Increasing Certainty in Formation Evaluation Utilizing Advanced Mud Logging Gas Analysis

Abstract

Recent developments in surface logging and the need for sophisticated information on reservoir content and type in the oil industry have led to the availability of real-time advanced fluid solutions assisting in informed decisions while drilling. The objective of this study was to identify possible fluid contacts and acquire PVT quality sample data while drilling Paleozoic formations. This is accomplished by extracting and analysing formation gas from the drilling fluid employing the Advanced Formation Gas Extraction System for formation evaluation with a high-resolution chromatograph. The Advanced Formation Gas Extraction System provided consistent flow and heated mud and maintained constant temperature conditions. Thus, it provided an accurate chromatographic breakdown of the formation gas extracted from the drilling fluid at surface. The chromatograph was able to detect the hydrocarbons from the light to heavy factions, methane (C1) to pentane (C5), and also extended the detection range to include the dominant C6, C7, C8, aromatics and lighter alkenes. Gas ratio analysis of the detected hydrocarbon components enabled us to evaluate the reservoir fluid content and to identify and characterize the formation fluid and possible fluid contacts. The results, validated by correlation and comparison with other data such as wireline logs, well tests and PVT results assisted in the characterization of lithological changes, possible fluid contacts, vertical fluid differentiation in multi-layered intervals, and drill bit metamorphism (thermal cracking) effect. The comparison between surface gas data analysis and PVT data confirms the consistency between the gas show and the corresponding reservoir fluid composition.