalization which impacts the ability to drain the reserves.

This paper presents a comprehensive study to understand reservoir connectivity in a gas and oil field located in Southeast Asia. This particular field has variation of the CO2 content even in the same zone ranging from less than 10% to more than 80%wt. A key for production strategy and facility design is to be able to accurately quantify the CO2 in each reservoir. Initially, the CO2 study aimed to quantify the CO2 content for each reservoir using an advanced Downhole Fluid Analyzer (DFA), and then to use the DFA measurement as well as the available PVT data from nearby wells to understand reservoir connectivity through the use of a compositional gradient concept. Reservoir fluids that deviated from the compositional gradient were considered to not be in equilibrium with each other, and subsequently indicate possible compartmentalization. In addition, the use of a PVT thermodynamic calculation with a non-isothermal solution resulted in a possible identification of the CO2 charging location. Although the geochemistry study was conducted to understand the source of CO2 and hydrocarbon isotopes, it did not provide a conclusive result of the reservoir connectivity. As expected, the hydrocarbon and non hydrocarbon gases had different charge sources. Since the geochemistry study was inconclusive, a mass transportation simulation was performed to understand the reservoir connectivity and this information has had a great impact on understanding the production mechanism of this field.

In addition to the discussion of the horizontal connectivity between wells in the reservoir, a preliminary study of the asphaltene composition and concentration in one of the wells was provided as part of this paper to determine the extent of vertical connectivity between the thin beds. Specifically, the effect of the asphaltene content on the reservoir fluid's optical properties versus depth was analyzed and is presented in this paper.

A systematic process is provided to understand the reservoir connectivity by using integrated reservoir data including pressure, DFA, PVT fluid properties, geochemistry, asphaltene science, as well as the geological and geophysical interpretations of the reservoir. This paper offers an efficient way for reservoir characterization for proper field management for an important hydrocarbon discovery in Southeast Asia.

Introduction

Carbon dioxide and Its Origin

The occurrence of CO2 in gas fields can be of primary or secondary origin, or a combination of both. For example, primary origin(s) of CO2 are1,3,4,6–9,15–17,19;