HQ vs NQ vs PQ Diamond Drilling: Which Core Size Should You Use?

PQ gives you more core. NQ gives you a smaller hole. HQ sits somewhere in the middle.

Excellent.

We've successfully reduced decades of diamond drilling experience to one useless paragraph.

Because choosing between PQ, HQ and NQ diamond drilling isn't really about deciding which core size is “best”.

It's about what happens when your geology, target depth, sample requirements, ground conditions, rig capability and budget all start arguing with each other.

And occasionally the correct answer is:

Start big. Finish small.

Here's why.

HQ vs NQ vs PQ: What's the Difference?

PQ, HQ and NQ are commonly used wireline diamond drilling sizes.

The most obvious difference is diameter.

Using standard Q-series dimensions as a general reference:

Size	Approx. Hole Diameter	Approx. Core Diameter
NQ	75.7 mm	47.6 mm
HQ	96.0 mm	63.5 mm
PQ	122.6 mm	85.0 mm

Exact dimensions depend on the specific tooling system being used, so always confirm against the manufacturer's specifications.

But the basic relationship is simple:

PQ = biggest.HQ = middle.NQ = smallest of the three.

And bigger isn't automatically better.

What Is NQ Diamond Drilling?

NQ is one of the workhorses of mineral exploration.

It produces core around 47.6 mm in diameter while requiring a substantially smaller hole than HQ or PQ.

That creates several advantages.

A smaller hole generally means less rock needs to be removed.

It also means smaller:

• rods;

• barrels;

• bits;

• reamers;

• casing;

• core.

For deeper exploration drilling, those differences start becoming important.

Every rod has weight.

Every litre of drilling fluid has to circulate.

Every trip has to move the drill string.

Every bit has to remove rock.

NQ therefore provides a useful balance between sample size, drilling efficiency and depth capability.

That's one reason you'll see a lot of it.

A side-by-side comparison of NQ, HQ and PQ diamond drilling equipment and corresponding core diameters. The image demonstrates the difference in size between three of the most commonly used wireline diamond drilling systems in mineral exploration.

What Is HQ Diamond Drilling?

HQ produces considerably larger core—around 63.5 mm in standard Q-series tooling.

That extra diameter can be extremely useful.

You have more physical sample.

Geological features can be easier to observe.

There is more material available for laboratory work.

And larger-diameter tooling can sometimes play an important role when dealing with difficult upper-hole conditions.

But you're also drilling a larger hole.

That means more material to cut and remove.

The drill string is heavier.

The tooling is larger.

Consumables generally cost more.

Water and drilling-fluid requirements can increase.

The rig has more work to do.

More core isn't free.

What Is PQ Diamond Drilling?

PQ takes things another step up.

Standard PQ core is approximately 85 mm in diameter.

That is a serious piece of rock.

There are programmes where that additional sample volume is extremely valuable, including certain:

• geotechnical programmes;

• metallurgical programmes;

• bulk-sampling requirements;

• difficult ground applications;

• specialist geological investigations.

PQ can also provide useful flexibility at the top of a hole when the drilling strategy involves reducing diameter later.

But the trade-off becomes increasingly obvious.

PQ equipment is bigger.

Heavier.

More expensive.

And substantially more demanding on the drill rig.

If your geological objective can be achieved perfectly well with NQ, drilling the entire hole PQ simply because “bigger core looks nicer” may be an expensive way of filling core trays.

Why Would You Choose HQ Instead of NQ?

There are several reasons.

1. More sample

HQ gives the geologist substantially more rock to work with.

That can matter where mineralisation is heterogeneous or where additional sample material is required.

2. Better visibility

A larger core diameter can make geological and structural features easier to examine.

3. Difficult ground

Larger diameter drilling can sometimes form part of the strategy for stabilising problematic upper sections.

4. Future reduction

Starting with HQ provides the option of reducing to NQ deeper in the hole.

That last point is extremely important.

Because diamond drilling doesn't necessarily mean choosing one diameter and stubbornly using it from collar to end of hole.

Why Reduce From HQ to NQ?

Imagine the first 150 metres of your hole are horrible.

Weathered material.

Clay.

Fractures.

Broken formations.

Water loss.

The hole would quite happily collapse the moment everyone stops looking at it.

Below that?

Beautiful competent rock.

One possible drilling strategy is to establish and stabilise the upper hole at a larger diameter before reducing to NQ once conditions improve.

The larger string effectively preserves options.

Once you're successfully inside stable ground, the smaller NQ system can then continue deeper.

This is called telescoping the hole.

And it's one of the reasons starting diameter matters.

PQ → HQ → NQ: Building Yourself Options

Now extend that idea.

