Drilling mud gel strength

Gel strength is the shear stress of drilling mud which is measured at a low shear rate after the drilling mud remains static for a certain period of time. Gel strength is one of the important properties of drilling fluid because it demonstrates the ability of drilling mud to suspend drilling solids and weight material when circulation ceases.

How can we get the strength?

We use the 3 rpm reading that will be recorded after stirring the drilling fluid at 600 rpm with a rheometer. Normally, the first reading is noted after the mud is in a static condition for 10 seconds. The second reading and the third reading will be 10 minutes and 30 minutes, respectively. You may be wondering why we need to record the 3 rpm reading after 30 minutes.

The reason is that the 30 minute reading will tell us whether or not the mud will gel heavily over an extended static period. If the mud has high gel strength, it will create a high pump pressure to stop circulation after the mud is static for a long time. In addition, the increase in a gel strength trend at 30 minutes indicates an accumulation of ultrafine solids. Therefore, the mud must be treated by adding chemicals or by diluting it with new base fluid.

The following causes will result in high gel strength in water-based mud.

• Bacteria

• Solid drill

• Salt

• Chemical contamination such as lime, plaster, cement and anhydrite

• Acid gases such as carbon dioxide and hydrogen sulfide

For an oil-based drilling fluid, there are several points that will cause high gel strength in the mud system as listed below.

• Overtreatment with organic gelling material

• Accumulation of fine solid particles in the sludge

The operational impacts of gel strength are as follows:

Cutting suspension capacity – Low strength drilling mud will not be able to efficiently suspend cuttings; therefore, the cutoff will fall rapidly once the pumps are turned off. This can lead to various problems such as stuck pipe, hole plugging, and cutting bed build-up.

barite buckling – The problem of barite sinking occurred mainly due to the low force of the drilling fluid. Therefore, the weight of the mud in the well will not be constant. You will see that the lowest mud weight will be seen at shallow depth, but the heaviest mud weight will be seen in the deepest section of the hole. This situation could possibly lead to a well control incident due to insufficient mud weight to balance formation pressure in the shallow section of the well.

Break circulation pressure – If you have a highly progressive gel strength fluid, it will take a lot of pressure to break circulation. Once high pumping pressure is applied, it could cause cracks to form and result in a lost circulation problem.