The gel strength is the shear stress of drilling mud that is measured at low shear rate after the drilling mud is static for a certain period of time. The gel strength is one of the important drilling fluid properties because it demonstrates the ability of the drilling mud to suspend drill solid and weighting material when circulation is ceased.
How can we get the get strength?
We use the 3-rpm reading which will be recorded after stirring the drilling fluid at 600 rpm from a rheometer. Normally, the first reading is noted after the mud is in a static condition for 10 second. The second reading and the third reading will be 10 minuets and 30 minutes, respectively. You may wonder why we need to record the 3-rpm reading after 30 minutes.
The reason is that the 30 minute-reading will tell us whether the mud will greatly form the gel during an extensive static period or not. If the mud has the high gel strength, it will create high pump pressure in order to break circulation after the mud is static for long time. Furthermore, increasing in a trend of 30-minute gel strength indicates a build up of ultra fine solid. Therefore, the mud must be treated by adding chemicals or diluting with fresh base fluid.
The following causes will result in the high gel strength in the water base mud.
• Drill solid
• Chemical contamination as lime, gypsum, cement, and anhydrite
• Acid gases as Carbon Dioxide (CO2), and Hydrogen Sulphide (H2S)
For an oil base drilling fluid, there are several points that will cause the high gel strength in the mud system as follows.
• Over treatment with organic gelling material
• Build up of fine solid particles in the mud
Reference Books: Drilling Fluids Book