An Innovative Approach to Integrated Fluid Typing in Depleted Reservoirs
Authors: Ramli Asari , Ashqar Ayham , Karim M. Azan 
Published: March 2021

The economic value of completing a reservoir is strongly influenced by the fluid type. Wells drilled in developed brown field penetrate reservoirs with significant pressure loss due to offset production. A major challenge in evaluating mature reservoirs is the uncertainty introduced by pore fluids with unknown or varying petrophysical properties, such as change hydrocarbon gravity, diminishing pore pressures, and low to absent gas level indication. These are prone to error and uncertainty. Accurate understanding of reservoir fluid properties is therefore a key requirement for successful reservoir management. This manuscript illustrates a successful integrated workflow to ascertain. An integration between LWD triple combo data, near/far neutron, mud logs, pressure measurement, and production history of neighbouring wells, are critical to confirm fluid type within the drilled reservoirs. Cross plots, ratios and confidence analysis are required to ascertain the confidence level. Acquired data was ranked according to uncertainty associated with the acquisition technique, rate of penetration, lag time, mud type, and pre-test drawdown. Mobility was used as an indicator of fluid type or phase change in absence of any major rock type changes. Gas data were verified for any mud contamination and analysed using ratios to verify Hydrocarbon wetness. Data was ranked based on confidence factor determined through data precision and reservoir propertied. We also highlight the uncertainty in measurements. The fluid typing workflow used successfully identified the correct fluid typing, and reduced the reliance on single conventional method, or the need to run pre-test measurements. Data in intervals dominated with residual oil saturation showed misleading fluid type, same applies in high permeability sand, corrected gas data analysis gave a good indication of fluid type and mapped the change in fluid phase when combined with log data, while near/ far neutron aided to correlate the different sands, however due to its relationship with porosity, there is no one correlation could be derived.

This paper illustrates that standard petrophysical techniques, such as analysis of density and neutron porosity logs, near/far neutrons, pretest can give misleading results if used in solo without consideration to the uncertainty associated with the measurement. The integration of fundamentally different data has resulted in identifying the fluid typing and its distribution in the reservoir and without integrating other measurements. A fluid typing systematic was developed to ensure the best and cost-effective model to assure the correct fluid type is identified.

In this paper, a methodology is proposed which uses the geodesic transform, and integrate various source fundamentally different data, which is routinely acquired, then develop a systematic reasoning of confidence on data precision and accuracy. The system followed ensured the correct mapping of fluid typing in various reservoirs with different petrophysical properties. It is the first time such workflow is followed, and an integrated approach is consistently used in different sandstone reservoirs.