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The Philosopher’s Stone is a legendary alchemical substance that has captured the imagination of scientists, philosophers, and mystics for centuries. At its core, it represents the ultimate achievement of alchemy: the ability to transform the ordinary into the extraordinary. Though its existence has never been proven, the Philosopher’s Stone has become a symbol of transformation, potential, and the pursuit of knowledge.

The idea of the Philosopher’s Stone is rooted in the principles of alchemy, an ancient practice that combined elements of science and philosophy. It was said that the Philosopher’s Stone could accelerate the natural process of transforming less perfect substances, such as lead into gold, essentially unlocking their hidden potential.

Although modern science has dismissed the existence of the Philosopher’s Stone, its influence endures. The Stone represents a universal human aspiration: the ability to find value and potential in unexpected places. It embodies the enduring quest to extract something valuable from the seemingly mundane, to transcend limitations, and to uncover the hidden potential in both the natural world and ourselves.

This story brings me back to my mud logging days (#doublebagger), where a fundamental responsibility was to operate the wellsite gas system to capture the gas signature of the well. A vital data set that is used to understand not only the geochemistry, but also a useful indication of the well engineering. But what if we could employ the Philosopher’s Stone to uncover the hidden potential gas bubbles in drilling mud have and find value in an unexpected place?


Advanced Gas Interpretation

One way to gain valuable information for (almost) nothing is to employ the advanced gas data interpretation techniques offer by HRH Geology.

Standard mud gas analysis relies on Total Gas readings and Gas Chromatography C1-C5 alkanes (methane, ethane, propane, butane, pentane), including isomers (iC4, nC4, iC5, nC5). This data is paired with other datasets, such as lithological cutting samples, Logging While Drilling (LWD) log responses, and wireline sampling runs, to help subsurface teams understand geology and reservoir properties. This includes, among other things, depths of gas/oil/water contacts, identifying reservoir compartments, and understanding the porosity and permeability of the seal/cap rocks.

Advanced Gas Analysis services typically analyse gas species in addition to the standard C1-C5. Depending on the provider and setup used, the additional species can vary and include heavier alkanes (C6+), Cyclic hydrocarbons, aromatic hydrocarbons, CO2, Helium, Nitrogen, H2, ethene and/or propene. An advanced gas service will give a larger data set, and generally a more reliable service with better calibrated and maintained equipment.

Advanced mud gas data interpretation techniques can be employed in all wells with active gas monitoring systems with any and all vendors, so long as Gas Chromatography C1-C5 or ‘advanced gas’ is used. The big differentiator, however, is an in-depth interpretation by a subject matter expert.

This process involves delving deep into the available gas data, often scrutinising anomalies, trends, and relationships, rather than focusing on absolute values which are influenced by uncontrollable acquisition factors. The strength of this approach lies in its adaptability; it can be employed on newly acquired datasets or even legacy data from wells drilled decades ago, and employed in oil & gas wells, carbon capture, utilisation, and storage (#CCUS) wells, hydrogen or helium drilling, and deep geothermal wells.


Extracting Meaningful Results

Figure: Plotting mud gas data in Gravitas Edge.

The output from advanced gas interpretation is tailored for subsurface teams, focusing on geological attributes like porosity, permeability, fault analysis and seal efficiency, rather than purely chemical aspects alone. Interpretation is software-aided; tools like Gravitas Edge streamline data processing and visualise results on logs and plots. However, the real value lies in the expertise of geologists and geochemists who recognise and interpret patterns and trends before drawing meaningful conclusions related to formation fluid characteristics and reservoir geology.

Mud gas data is a qualitative rather than quantitative method. This is mainly due to the difficult balance of completely degassing the drilling fluid while also maintaining a continuous, high resolution real-time data feed. Hence rather than focusing on specific ppm concentrations of gases, the analysis looks at overall trends and ratios between gas species. These can be industry standard ratios like wetness, balance and Pixler, they can be custom ratios that work well for a particular data set or field, or they can ‘calibrate’ the industry standard ratios to the well/field to make the results easier to interpret for non-experts. If mud gas data is combined with Pressure-Volume-Temperature (PVT) samples, the gas data can be calibrated against the PVT samples to allow for a better quantitative approximation of results.

