The gel strength of drilling mud (or drilling fluid) is the shear stress in the fluid at a low shear rate after it has been static for a specific period of time. The parameter gives a measure of the inter-particle forces in the mud once drilling has stopped.
The gel strength is one of the most critical properties during the drilling process since it demonstrates:
- The ability of the mud to suspend the cuttings when circulation has ceased
- The amount of pump pressure required to restart circulation or "break" the gel strength.
The more the mud gels during drilling shutdown, the "stiffer" it becomes. Therefore, the greater the gel strength, the higher pump pressure needed to get it circulating again.
Conversely, muds with lower gel strengths are easier to break, requiring lower pump pressures to resume circulation.
Gel strength is measured in the laboratory by first agitating the fluid sample, then leaving it to settle until it is in a static condition. Its shear strength is then determined using a rotational viscometer, or rheometer, which tests the sample for specific durations (10 seconds, 10 minutes, and sometimes 30 minutes).
Gel strengths are measured in units of lbf/100ft2, and the test values are termed the 10-second, 10-minute, and 30-minute gel strengths. Gel strength typically falls under two categories: progressive and flat.
Progressive gel strength describes the condition where the 10-second and 10-minute gel strengths differ significantly. In this case, the 10-minute strength is much higher than the 10-second strength. This result indicates that the drilling mud gels or gains strength as time progresses.
Progressive gel strength is an undesirable feature for drilling mud since excessive pump pressures may be required to restart circulation.
Flat gel strength, on the other hand, describes a situation where the 10-second and 10-minute gel strengths of a drilling mud are similar. This result indicates that the shear strength of the fluid will remain relatively constant over time and should be pumpable if left static in the hole.
Flat gel strengths are preferred since the circulation of the drilling mud can resume with minimal pump pressure.
While flat gels are ideal, gel strengths which are too low can cause the solids or cuttings to settle. If both the 10-second and 10-minute gel strength values are near zero when tested according to standardized test procedures, the drilling fluid is termed a zero-zero gels mud.