Drill-bit metamorphism (DBM), the thermal degradation of drilling fluid at the drill-bit and rock interface because of drill-bit overheating and inefficient drilling, causes generation of nonreservoir gases, such as alkenes (ethene, propene, etc.), as well as compounds equivalent to petroleum reservoir species, such as alkanes (methane, ethane, etc.) and aromatics (benzene). This process is mainly associated with oil-based mud (OBM), enhanced use of drill bits containing diamonds and turbodrilling, deeper wells, and harder rocks. Intense DBM biases mud gas data interpretation while drilling and in extreme cases, it can generate so-called "ghost reservoirs," for example in tight hydrocarbon-poor zones. Presence of alkene species at significant concentrations and with good correlation between them implies cracking of OBM and consequently, generation of nonreservoir alkanes as well. Preservation of reservoir information in mud gas may require an adjustment in drilling practice once the DBM is flagged. Presented methodology adapts gas chromatography-mass spectrometry procedure to calculate ethene concentration, thanks to detection of mass/charge ratio of 26 and 30. Additionally, a new compact gas chromatograph flame ionization detector analyzer for ethene and propene with short cycle time has been developed for efficient DBM flagging. In case the drilling practice cannot be adjusted and drilling continues with varying extent of DBM, an in-house methodology subtracts the impact of DBM on the reservoir-derived mud gas. The multi-alkene sensor is a key enabler of this DBM correction. Furthermore, the ability to log clear changes in the carbon-isotope composition of DBM-impacted methane (δ13C−C1), ethane (δ13C−C2), etc. can be alternatively used to improve the DBM correction of mud gas to represent the indigenous reservoir fluid.
Since the 1980s, drill-bit metamorphism (DBM) has been suspected of impacting mud gas data while drilling [1]. A decade ago, evidence of DBM's impact on molecular and isotopic composition of mud gas appeared in publications [2]. Generation of nonreservoir gases, such as alkenes (ethene, propene, etc.), as well as compounds equivalent to petroleum reservoir species, such as alkanes (methane, ethane, etc.) and aromatics (benzene), at the drill-bit and rock interface was deduced from field mud gas samples and laboratory pyrolysis experiments [2] on base oil.