Data sets are normalized for drilling parameters to erase their effect from the data. This makes it easier to compare data from different depth intervals or from well to well. Lack of normalization could lead to false interpretation of gas shows. For example, assuming the same rock type, a larger gas show will be seen when drilling at high ROP versus low ROP. Normalizing the data eliminates this effect, with the same principle applying to different hole sizes, and the inverse being true for flow rate changes.

Figure: Cross plot of C1 versus C2 mud gas concentrations. The two reservoir compartments have similar but distinct trends. Data from an overburden limestone and secondary target have been plotted for comparison

A recent case study illustrates how the above techniques are applied. Fluid fingerprinting for an appraisal well in continental Europe used C1-C5 data to reveal a small difference in chemical signature for the top versus bottom half of the reservoir. The difference could not be explained by density stratification of hydrocarbons. Moreover, pump off gas peaks were only seen in the lower half of the reservoir while the reservoir section was drilled with the same mud weight. These observations led to the conclusion that the reservoir is split into two compartments with no fluid or pressure communication between them. The higher pressure in the lower compartment explains the different fluid fingerprint. In a broader geological perspective, the team concluded that a sealing fault with minimal offset split the reservoir in two. The fault had not been identified in seismic data.

Figure: Advanced Gas data interpretation log. Changes in total gas and gas ratios indicate the depths of the compartments and fluid contacts.

The Philosophers Stone Test

By applying the Philosophers Stone principals, any investment should result in turning something ‘lesser’ into something of greater value. Depending on the specific well, geology or reservoir we’re dealing with, the benefits of better understanding the mud gas data, and hence the geology and reservoir characteristics include:

  • Formation fluid: Mud gas chemistry gives an indication of the fluid present; dry gas, wet gas, light oil, residual oil, biodegraded hydrocarbons, etc. As well as the depth of original or moved gas/oil/water contacts and relative water cut associated with the hydrocarbons.
  • Geological context: Provides greater understanding for reservoir and geological interpretation, e.g. sealing vs. open faults.
  • Reservoir structures: Identify seals, baffles, compartments, and relative porosity and permeability variations.
  • Pore pressure: Pump off gas can help confirm correct mud weights, permeability of formation and/or give a more detailed insight into the formation fluid composition.
  • Geochemical steering: Real-time gas data can be used as a geochemical steering aid by tracking the sweet spot fingerprint and using proximity to pay indicators.
  • 100% section coverage: Where wireline tools require a logging sump and LWD tools are behind the bit, mud gas data always extends all the way to TD, providing full data coverage.
  • Wireline planning: Use real-time gas data to optimise wireline operations e.g. pick PVT sample depths.
  • Detect bit burn: Ethene and propene may form due to friction and high temperatures at the drill bit. Their readings can be used to optimise ROP, avoid bit damage and/or incorporate timely bit trips. Gas data can also be used to identify successful milling and side-tracking operations.
  • Gas data QC: Fingerprinting data quality markers such as offsets in the set up or calibration of the Gas Chromatographs and the Total Analysers, optimise bad set ups and improve future mud logging requirements.
Figure: Advanced Gas data interpretation log. The depth of the gas-oil contact (GOC) and moved oil-water contact (OWC) are deduced from the gas composition and gas ratio changes.

Applying the Philosophers Stone test and advanced gas data analysis to established reservoirs can have just as much benefit as new or exploration wells. One such example was a pilot well drilled in the central North Sea in a field that had been in production for decades. The gas data revealed the depth of the pre-production oil-water contact (original OWC) as well as the moved oil-water contact at a noticeably shallower depth. Although oil had been clearly drained from this part of the field over the years, the moved Oil-Water contact was still 8m lower than the models had predicted. This result gave the operator confidence to proceed with the riskier side-track option.

