Gas Logging Applications: A Methodology for Porosity and Water Saturation Calculations for Successful and Cost-Effective Use in Reservoir Model, and Real-Time Identification of Gas Injection Impact on Fluid Composition.
Authors: Hanif Rehan , Ahsan Syed Asif , Baudot Pascal 
Published: November 2018

Abstract

Objective/Scope
In the recent past, budgetary constrained E&P market forced companies to find cost optimized solutions to their activities. Here, we present a case study from a field in Abu Dhabi Onshore to demonstrate how cost optimization in logging activities can potentially be achieved. In mature developed onshore fields where significant amount of static data is available, acquisition of additional in-fill cost optimized data has potential to add value to enhance confidence on reservoir model. Instead of acquiring conventional suite of logs, the well cost could be optimized by obtaining low cost yet effective mud log data. Additionally, fluid composition and its phase, in gas injection area for production optimization, can be obtained in real-time while drilling.

Method, Procedures, and Process
Gas chromatography is a proven, accurate and sensitive technique for separating and quantifying dissolved hydrocarbon and non- hydrocarbon gases in mud samples. Hydrocarbon gas dissolved in mud can be used to calculate ratios which in turn provide an assessment of hydrocarbon phase (gas, condensate, oil) in reservoirs during drilling as the gas dissolved in mud is circulated on the surface.
Gas dissolved in the mud and circulated to the surface is measured by an automated gas detection system. The primary gas measurement tool is a Gas Chromatograph with Flame Ionization Detector (GC-FID), which measures hydrocarbon gas concentrations as low as 5 ppm. A significant increase, above the background concentration, in detected gas is possibly associated with a zone of increased porosity and permeability and hydrocarbon content. The fractional components of gas are separated by gas chromatograph according to their molecular weight. Common components are classified in Alkane group, i.e. C1 (methane), C2 (ethane), C3 (propane), nC4 (normal butane), iC4 (iso-butane), nC5 (normal pentane), iC5 (iso-pentane).
This paper describes application of mud gas ratios to characterize hydrocarbon-bearing and water-saturated zones. Provided sufficient calibration from open hole logs, mud gas data can be used in quantitative way by establishing mathematical expression based correlations with porosity and fluid saturation. The work flow presented in the study can be applied to other mature fields and reservoirs with mathematical expressions developed that are unique to the reservoirs.

Results, Observations, Conclusions
Based on mud gas data, equations for porosity and saturation estimation were built in one pilot well. Results were found encouraging and can be actually used for reservoir modelling with local tuning. In addition, blind test to assess hydrocarbon phase was performed on two other wells with available gas data. Phase prediction from both wells was successful. Correlation between porosity and saturation derived from mud gas data is reasonable with that of LWD derived porosity and saturation. Hence, the mud gas data based porosity and saturation estimation can be used to produce upscaled average input needed in reservoir model.
This paper demonstrates application of basic gas composition data in almost real time fluid identification and its variation while drilling during early formation evaluation phase. The results of fluid identification using mud gas ratios were compared with pulsed neutron interpretation. The two independent measurements confirmed the zone of interest affected by Gas injection.