HOW DO WE MEASURE STATIC GEL STRENGTH DEVELOPMENT

Historically, the SGS of a cement slurry was determined by a method using a couette-type rotational viscometer. Today, more specialized instruments have been developed that allow the measurements to be done under conditions of high temperature and pressure.

API-10B6 was developed to establish the testing protocols to determine SGS by different mechanisms, including a rotating-type apparatus, an intermittent rotation-type apparatus and an ultrasonic-type apparatus (removed in the latest API adoption due to patents exclusivity).

1. Test method using rotating-type static gel strength apparatus
   The apparatus contains a pressure chamber that can be heated and pressurized according to a simulated cement job schedule. The SGS is calculated from the torque required to rotate a paddle of known geometry at very low speed. The rotation speed of the paddle during the SGS measurement portion of the test is usually a continuous 0,2 r/min. The initial stirring to simulate placement in the well is typically conducted at 150 r/min.
2. Test method using intermittent rotation-type static gel strength apparatus
   This apparatus works on the same principles/methods as the previous one with the sole difference that this it operates intermittently during the SGS testing phase at 0,01 r/min for 6s after a time interval adjustable between 1 min and 10 min. In general, an intermittent rotation every 3 min is used.
3. Test method using ultrasonic-type static gel strength apparatus
   The instrument measures the static gel strength of API cement under high temperature and high-pressure conditions. The instrument is equipped with an internal processor board that sends and receives an ultrasonic pulse through the slurry, then performs post processing of the data to determine the static gel strength (SGS) versus time plot. Additionally, as an option, the instrument may be used to determine the compressive strength of the cement using the same algorithms and method found in a conventional Ultrasonic Cement Analyzer (UCA). This testing methodology was included in API10B6 original version but was later removed as it’s patent protected and exclusive to Chandler Ametek. The machine is known as Static Gel Strength Analyzer (SGSA).

 

POROSITY/PERMEABILITY OF THE GELLED CEMENT.

In 1963, Guyvoronsi and Farukshin measured the cement matrix permeability during the period of hydrostatic pressure reduction (Transition time) to be as high as 300 mD. In their paper, they were the first ones to introduce the concept of gas percolating through the pore structure of a very permeable gelled or set cement. In general, the higher the porosity, the easier path for gas migration through its pore structure.

For those slurry systems that rely more on porosity reduction to block gas from flowing (i.e. Microsilica systems) than on the development of gel structures, additional tests are required to ensure gas-tight capabilities. In those systems, small particles appear to pack between the larger cement particles and create a physical blockage, which can prevent gas from mobilizing the pore water.

For these systems, not depending on gel development, tests that evaluate the resistance to gas migration in the lab are more relevant. These tests offer “a closed system in which nitrogen gas is injected into the bottom of a cement slurry during its hydration. The cement’s susceptibility to gas migration is determined by whether or not the nitrogen gas injection pressure is transmitted up through the column of cement to the opposite (top) side of the sample where the pore pressure is measured. If no gas migration occurs, the pore pressure will continue to drop (due to the shrinkage and loss of fluid communication through the sample)”.

The two more popular devices in the market are the “Cement Hydration Analyzer” (CHA) and the “Fluid Migration Analyzer” (FMA). The first one, perhaps the more widely used, is capable of managing temperatures of 325°F and a ΔP of 1,000 psi. The second one, a more recent invention, is capable of managing 400°F and a ΔP of 2,000 psi. The FMA test cell itself is similar to an API HTHP fluid loss cell. The filtrate from the cement slurry can be collected from the bottom and the top of the cell through screens or rock core. The test cell can be rotated to simulate actual wellbore angles

These tests may not be available at your local lab but on a regional client support lab instead. Regardless the case, it is very important to use representative samples of cement, water, and additives for the test results to really endorse the gas-tight characteristics of the slurry that you are about to pump downhole.