Abstract

Direct quadrupole mass spectrometry (DQMS) of mud gas allows rapid, real time analysis of C₁-C₁₀ hydrocarbons and inorganic species. The technique has been successfully applied to wells drilled with a variety of fluids, and a variety of drilling/analytical methods. Here, we discuss some unique applications of the technique.

1. Prediction of produced gas composition in shales: The introduction of direct quadrupole mass spectrometry (DQMS) and its field application on horizontal wells has been used to improve application of traditional component ratios (Wh, Bh and Ch) using the significantly better precision and more linear data of the DQMS. In certain shale plays, significant variation in produced fluid composition exists within a restricted geographical area. Operators typically may not collect electric log suites in shale laterals due to expense and risk factors. DQMS data were compared to production data for 105 wells from 5 basins within the mid-continent region. Of the 105 wells, 52 were drilled with water-based drilling mud (WBM) and 53 were drilled with oil-based drilling mud (OBM). The use of OBM and hydrocarbon-containing mud additives contributes the higher mass fraction and needs to be accounted for; thus, DQMS data was mathematically transformed to standardize the data between OBM and WBM. This analysis suggests that a clear distinction between wells that produce significant liquids and those that produce only gas can be made with greater than 95% confidence (101 of 105 wells were sorted accurately), by simply considering the DQMS generated Wh value. Additional applications are in progress to allow more detailed prediction of produced fluid composition (C1-C6+, BTU value, etc.). This data is available as soon as a well reaches TD, thus can be used in planning completions and surface facilities.

2. Distinction between formation gas and bit generated gas: Drill bit gas (Drill Bit Metamorphism; DBM) is produced by employing polycrystalline-diamond compact (PDC) or diamond-impregnated bits and a down-hole mud motor (or turbo drill) in place of the more conventional roller-cone bits and top-drive configuration in order to increase ROP, reduce drill bit trips and facilitate directional steering. It results in numerous artifacts in mud gas data, and irreparably compromises evaluation of rock cuttings by geochemical, petrophysical or petrological means. The changes to the gas chemistry are almost universally restricted situations in which OBM is being used as the drilling fluid. High temperatures at the drill bit degrade the mud, creating some unique species not found in natural systems, as well as some that are typically associated with thermochemical sulfate reduction and/or oil to gas cracking in natural systems. The former include alkenes (a.k.a. olefins) and CO (carbon monoxide), while natural species include hydrogen, CO2, Benzene, COS and CS2. These species are generally not detectable with standard GC instrumentation at well site, and generally require lab analysis of isotube samples. Carbon isotopic characteristics of bit-generated gas are also distinct (typically shifted to higher, more mature values). Discriminating between bit-generated gas and formation gas is possible with DQMS, although the exact methodology is currently still being evaluated. There are two approaches. The first involves empirically identifying a gas that is not produced by DBM, but is part of the natural formation gas. Helium is the most definitive species, when present. The other method is to identify a natural or artificial compound that is not part of the formation gas (or is present in a different relative proportion), but is generated by the bit. Hydrogen is the most identifiable species here, and seems to be universally present when bit generated gas is present. Historically, one of the uses for monitoring hydrogen is in identifying bit wear. Worn bits ineffectively cut rock and can produce bit-burn type anomalies even with conventional drilling configurations. Specific fragmentations appear to track alkane/alkene ratios. There is a tendency for the ratio of C1/C2 to be lower in bit generated gas zones, as there appears to be excess C2 and in some cases C3 (perhaps partially ethene and propene) as compared to what is normally found in formation gas. This is the case even in quite dry shows. The gas may also get drier within bit affected zones, due to conversion of higher molecular weight species to lower molecular weight gases. Distinction between formation gas and drill bit gas is obviously desirable in order to accurately interpret mud gas show data.

3. Comparison of mud gas and trapped gas (fluid inclusion) DQMS data: Fluid inclusions are small isolated pores in rocks that contain trapped fluids that were originally connected to the natural poroperm network. As such, they contain a record of both present day and paleo-fluid events. Comparison of DQMS data from mud gas generated in real time with analogous Fluid Inclusion Stratigraphy (FIS) DQMS data collected on cuttings in the lab enables unique conclusions to be made regarding current and past distribution of petroleum, hydrocarbon type, gravity and maturity. It can also help with certain equivocal drill results, particularly when testing is limited or non-existent, and conventional fluid samples are not available. Shale gas composition and maturity can also be estimated in many cases from the trapped gas composition, although here a combination of conventional fluid inclusions and adsorbed gases are being analyzed. Drill bit gas is obvious in FIS data.