Why it pays to log cement


Published 28 January 2020

A previous post discussed the geeky reasons why we should log cement: some sort of moral imperative that urges us to properly understand and safely manage our wells.



Matteo Loizzo
Well Integrity Consultant, Germany


In fact, evaluating wells makes great economic sense: a single analysis, at a cost of $25K, typically brings in savings exceeding 10 times the investment. And of course you are now sure that the risk of a catastrophic leak, and the ensuing huge losses and reputational damage, is properly managed.

Before we look in detail at why the returns on investment are so high, by taking as an example the evaluation of a well, a couple of disclaimers:

  • I will be focusing on a particular flavor of evaluation, called Advanced Integrity management (AIm). AIm technologies harness the power of innovative models and structured methods to help take efficient and effective well integrity decisions throughout a well's life cycle (see picture below). It may not come as a surprise that I consider AIm as the best tool around, after all it's not called "advanced" for nothing, but it will be easy to spot general concepts and read "quantitative well analysis" whenever I write AIm.


  • We started talking about logs, and now the subject has been subtly changed to evaluation. Fishy? Not really: logs are data, soulless and dispassionate measurements. Information needs to be distilled from data to shape understanding, and ultimately drive decisions and actions. If we spend $30-50K on a log, and then we don't squeeze information out of it, then it's pretty much waste, even if "regulators made me do it". But more about it later on.

How best to manage sustained annular pressure?

First off, our scenario: we have sustained annular pressure and need to decide how to deal with it. As the decision tree at the top of this post shows, the choices before us are deceptively simple:

  • Repair the failed barrier.


  • Manage the leak; either by shutting it in, venting it, or by letting pressure build up to a set point before bleeding it off again.

In theory we should pick the best decision after assessing the risks and performing a cost/benefit analysis. In reality, we should add two dimensions to our decision-making:

  • First, regulators, international standards or corporate rules may impose a particular choice, such as attempting to repair the well, or even abandon it. We should also remember that many rules do not require sustained pressure to come into force, since leaks can leave the well envelope without being detected: “bad” logs are often enough to trigger integrity requirements.


  • Second, not all decisions concern the leaky well: lessons learned are essential to eliminate failures (effectiveness) and waste (efficiency) in future wells.

Evaluating the risk, and thus estimating the benefit of each management strategy, depends on us knowing what can go wrong:

  • The leak can be serious enough that it can cause damage on its own if wellhead barriers fail. A rough order of magnitude for this alarm threshold is 10,000 tons per year: the only way that a cemented annulus can let so much fluid through nowadays, is if we experienced gas migration while cement sets


  • The leak is small enough, 100 tons per year or less, not to be of major concern. Still, improper management can lead to two catastrophic scenarios:
    1. Leaks are small because they must pass through hairline gaps at the interface between cement and casing, or rock; if fluids corrode the steel wall and flow through the casing, even jumping back out into the annulus through a second corrosion hole, then the leak rate is not mitigated any more. This has led to multi-million dollar accidents, and potentially fatalities.
    2.  Even a small gas leak can accumulate underground, in a collector zone, and then break through the thin rock crust with resulting fires and explosions. This scenario has actually happened more then once in the gas storage industry.
  • Even if we choose to fix the leak, we still have residual risks:
    1. Repairs have an uncertain (and not very high) success rate, even if we attempt the operation a few times. We may end up losing a few $100K and still have to live with the leak.
    2. Unless we stop the leak at the source, we may still have unwanted flow: we just don’t see it any more at the wellhead. If that’s the case, we lose the ability to manage the leak, and may still suffer from the collector zone scenario.

So, why does it pay to use AIm? We stand to save substantial money – many times the cost of an Aim study – through gains on three fronts, as the figure below depicts:

  • Shorter, effective decision-making.
  • Waste elimination through leak management and improvements in well construction.
  • Better risk management.

Shorter, effective decision-making

When diagnosing a leak, you want to bring advanced technology to the table, especially when interpreting logs and modeling the response of the system. And you want a proven method to provide reliable answers.

Instead, excellent engineers that know a lot about drilling or production, but little about integrity failure or cement-evaluation logs, spend an inordinate amount of time trying to figure out what’s wrong and what to do. Logging and cement companies are almost always brought in for advise, though this is often wasted time: their business model is selling measurements and chemical products, not solving problems; and the time they spend in investigations ends up being paid by operating companies one way or another.

Part of the problem is that the relationship between leaks and logs is not simple, as the figure below sketches.


The direct costs of such an investigation easily exceeds $20K, and may entail new data acquisition and a substantial amount of testing that can get the amount closer to the $100K mark. Often, though, this pails in comparison to the opportunity cost of not knowing what to do for a long time, which may delay production, make repairs more expensive or put unnecessary constraints on the most effective management strategy.

Using AIm, the analysis time is short, its cost is known, and valuable drilling and production resources are not distracted from doing their job.

Waste elimination

Whatever the time and cost taken to reach a decision, we expect it to be lean, i.e. without the waste of useless expenses. As we mentioned, there are two reasons we may not get rid of waste:

  • The choice we took to manage the leak is not optimal, regardless of its risk implications: we may measure too much, implement controls that are too complex (and unreliable), or spend $100K or more on ultimately unsuccessful repairs.
  • We don’t learn the right lessons for the next wells. This may take the form of the same sustained pressure problem happening again, and again. Or adding expenses that do little to cure the issue: cementing accounts for around 10% of the cost of a well, so there is scope to increase costs by adopting expensive but inefficient solutions. Cement logging is cheap in comparison, of the order of $30K per string, but a pernicious form of waste could be lurking: if we want to save too much and end up not collecting enough data to take the right decisions, then whatever we end up spending is a waste, since we don’t benefit from it. The same goes of course for logs from which we don't even attempt to extract information.

Using AIm, operators have realized actual, direct savings, of multiple times $100K, compared to a fixed cost of $25K; that’s more than a factor 10, without counting the future benefits as we cumulate the savings on many wells. Here are some examples:

  • Regulators were convinced to adopt risk-based integrity logging rules, instead of a fixed, expensive calendar.
  • Controlled venting was chosen over very costly repair operations.
  • A logging campaign that had taken up to a week was replaced by a single-day, safer program, at the same time improving the quality of actionable data acquired.

Better risk management

In the end, well integrity is also about preventing catastrophic accidents, however infrequent we may hope them to be. We shouldn’t forget that besides the losses and human suffering that these events cause (be they a major gas release, a polluted aquifer, ground movement or a long list of unfortunate scenarios that have actually happened), the consequences on reputation and the social license to operate can be significant, easily exceeding $100 million.

AIm can help manage risk better in many ways:

  • The inclusion of advanced risk management tools in the diagnostic method means we handle probability and uncertainty correctly, right from the start, and in a quantitative way.
  • We also consider all applicable scenarios in a complete and exhaustive way, without weighing down the analysis with improbable clutter.
  • Advanced interpretation of logs and measurements means that uncertainty is reduced to a minimum.
  • Quantitative failure models allow us to specifically assess a well and its conditions, including geological barriers. The alternative is using generic, vague (and ultimately unhelpful) qualitative indicators: one size fits all usually doesn’t fit anyone.

Ultimately, risk is not distributed fairly or uniformly: a few fields, and wells, account for the majority of leaks. So whereas the global risk of a catastrophic outcome may be tolerable, it can become unacceptable in specific areas. AIm can assist in prioritizing and allocating resources where they are really needed to control risks. Again: eliminating waste, but this time from risk management.


Using AIm whenever you find yourself confronted with Sustained Casing Pressure brings returns easily exceeding 10 times the investment. Indeed, even if you want to know all that logs can tell you (a cheaper exercise, at around $6K), AIm will pay for itself many times over.

Mind that we haven't embarked on complex Net Present Value calculations, with a lengthy debate on the appropriate discount rate: AIm pays back now.

Please don't hesitate to start a discussion, or share your questions and opinions. And don't hold back if you think my reasoning came straight out of a bovine: I'm a Roman exiled in Berlin, so I can take sarcasm with a smile. And I eat trolls for breakfast.