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# Reservoir Geomechanics Tips

The following tips and suggestions are product of more than twenty years experience in the oil and gas industry. These tips provide proved specific advices to achieve a consistent geomechanics model.

You may also want to check our general reservoir modeling tips index as well. These tips are biased to stress unique capabilities and features of GeolOil software.

You are welcome to provide your feedback, comments or contributions, subject to editorial review. Just refer the tip number and fill the contact form.

 `If you have to estimate the stress tensor, make sure you use tensorial math` No matter how expensive or sophisticated seems to be a reservoir modeling software, if you work on a geomechanics project and need to estimate stress horizontal directions you should not use a package that only handles regular, standard scalar geostatistics algebra. GeolOil geomechanics module properly handles the estimation of the stress horizontal direction field, by using tensorial algebra. You can not simply use neither scalar algebra nor even vectorial algebra to conclude that an average stress direction of an angle of 0 degrees and an angle of 90 degrees lies on a 45 degrees angle!. That is wrong for a gemechanics tensor. `Filter rock mechanics log properties per lithology or facies` If you have derived rock mechanics properties from log correlations, then you have the opportunity to easily analyze the depth column data. However, avoid the temptation to reach global conclusions on the whole formation column. Instead filter your log curve by lithology and facies, and compute rock properties summaries per litho-facies. The GeolOil petrophysical summaries module allows you to process those filters. `Do not heavily rely on rock mechanics extremes` Rock mechanics properties are inherently unstable at extremes. You may take a plug to the laboratory, measure its UCS (Unconfined Compressive Strength), record a value, come back the next day to test a similar neighbor plug (UCS tests are destructive and non-repeatable), and record a completely different value, perhaps with a difference or 50% or more. Minor internal invisible cracks could be the culprit for this behaviour. Even dynamic rock mechanics property extremes derived from logs could be unstable, noisy or erratic. Why do you have to rely on unstable rock mechanics maximum or minimum property extremes? Instead, compute quantile distribution values. They are far more stable (lower statistical standard deviation), capture the behaviour you want, and are their computation is not based on just one single possible unreliable value. Consider for instance to compute left and right tail quantiles of 0.05 to 0.10 (left and right percentiles of 5% to 10%). The GeolOil petrophysical summaries module allows you to compute quantiles and cutoffs values. `Only have old sonic compressional log curve? Explore it` If you don't have modern, full shear wave sonic log curves to derive dynamic rock mechanics properties, you may still try to define rock index trends. If a more compacted rock is associated with a shorter sonic travel time, you may compute a pseudo net-to-gross stiffness rock index. Define a pseudo "net" stiffness thickness as the sum of rock column interval depths dz in the formation with sonic transit travel time lower than a cutoff value (for instance 10% left tail percentile distribution). Then divide this net thickness by the gross formation or zone thickness. A ratio close to 1.0 could have a good correlation with real stiffness, and a ratio closer to 0.0 might be related to weaker rocks.
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