CO2 and Ethane-Based Enhanced Oil Recovery (EOR): Laboratory Evaluations and Simulation Studies for a Matured Carbonate Reservoir in Bahrain
Authors: Alanaisi Isa , Konwar Lakshi , AlMeftah Ali , Alawainati Ebrahim 
Published: October 2022

The Mauddud reservoir discovered in 1932 in Bahrain is now in a mature stage of development. Crestal gas injection (GI) in the oil bearing, under saturated, layered, and heavily faulted carbonate reservoir has continued to be the dominant drive mechanism since 1938. Current strategy for maximizing reservoir potential and reduce production decline with infill drilling, workovers, and routine maintenance of wells are not adequate for a matured reservoir like Mauddud. As such, a detailed feasibility study is being carried out to identify the most appropriate enhanced oil recovery (EOR) process for this reservoir and define a strategy for further evaluation and implementation of the most promising EOR options. This paper aims to present detailed design and results of laboratory experiments using CO2 and ethane gas en route to sector modeling studies in three (3) selected large areas. A high-level cost estimate is also performed using the results from the pattern simulations.

The gas EOR laboratory study consisted of performing swelling and slim tube tests using the recombined Mauddud live oil and two injection gases: carbon dioxide (CO2) and ethane (C2). The swelling tests and the corresponding constant composition experiments (CCE) were matched using a 16-component equation of state (EOS) model. Slim tube simulations performed with the tuned EOS were able to replicate the oil recovery values from the slim tube tests. Representative sector and pattern simulation models were developed to estimate the EOR production potential from the Mauddud reservoir. The sector model developed was calibrated to the historical production, injection, and pressure data. An extensive sensitivity study was conducted to match the fluid flow dynamics of the reservoir. The history matched sector model was used to select and develop pattern simulation models that were used to estimated EOR production potential.

Swelling tests conducted with CO2 and Ethane shows the effect of oil swelling and changes in oil properties such as density, viscosity, formation volume factor, and solution GOR. The elevation in swelling factors and the reduction in oil viscosity exhibit the benefits of using CO2 and ethane as injection sources for Mauddud. Solid precipitation on the PVT cell window was observed, indicating the possibility of asphaltene precipitation with CO2 and C2 injection. CO2 slim tube tests showed a minimum miscibility pressure (MMP) of about 1,762 psig, which is around 800 psi higher than the current reservoir pressure. Therefore, CO2 injection under miscible conditions is not viable in Mauddud reservoir. Ethane gas mixture and Mauddud reservoir live oil showed an MMP of 1,022 psig. Ethane pattern simulations showed incremental oil recovery factors over the no-further-activities (NFA) between 17.7 and 27.6 percent of the original oil-in-place (OOIP).

The laboratory and sector simulation results are crucial to explore the feasibility of any EOR project and will serve as inputs to detailed economic evaluation as well as pilot design and facilities planning.