Figure: Spider plots of the gas cap on the left and oil leg on the right. The gas cap demonstrates density stratification; the largest web is from the top of the reservoir, subsequently webs gradually decrease in size to the smallest one just above the gas-oil contact. Webs from the oil leg are very uniform in shape due to the homogenous composition of the fluid.

When to employ?

While advanced gas interpretation is suitable for almost any well, there are some situations where the benefits or business cases stand out as a differentiator:

  • Inclined, ERD, or horizontal wells where wireline conveyance is limited.
  • Ultra HPHT wells where MLWD tools are not rated for the downhole environment.
  • Deteriorating hole conditions prohibiting the use of expensive logging strings.
  • Remote wells where logistics are restricted; the mud logger oversees the gas chromatograph while the data interpretation is an office job.
  • Wells with long tripping times where complex BHAs may have a significant impact in the event of tool failures. This justifies simplistic BHAs aided by advanced gas analysis and interpretation.

Speaking specifically about HPHT wells, a prime example of the advantage of detailed gas data interpretation under difficult well conditions is a deep HPHT well in Middle East. The well was in an established field and the operator did not expect any particular problems, hence they opted for a slimmed down, minimalist BHA design to reduce the risk of damaging tools. As mud gas analysis is carried out at surface, this was one of the few (near) real-time data sets available to them. In the formation that separated the two main pay zones, large connection (pump off) gas peaks were noticed. Pump off gas can be used to gather a lot of information, in this case showing that drilling was near balance, giving a good estimate of formation pressure. The large gas peaks also gave a more detailed view of the composition of the formation fluid as several heavier hydrocarbon species were only visible above the detection limit during these events. The flow of pump off gas also indicated that the formation had better permeability than anticipated. Thanks to the results of advanced mud gas data analysis, additional wireline testing was done over this particular depth interval.


Practical Applications Across All Well Phases

Advanced gas data interpretation provides value across the entire lifecycle of a well, from pre-well onwards, to real-time decision making and post-well data analysis and review:

  • Legacy data: Review historic data sets; was anything missed? Was there a separate reservoir or did the gas come from somewhere else, i.e. a fault? Review field for development. Forward modelling of expected gas results aids in planning and optimising system setups.
  • Real-time data: Enhances decision making during drilling by aiding well engineering and operations. Identifies gas breakout which may affect mud weights, fluid contact depths and/or bit burn gases. Detects gas system issues and suggest improvements. Optimises the wireline program.
  • Post well analysis: In-depth post well geochemical and geological analysis, field development work, model optimisations, production strategies, etc.
  • Future well advice: Advice on best analytical methods to use for the well and budget. Breaking into new markets such as hydrogen or helium drilling.

Bridging Gaps in Data

Advanced gas data interpretation addresses challenges posed by incomplete or poor-quality datasets and often requires some detective work to understand the root cause of the problem. Not all data sets are equal, and depending on how high gas data collection was on the mud logger’s agenda, there are typically fluctuations in quality.

Mud gas data is not quantitative. This is mainly due to the degasser that ‘outgasses’ the drilling mud. Methane is very easy to degas but as hydrocarbon species get heavier it is progressively more difficult to separate them from the mud. The oil species in particular are difficult to outgas. This is why most of the data interpretation is based on C1-C5 and why advanced gas analysis doesn’t always add a lot of value. For example, cyclic hydrocarbons are difficult to degas from oil based mud while aromatic hydrocarbons are nearly impossible to degas from water based mud.

While the technology of standard mud logging gas chromatograph (GC) has not significantly changed over the years (a future #TT post?), more advances have been made on the degasser front. Constant volume degassers (CVDs) have become standard, providing a better gas level baseline by always outgassing the same volume of mud. A step up, which is not commonly used, is the heated constant volume and temperature gas trap, which improves the baseline even further, allowing for easier comparative analysis.

