Rate of dissolving/dissolution defines how quickly a solute dissolve in solvent. A dissolvable frac plug can take up to hours or even weeks to dissolve under a dynamic down hole environment. In our experience, ROD (rate of dissolving) cannot accurately represent the total time for a frac plug to be fully dissolved. In this post, let us take a look at the differences between ROD and TDT (total dissolving time), and talk about why TDT is more down-to-earth.
ROD of a dissolvable material is a major indicator to determine if it is fit for the job, aside from strength and elongation properties. Most dissolvable frac plugs are made up of different parts, these parts have different functions to perform. Some parts need to last through the whole frac job (slips), some parts just need to survive pump down (nose cap). Therefore, it won’t make sense if we use the same material for the whole thing. To balance dissolving and performance, different ROD materials are selected for different parts, and these parts will dissolve at a different rate.
Most ROD tests are conducted under lab conditions, whereas the maximum temperature and fluid property is static. These tests cannot represent the dynamic aspects of the operation such as pump down, rig up for frac, frac, shut-in, flow back, fluid flow regime, cooling effect and warm back. Furthermore, dissolved particles (magnesium hydroxide) can cover up the sample surface, without external forces, the solvent can’t pull them away. This is another reason if lab ROD test results don’t match with actual dissolving performance, because we know flow back can help frac plugs to dissolve.
Whenever a client asks what is the ROD of our frac plugs, we find it difficult to answer, not because we don’t know, but because the answer can be a bit complex. So, we came up with TDT to make things simpler.
TDT represents the performance of the whole tool instead of individual parts, it simply means the total dissolving time (hours or days) of a frac plug inside a live well, by taking dynamic factors into consideration. It is more arcuate, simple to understand, and practical.
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