Starting PQ can potentially allow a drilling programme to reduce:

PQ → HQ → NQ

as depth and ground conditions dictate.

You have effectively created several opportunities to save the hole.

That's useful when you're drilling expensive deep exploration holes where abandoning the hole after hundreds of metres would be particularly painful.

But there's a catch.

You paid for that flexibility.

The upper section required larger tooling, more powerful equipment and more expensive consumables.

So the decision becomes a risk calculation.

How much is preserving the hole worth?

That's a much more interesting question than:

“Which core size is cheapest?”

Ground Conditions Should Influence Core Size

This is where field experience starts beating spreadsheet logic.

If the ground is competent and predictable, smaller-diameter drilling may perform beautifully.

But highly fractured, weathered, swelling or unconsolidated formations can completely change the strategy.

Problems may include:

• collapsing formations;

• squeezing ground;

• excessive water loss;

• poor core recovery;

• blocked barrels;

• high torque;

• difficult rod movement;

• repeated reaming.

Sometimes the smartest move isn't immediately reducing diameter.

Sometimes maintaining the larger hole until you've passed the problematic zone gives you more options later.

Because once you've reduced:

You can't magically make the hole bigger again at depth.

Well, not without considerably more work than anybody wanted that morning.

A diamond core drilling rig operating on a remote mineral exploration site with drill rods prepared alongside the rig. Rig capacity, rod diameter, ground conditions and target depth are important considerations when choosing between NQ, HQ and PQ drilling.

Core Recovery: Is Bigger Always Better?

Not necessarily.

Core recovery depends on far more than diameter.

It can be influenced by:

• geology;

• drilling technique;

• bit selection;

• barrel configuration;

• drilling-fluid programme;

• penetration rate;

• run length;

• ground disturbance;

• operator experience.

A beautifully drilled NQ hole can produce excellent core.

A badly managed HQ hole can produce expensive rubble.

Diameter is part of the equation.

It isn't the whole equation.

What About Triple-Tube Core Barrels?

When formations are particularly broken, fractured or sensitive, triple-tube core barrel systems can help improve recovery and preserve sample integrity.

These systems add another inner liner arrangement intended to reduce disturbance to the recovered core.

They're commonly considered in ground where maintaining core condition is particularly important.

Again, this illustrates why the question shouldn't simply be:

HQ or NQ?

It should be:

What combination of diameter, barrel, bit, fluids and drilling technique gives us the geological sample we actually need?

Core Diameter Affects the Laboratory Too

Here's something that doesn't always get discussed at the rig.

More core means more sample material.

That can matter when samples need to support:

• assay;

• density measurements;

• geotechnical testing;

• metallurgical testing;

• structural interpretation;

• additional verification work.

If the programme requires significant test material, larger-diameter core may justify its additional drilling cost.

Conversely, if you're drilling straightforward exploration holes where NQ provides perfectly adequate sample volume, the additional cost of larger core may not create much additional value.

Diamond drill core stored in geological core trays following recovery from an exploration drill hole. Core diameter affects the amount of geological sample recovered, with larger HQ and PQ core providing more sample material than NQ drilling.

Sample requirements should influence drilling design before the rig arrives.

Not after somebody discovers there isn't enough material for the test work.

How Does Core Size Affect Drilling Cost?

Generally:

Larger diameter = higher drilling cost.

Not universally.

But it's a sensible starting assumption.

Why?

Because larger diameter can mean:

• larger bits;

• larger reamers;

• larger core barrels;

• heavier rods;

• greater rig load;

• more energy required;

• potentially slower penetration;

• increased water requirements;

• increased consumable costs;

• increased handling requirements.

This links directly back to our previous CED article on diamond drilling cost per metre.

A quotation for NQ drilling and a quotation for PQ drilling shouldn't be expected to look identical.

You're not drilling the same hole.

Your Drill Rig Has an Opinion Too

This part occasionally gets forgotten.

Every diamond drill rig has limits.

Depth capacity isn't simply determined by whether the engine can turn the rods.

You need to consider things such as:

• pullback capacity;

• feed force;

• torque;

• rotation speed;

• chuck/head capability;

• mast capacity;

• rod handling;

• pump capability;

• overall rig stability.

A machine capable of drilling deep NQ isn't automatically capable of drilling the same depth in HQ.

And it certainly doesn't follow that it can do it in PQ.

The drill string becomes heavier as diameter increases.

So when somebody says:

“This rig drills 1,000 metres.”

Our next question is:

1,000 metres of what?

NQ?

HQ?

PQ?

Because that little detail matters enormously.