The benefit of the C1-C5 GC is that it is included in the standard mud logging package; there is no additional cost or increased rig footprint needed to provide the data. In contrast, most ‘advanced gas analysis’ services require rig space, along with longer rig up times, additional personnel to operate and maintain equipment, and operational costs to run the service. However, advanced gas data interpretation is capable of distilling in-depth and meaningful information from ‘basic’ C1-C5 results, rivalling with the interpretations that can be made from data sets including species above and beyond C5. In general, the same conclusions can be drawn from ‘basic’ and ‘advanced’ data sets, but the difference is that conclusions are drawn more confidently when additional gas species are analysed.

In a world of ultra deep resistivity reservoir mapping, wireline rotary side wall coring or MPD dynamic pore pressure tests, gas data is often an underutilised resource. With all eyes on lithology logs, paleo stratigraphy, LWD and wireline data, one forgets that the humble GC can still punch above its weight with the quality and quantity of unlockable data.

Gas analysis and data interpretation are techniques to complement, not replace, LWD and wireline. Gas data is a single piece of the puzzle that needs to be added to the other subsurface data sets. Fitting all observations together will build a strong case and gives the subsurface team confidence in their final interpretation and models.


Figure: Geology is complex to understand and visualise, using gas data can help understand the geological structures which might be missed with other data sources such as sample cuttings, LWD or wireline.

Data Inputs & Deliverables

The service uses Gravitas Edge software platform to collate, sort and display data sets. The results are then used for manipulation and interpretation. Turnaround times depend on the individual well, client and data objectives. Projects can range from ‘quick look’ interpretations to full reservoir or field studies. It is important to note that the software is only a tool; it’s the experience and knowledge behind the interpretation that is the real Alchemist’s Stone.

The interpretation and expert analysis are provided by a PhD level Geochemist. The deliverables are designed to be used by the end user, the subsurface team.  Value is demonstrated by actionable information about the subsurface environment. Interpretations are grounded in reservoir scale geology, small scale rock properties and geochemistry.

Cost Efficiency

Investing in advanced gas data interpretation offers significant returns. It enables better geological understanding, identifies cost-saving opportunities, and enhances reservoir evaluation. By optimising use of existing data streams, companies can make faster decisions on dry hole or success case wireline logging requirements, make informed decisions on mud weight adjustments, and reduce unnecessary expenditures such as calling TD needlessly late.

The value of this approach is evident in real-world examples. For instance, advanced gas data analysis has helped confirm reservoir compartmentalisation and depth of fluid contacts, it has recognised worn drill bits from bit burn gas, provided information on fluid migration pathways from dry well data, assisted in geochemically steering horizontal wells, and so much more.

Generally, it is assumed that the most interesting data is at and below the reservoir seal with the data from the overburden paid the same level of attention. However, mud gas can help with shallow gas mitigation, faults identification, and even recognise secondary pay zones not previously identified or explored. Data sets can be interpreted as a whole, with no extra time needed to plot overburden data in Gravitas Edge, giving a holistic view of the well.


I Gas, Therefore I am Reservoir?

With ancient philosophers, the quest for knowledge was constant and to broaden their understanding of the world was a key driving force behind their work. Today we are no different in the quest for a better understanding of the world around us, and underneath us. While the existence of the Philosophers Stone ultimately remains unproven and its abilities unrealised, the philosophy behind it endure as a symbol of humanity’s quest for knowledge and our potential to find value in unexpected places.

This concept resonates strongly with mud logging, where gas bubbling out of drilling fluid is used to construct an image of the reservoir kilometres below. By metaphorically applying the Philosopher’s Stone, we can uncover hidden potential and create significant value from what might otherwise be overlooked: that the gas signature of a well provides critical insights into geochemistry and reservoir properties.

Why not dig out the data sets you already have and unlock the extra value hidden in historic, and future wells. As a cost efficient, often underutilised data set, gas data interpretation may unlock a yet unrecognised secondary pay zone, improve understanding of reservoir geology, optimise your production strategy, or help with your reservoir volume calculator. Now that really is turning something lesser into something more.

#C1toC5 #SME #Gas #MudLogging #GeoChemistry #Petrophysics #LWD #Wireline

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