Real-time determination of fluid contacts during horizontal well drilling using mud gas logging

Abstract

The article analyzes real-time detection of gas-oil (GOC) and oil-water (OWC) contacts during horizontal drilling in clastic reservoirs. Timely localization ensures high-quality geosteering, keeping the wellbore within the pay zone and preventing undesirable fluid intersections. Relying solely on conventional logging-while-drilling (LWD) is often insufficient. Limitations arise from electrical macroanisotropy distorting resistivity data, the clay electrical double layer masking responses, and tool «blind zones» causing delayed observations, which poses a critical risk in horizontal drilling. To address this, advanced mud gas logging is proposed. Instead of indirect electrophysical features, this method captures actual reservoir fluid dynamics. By using chromatographic analysis of C1–C5 fractions and calculating Wetness, Balance, and Pixler ratios, characteristic fluid signatures and transition zones are identified. Crucially, reliable interpretation requires mathematical normalization of gas data. Without it, operational artifacts from rate of penetration (ROP) and mud flow rate fluctuations create false anomalies or obscure actual boundaries. An Eastern Siberian field case study demonstrates that normalized mud gas logging achieved real-time GOC localization with sub-meter accuracy in a low-contrast reservoir. Validated by pulsed neutron logging (PNL), this method proves to be an effective geosteering and risk management tool. Keywords: mud gas logging; horizontal wells; gas-oil contact (GOC); oil-water contact (OWC); geosteering; mud logging; gas chromatography; gas data normalization.