Water Pumps Matter Too

Larger holes require the drilling-fluid system to move sufficient volume through the hole.

The pump therefore needs to suit:

• drilling diameter;

• depth;

• formation;

• fluid programme;

• expected losses.

A diamond rig is only one component of the drilling package.

We've said the same thing about RC rigs and compressors.

We'll keep saying it because it keeps being true.

Drilling systems outperform collections of unrelated equipment.

What About Drill Rod Weight?

Let's take a simple concept.

A deeper hole means more rods.

Larger rods weigh more.

That matters every time the rig:

• rotates the string;

• feeds the string;

• pulls the string;

• trips for a bit change;

• recovers from a problem.

• 
  

At significant depth, rod-string weight becomes a major part of rig selection.

This is why the maximum advertised drilling depth on a rig specification should always be read alongside the core size.

“1,000 m capacity” without a diameter is marketing.

“1,000 m NQ under specified conditions” is engineering.

NQ vs HQ for Deep Diamond Drilling

If the objective is significant depth and the geology allows it, NQ can become attractive because the drill string and hole are smaller.

But deep holes also increase the cost of failure.

Lose a shallow hole?

Annoying.

Lose a hole after 800 metres?

Everyone suddenly becomes very interested in the drilling report.

That means deep-hole planning may deliberately preserve larger diameters in the upper section to provide contingency.

The drilling strategy becomes a balance between:

Efficiency now versus options later.

There isn't a universal answer.

When Would We Choose NQ?

NQ deserves serious consideration when:

• adequate sample volume can be achieved;

• the ground is reasonably drillable;

• significant depth is required;

• efficiency matters;

• rig capacity is a consideration;

• larger-diameter core isn't technically necessary.

For a lot of mineral exploration programmes, that's a compelling combination.

When Would We Choose HQ?

HQ becomes attractive where:

• larger core is valuable;

• additional sample volume is required;

• geology benefits from better visual exposure;

• upper-hole conditions are challenging;

• reduction to NQ may be planned later;

• the rig and budget comfortably support the larger diameter.

HQ is often a very useful compromise.

Which explains why you see rather a lot of it.

When Would We Choose PQ?

PQ may make sense when:

• maximum sample volume is important;

• specialist testing requires larger core;

• ground conditions justify the larger diameter;

• the hole design needs multiple reduction options;

• geotechnical or metallurgical requirements demand it.

But PQ shouldn't be selected because it sounds impressive.

The geology doesn't care how big your core box looks on LinkedIn.

The CED Approach: Design the Hole Backwards

When CED discusses a diamond drilling programme, we don't want to begin with:

“Which rig do you want?”

We want to know:

Target depth?

Expected geology?

Core diameter?

Hole angle?

Ground conditions?

Sample requirements?

Access?

Water?

Total programme metres?

Then we can work backwards.

That might lead to a compact man-portable or heli-portable diamond rig.

It might require a larger surface diamond drilling system.

It might require PQ through the upper section before reducing to HQ and eventually NQ.

And the equipment package can then be built around that strategy.

CED can supply:

• diamond drilling rigs;

• PQ, HQ and NQ drill rods;

• core barrels;

• inner tubes;

• bits;

• reamers;

• casing;

• casing shoes;

• drilling muds;

• pumps;

• spare parts;

• technical personnel;

• complete turnkey drilling packages.

Or we can operate the drilling programme ourselves.

That's the difference between selling somebody a machine and understanding what they're trying to do with it.

So: HQ, NQ or PQ?

Here's the simplest answer we can give.

Choose NQ when:

You need efficient exploration drilling and the sample size and ground conditions allow it.

Choose HQ when:

You want more sample, additional hole stability/options or intend to reduce to NQ later.

Choose PQ when:

You need substantial sample volume, specialist testing or maximum flexibility to telescope the hole.

But don't select the diameter in isolation.

Think about:

Geology. Depth. Core recovery. Sample requirements. Rig capability. Water. Consumables. Cost. And what happens if the hole starts misbehaving.

Because the best core size isn't the biggest.

And it isn't necessarily the cheapest.

It's the one that gets the geological information you need while keeping enough options available to finish the hole.

Over to the diamond drillers

When you're expecting genuinely horrible upper-hole ground, would you rather start larger and preserve the option to reduce—or attack it immediately with the smallest practical diameter?

There should be some strong opinions on that one.

Planning a diamond drilling programme?

Cylinder Exploration Drilling can supply the rig, PQ/HQ/NQ drill string, barrels, bits, casing, drilling fluids, pumps, spares and personnel—or manage the drilling programme as a complete turnkey operation.

Tell us the geology.

We'll start there.