The gel strength is the shear stress of drilling mud that is measured at a low shear rate after the drilling mud has been static for a certain period of time. The gel strength is one of the most 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 gel strength is measured?
Gel strength measurement is made on viscometer using the 3-rpm reading, which will be recorded after stirring the drilling fluid at 600 rpm to break gel. 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.
Why do 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 significantly form the gel during extensive static periods like tripping out BHA or not. If the mud has high gel strength, it will create a high pump pressure in order to break circulation after the mud is static for a long time. Furthermore, increasing in a trend of 30-minute gel strength indicates a buildup of an ultra-fine solid. Therefore, the mud must be treated by adding chemicals or diluting it with fresh base fluid.
Causes of Increasing in Gel Strength of Water Based Mud
The following causes will lead to high gel strength in the water based mud.
- Bacteria
- Ultra fine solid
- Salt
- Chemical contamination such as lime, gypsum, cement, and anhydrite
- Acid gases such as Carbon Dioxide (CO2), and Hydrogen Sulphide (H2S)
Causes of Increasing in Gel Strength of Oil Based Mud
For an oil based drilling fluid, there are several points that will cause high gel strength in the mud system as follows.
- Over treatment with organic gelling material
- Build up of fine solid particles in the mud
Operational Impact of Excessive Gel Strenght
Operational Impacts of gel strengths are as follows;
Circulating Pressure
Excessive gel strength will lead to high pump initiation pressure to break circulation after mud is in a static condition for a period of time. High pump pressure may result in formation fracture and lost circulation.
Cutting Suspension
Low gel strength indicates inability to suspend cuttings. It can lead to pipe stuck and hole pack off due to insufficient cutting suspension.
Barite Sag
Barite sag is a situation where barite cannot be suspended by drilling mud because of low gel strength. It can be seen that when large fluctuation of mud density coming out of hole.
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