posted by Rod McCoremick, Blog author, Fordia Powered by Epiroc
November 16, 2021
For optimal drilling performance, you need a drill bit that is designed to handle a variety of variables that are particular to your drilling project. Choosing the right core bit is one of the most important decisions you will make, and the type of ground is an important consideration. The hardness of the ground can be determined by performing a scratch test with an etcher kit, but this blog will explore the other factors that affect the drillability of rock, such as competency.
Rocks are classified based on chemistry and structure. The hardness is classified on a relative scale referred to as Mohs hardness scale. To choose the right matrix for the hardness of the rock, you refer to a matrix selection chart that gives you options for each category of hardness. Adding to the complexity is the fact that a specific rock type can change drastically within the same borehole. This known as variability and manufacturers have designed core bits that are excellent at dealing with ground variability, such as the Viking line.
In addition to hardness and variability, you will also need to consider other variables that affect how drillable your rock will be. Other factors that are most important are grain size, weathering and fracturing. The strength of the rock is reduced when there is weathering.
So how do you know where your rock lies when it comes to abrasiveness and competency? Let’s look at some examples of rock and suggested core bits.
Very abrasive badly fractured rock
Common types of badly fractured and very abrasive ground are sandstone, shale, limestone and conglomerate, especially with a high silica content.
Moderately fractured and/or abrasive rock
Weathered granite and gneiss, dolomite, tuff and schist are all common types of moderately fractured rock, with or without abrasiveness.
Moderately abrasive rock
Common examples of moderately abrasive rock include basalt, gabbro, peridotite, diorite and gneiss
Competent, moderately abrasive rock
Rock formations that are competent and moderately abrasive are metabasalt, amphibolite, diorite, granite and diabase.
Hard, competent and slightly abrasive rock
Granite and pegmatite are examples of hard, competent rock with slight abrasiveness.
Very hard, competent and slightly abrasive rock
Metamorphozed granite and quartz rich gneiss are examples of rock that is very hard and competent, with slight abrasiveness.
Extremely hard, very competent rock
Chert, jasperite, quartzite and metamorphosed volcanics are all common rock formations that are extremely hard and very competent.