Accuracy of EOS Model Approach to Restore Depleted Reservoir Composition

Abstract

Oil and gas reserves calculation is highly dependent on various factors such as the properties of rock physics, reservoir fluid and reservoir size. Reservoir simulation methods are most chosen within the oil and gas companies to obtain maximum accuracy of field performance.

In Indonesia, many mature fields are still producing. Despite of small field production that may be uneconomic; the remaining reserves are still potential. Many oil and gas recovery techniques are used to boost more production such as water injection, gas injection (HC, CO2, N2) and chemical injection. However, prior to the recovery technique application, remaining reserves must be estimated in order to value the economic field development projects. One of the methods is using Material Balance Analysis that leads to the real Initial Reserves (N) of a mature field. The most important key is information on the Initial Composition (Pi) that is absence on the database.

A possible theoretical approach is to use Eos Model to recover the fluid composition of the depleted sample. For oil reservoir experiment, a routine Differential Liberation Test (DL Test) is used. This method will be measured Oil Formation Volume Factor (B), Gas Formation Volume Factor (B), Density and Viscosity changes from hydrocarbon fluid to reservoir temperature under several pressure changes. Gas will be removed from the experimental sample and the oil volume change will be measured until it is reached the Temperature (T) and Pressure (P) at standard condition. This DL method will be reversed until Pi condition to get Boi, Bgi, Rsi and initial hydrocarbon composition to determine the Initial Reserves (N).