Reducing Uncertainties in Mud-Gas Data Acquisition While Drilling-Offshore Depo-Belt, Niger Delta

Abstract

Abstract Over the years, the acquisition of gas in mud is crucial to the exploration and production (E&P) industry for the evaluation of hydrocarbons presence in the geological formation, and for monitoring of drilling operations. The mudloging technique involves continuous gas extraction and analysis from a circulating drilling fluid for quantity and composition using surface gas trap, gas lines and gas analyzer. This paper presents major uncertainties that associated with gas acquisition during drilling, sources of the uncertainties, and the way forward to acquire highly accurate gas-in-mud data. During the process of acquiring gas-in-mud data, some uncertainties have been observed which can adversely impact real-time decision making as well as cascading effect on the geological models built with this data. In some of the recent wells drilled in offshore depo-belt of Niger delta the following uncertainties were observed; (1) Shift-in-the gas analysis window- Methane (C1) and heavy gas components (C4 and C5) were present in the absence of some light gas components (C2 and C3), (2) Misaligned gas data- gas values acquired misaligned with the resistivity readings of the reservoir being drilled, and (3) Gas in mud (volume and component) underestimation across the reservoirs; Lao-2 reservoir up to 35 units (0.7%) total gas, and gas composition- C1 to C3, and Mao-1 reservoir up to 27 units (0.54%) total gas, and gas composition- C1 to C2 were recoreded at well 43H drilled. This study showed that these uncertainties could result from a drop-in sample pressure or moisture in the gas line, real-time transmission of gas in mud data in bit depth instead of lagged depth and poor calibrations of gas analyzer. The issues then, are how to minimize these uncertainties to obtain accurate gas in mud that truly represent the reservoir being drilled. A systematic approach was deployed to minimize the abovementioned uncertainties. The methodology adopted significantly improved the data acquired in the new well drilled in the field. Highly accurate gas in mud data were obtained in the same reservoirs at the new well (44H) across Lao-2 reservoir: 190 units (3.8%) total gas, and gas composition- C1 to C4, and Mao-1 reservoir: 430 units (8,6%) total gas, and gas composition- C1 to C5. The new well is about 200m away from well 43H. The new well 44H was drilled under same drilling conditions as well 43H; same drilling fluid properties, rheology, mud weight, bottom hole assembly configuration and within the same fault block. Thus, we recommend; constant inspection of surface gas trap and gas agitator, periodic blow back of gas lines, multiple points gas calibrations, detailed gas analyzer calibrations for both total hydrocarbon analyzer and gas chromatograph prior to drilling operations, Maximum of five percent standard error margin, and periodic inspection of gas analyzer filament.