When the mud is static, the rheological properties should prevent solids (weighting material and drilled formation solids) from settling. The relevant property in this respect is the mud’s gel strength. The drilling mud gel strength is caused by the interactive forces between colloids (e.g. clay particles), polymer radicals, or emulsion droplets in mud which form a more or less rigid structure (gel). The gel strength, being the force required to initiate movement in a gelled fluid is measured with a FANN rotational viscometer (Check viscometer test procedure).
How Does Gel Strength Occur In Drilling Mud?
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Gel strength arises mainly from the attraction between particles and from friction between solids in suspension. Gels may be progressive or fragile. Progressive gels increase continuously for a long period of time. This may create problems during tripping (surge/swab pressures) and when resuming circulation. Fragile or “flat” gels reach a near-maximum in a brief time interval. You might also be interested in yield point of drilling mud.
Progressive Gel Strength
Normally the gel strength increases with time in a stagnant drilling mud (“progressive” gels), which is reflected in the difference between the “zero” and “ten minutes” gel strength values (measuring gels subsequently after ten seconds and ten minutes is a standard procedure). Some progressiveness in gel values is required to prevent solids from settling during long-stagnant periods.
The level and progressiveness of the gel strength values depend on concentration and type of interacting particles. The initial gel strength is mainly determined by the concentration of the particles, whereas the ten minutes gel is largely governed by the rigidity of the interparticle structure formed. In clay-based muds, the interparticle structures can be quite strong, which is reflected in progressive gel development in those muds. On the other hand, the bonds between polymers are rather weak, which results in “flat” gels in polymer muds. In IOEM the gels are largely provided by the emulsion droplets. The concentration of these droplets is high, which explains the characteristic high initial gel strength of IOEM. In addition to that, the presence of clays that have undergone treatment with a surfactant to make them oil dispersible is mainly responsible for the increased 10 minutes gel strength in these muds.
Decreasing Gel strength is one of the measures to avoid Lost circulation Problems
Gel Strength Values In Drilling Mud
Typical values of gel strength which should be aimed for are:
|Item||10’’ gel (lbs/100 ft2)||10’ gel (lbs/100 ft2)|
|For unweighted clay-based muds (bentonite or gypsum muds)||2- 5||5-10|
|For weighted clay-based muds||4- 8||8-20|
|For unweighted polymer muds (KC1-polymer, salt-saturated mud)||1- 2||3- 5|
|For weighted polymer muds||2- 5||6-10|
|For unweighted IOEM (OWR 75/25)||5-10||8-15|
|For weighted IOEM (OWR 75/25)||10-15||15-25|
Gel values are normally expressed in field units. Conversion into 51-units (Pa) can be established by multiplying the above values with 0.479.
Gel values should not be too high or too progressive as this could give problems during the following operations:
- When restarting mud pump & circulate after a prolonged stagnant period (e.g. a round trip). The force (pump pressure) required to break the gel structure might exceed the formation fracturing pressure and induce losses;
- During movement of pipe (up or down) in a stagnant fluid while tripping pipe, pressure surges might induce losses or influxes from formation fluids (Surging & Swapping in Drilling);
- Removal of solids and entrapped gases at the surface is hampered by a high gel strength.
It is, therefore, important to aim for the above gel values, particularly in areas with a known low formation strength or in case of a known small overbalance.
In general, a 10 minutes gel strength of 30-35 Ibs/100 ft2 should be considered as a maximum for all muds at all conditions. Higher gels also should be avoided and/or be reduced by remedial mud treatment (thinning or dilution, see
Application Of pressure To Gelled Mud
If the mud is left to stand, a gel will be formed due to its inherent thixotropy, and a definite pressure must then be applied to break up the gel and restore the fluidity of the mud. If the gel strength (long time gel strength) as determined in the Fann viscometer is g, the pressure which must be applied to restore the circulation of the mud can be found by the equation
- gt = long time gel strength (gt more than or equal g10)
- L = section length
- Do = hole diameter
- Di = external diameter of string
- Pr = pressure
The gel strength determined at the surface may, however, not be fully representative of the effective gel strength in the hole, especially if the gelation of the mud takes place under the influence of heat.
Moving the mud following a stagnant period (e.g. moving the drill string or starting the pump) should not produce a dangerous overpressure. If at all possible, the gel should be broken up by rotating the string before mud circulation is started.
Measurement & Calculations Of Gel Strength
The following instruments are used to measure viscosity,’ and or gel strength of drilling fluids: