Cuttings: Accurate sampling and sampling resolution

With the increased momentum and attention of lateral well cuttings generated data, I thought it would beneficial to share a snippet of CGG’s experience from over 200 lateral wells within onshore USA. As a company we have been working with cuttings samples for more than 50 years and know the true power they hold. Over the last 10 years we have been successfully implementing proprietary inorganic geochemistry tools and developed our own commercial cuttings analysis tool, RoqScan? (SEM-EDS).

For now, I would like to start at the basic, but fundamental starting point with cuttings analysis: sample collection. Many times during internal and external conversations, this question always arises “how do you know those samples have come from the depths assigned to them?” Personally, from field experience, it is easy to see how an incorrect depth is assigned to a cuttings bag. A single mud logger working long shifts, a sub-set of samples being collected from a bucket tied to the end of the shaker every hour or, drilling issues leading to mud pumps being switched off. These are just some of the potential issues we have witnessed.

Some of these events are out of our control but we do try to mitigate against potential depth assignment errors as much as we can. Whilst at wellsite, CGG personnel collect, clean, prepare and analyze the cuttings samples to minimize handover errors. When working at a well with operational issues and/or processing legacy cuttings, where depth control may be an issue, we rely on our consistent sample preparation technique, removing drilling artefacts, and our pseudo GR produced by RoqScan.

Below is an example of a lateral well drilled through the Upper Wolfcamp Formation, Permian Basin, Texas. Here we present the LWD GR curve (shaded) plotted against the measured RoqScan GR (red) from cuttings samples. You will note that though the trends of the two curves match very well, they are slightly offset from one another. This may be due to a miscalculation of the lag depth at wellsite or other issues as indicated in the above paragraph. To resolve this offset, we utilize the RoqScan derived GR to match those responses given from the LWD GR. From experience, this process can involve multiple depth shifts along a single lateral well. The blue curve represents a shifted (-30 ft.) version of the RoqScan GR, which has an improved match.  

Today we use the mineralogical and textural cuttings data, produced by RoqScan, to aid in the design of the completion. Ensuring our data is at the correct measured depth is critical, especially when stage boundaries and perforation placements may be located, utilizing these data.

30 ft. vs. 10 ft. sampling - How fit are the sample catchers?

Another question that typically pops up is “what sample resolution should we be collecting the samples at?” From experience, in lateral wells, 30 ft. sample catching is adequate as this provides enough lateral resolution, keeps the cost down and keeps the sample catcher happy. Even within a highly heterogeneous Wolfcamp lateral well, 30 ft. sampling has provided more than enough resolution to add value to the completion design strategy. The image below presents such a case, with the same lateral well displaying both 10 ft. and 30 ft. RoqScan data. Yes, the 10 ft. sampling provides improved resolution but the 30 ft. sampling still delivers bulk mineralogical volume turnovers that can be utilized in the completion design (stage placement). We have also included some fictitious geometric stages that would demonstrate where 100 ft, 200 ft. and 500 ft. stages would be placed along the lateral.

If you decide to utilize cuttings data for optimized completions, it is important that the correct QC procedures are applied to ensure correct depth assignment of each cuttings sample. The application of 30 ft. sampling is also sufficient to characterize bulk lithological variations throughout a lateral well.

Geology to production: Look how we are integrating cuttings with completion optimization https://www.cgg.com/technicalDocuments/cggv_0000029312.pdf?

For further information of our cuttings services, please visit https://www.cgg.com/cuttings