Drilling Cuttings (Why it may be difficult to examine)
Samples are examined under a microscope for:
- oil staining
The objective is to depict changes of lithology and appearance of new formations. The microscope and ultraviolet light are used as complementary tools in reconstructing the characteristics of the originating strata. An estimate of the percentages of lithology, staining, and porosity are made with great care since factors such as grain shape and size, color, distribution, etc. can affect the relative percentages Figure.1.
Sources Of Drilling Cuttings Contamination
There are many potential sources of contamination when undertaking estimates of lithology percentages. Some examples are:
Cavings can generally be recognized as large, splintery rock fragments that are often concave or convex in cross-section. They are lithologically identical with formations from higher sections in the open hole. If found in large quantities, it may indicate a serious underbalanced drilling mud condition or a situation where pipe rotation is too fast and Stabilizers are catching on the side of the hole.
2) Recycled cuttings
3) Mud chemicals
Causes Of Unrepresentative Cutting Samples & Good Logging Practices
In some cases, samples may be totally unrepresentative of the formation being drilled. For example, in evaporite sections drilled with a Water Based Mud, salts will dissolve and there will be no lithological indication of their presence in the lagged samples. However, evaporites can still be recognized using good logging practices:
- Evaporites generally have consistent drill rates
- Gas values through evaporites will be very low
- There will be poor or no returns at the shale shakers
- Limestones and dolomites are frequently found in association with evaporite deposits
- Anhydrite sections can usually be identified by BaCl solution which produces BaSO4 precipitate
- The chloride content of the drilling fluid will increase significantly
Other causes of unrepresentative samples are:
Cuttings may be absent when drilling soft clays/shales. These will tend to disperse/dissolve in the drilling fluid making a “native mud”. This can be recognized by the change in the drilling fluid’s properties, most commonly viscosity and mud density.
High rotary speed drilling (turbines and PDM’s) will often produce very fine “rock flour”. The same happens when drilling with diamond bits.
When drilling with diamond bits, the heat generated at the bottom of the hole can “burn” the cuttings to such a degree that they will have the appearance of metamorphosed rocks.