Definition. The time-dependent forces in the drilling mud cause an increase in viscosity as the fluid remains quiescent for a certain period of time. The gel strength is a measurement of the electrochemical forces within the fluid under static conditions. Its field unit is the same as that of the yield strength. The strength is a function of suspended solids, solid contents, temperature, chemical content and time. Usually it is caused by the high concentration of clay.
tuo(g) = gel strength, lbf/100 ft2
Gel strength Measurement. The gel strengths are also measured with the rotating viscometer. The sample should be stirred at high speed for 10 seconds (or constant reading), then allowed to stand undisturbed for 10 seconds. With the gears in neutral, a slow (about 3 rpm), steady motion on the hand wheel is applied. The maximum reading is the initial gel in pounds per 100 ft2. The mud is re-stirred for 10 seconds and allowed to stand for 10 minutes. The measurement is repeated as before and the maximum reading is recorded as the 10-minute gel strength in pounds per 100 ft2. When an electrically driven instrument is used, the gel readings are the maximum value obtained at the low-speed 3 rpm setting.
The dial readings from Fann VG viscometer are shown below and the last two readings are taken at 10 seconds, 10 minutes and 30 minutes.
The mud tend to build gelled structure when in idle state and so it needs additional force to break it when either the pipe is moved or fluid is pumped. When pumping fluid gel has to be broken inside the
The time for measurement such as 10 sec, 10 minutes and 30 minutes are used as a basis to measure the rate at which the mud is gelled.
Gel strength control. The gel strength is the measurement of attractive (electrochemical) forces within the mud system under no-flow conditions. It differs from the yield value because of its time dependence and is broken up after flow is limited. As with other properties, the ability to maintain the proper value of gel strength depends on effective solids control. Although, the type of treatment to control the gel strength will depend on the mud system in use, the primary means is flow line flocculation and minimal treatments with viscosifiers.
Difference between Gel Strength. Gel strength measurements denote the thixotropic properties of the mud. Gel strength will not exist once it is broken and the fluid starts moving whereas Yield stress will be present even if the fluid is moving. They are a measure of the attractive forces under static or-non-flow conditions whereas on the other hand Yield point is a measurement of attractive forces under flowing conditions.
More details can be obtained from 501 Solved Problems for Drilling Operations.