Viscosity – What can happen if we get it wrong
Viscosity is one of the key properties of a drilling fluid and the one that is the most easily controlled. On many occasions, drilling crews can misinterpret viscosity as they try to measure it visually or by running their hand through it instead of using a marsh cup and funnel.
The problem with this is that different products can give the fluids a different ‘feel’ and therefore look more viscous than it actually is. PHPA’s like Tiger Core or liquid polymers will give a ‘stringy’ feel to the fluid due to them being a long chain polymers compared to a fluid with only bentonite or with a PAC and is often seen as having a higher viscosity. However, when both fluids are measured with a cup and funnel, the viscosity is the same.
Higher viscosities are needed to clean the hole especially where the cuttings are heavier and the hole is deeper. Inadequate viscosity can lead to cuttings falling back down the annulus (space between the hole and the rods) and eventually leading to rods becoming bogged and stuck.
The viscosity required to clean a hole can vary depending on type of hole, size of hole and size of cuttings. Larger holes like waterwell holes at 12 to 26 inches will require significantly more viscosity than a diamond drilling hole at PQ or HQ. The volume and size of the cuttings will be larger and therefore a higher viscosity needed to carry it out to clean the hole.
Problems can occur when viscosities are too high as well.
When using viscosities that are high over 50 secs, it will increase the pressures required by pumps to move fluid through the system. It will also make it more difficult for cuttings to settle out in the pits and will eventually cause an increase in mud weight. If the mud weight gets too high, it leads to increased formation pressure down the hole which subsequently can cause formation fractures.
The higher viscosity will also increase chances of swabbing/surging down the hole which can cause the hole to collapse.
Below is a basic rule of thumb guide for viscosities for diamond drilling which can be applied to many situations but these can vary with different conditions.
Competent ground – PQ – 34 to 36 secs, HQ – 32 to 34 secs, NQ – 30 to 32 secs
Broken ground – PQ – 38 to 40 secs, HQ – 34 to 38 secs, NQ – 30 to 34 secs
For any advice on viscosities or any other mud related queries, contact us at firstname.lastname@example.org