I recently participated in a discussion about opting for the best alternative between conventional and reverse primary cementing to deal with losses and frequent difficulties to reach the required TOC in an intermediate well section.
The interesting thing in this discussion was that the majority of the participants had no experience in doing reverse cementing. There were many arguments; most of then showing the expected resistance to do a new practice. It was then when I immediately recalled my first time. Yes, my first time with reverse cementing.
First reverse cementing job
I had absolutely no experience with reverse cementing at the time, and I wasn’t the only one. Nevertheless, the customer wanted to do it for the first time and wanted me, not so unexpected, to do it successfully!!
The idea of a plan was there, but I remember having more questions than answers. Not to mention, I didn’t have software capable of simulating reverse cementing or even an actual case history to look at. In essence, I was starting from a minimal background and, as you can imagine, with even less time to put a design together.
In essence, one of the disadvantages of conventional cementing is that as the cement rises in the annulus, there is an important increase in hydrostatic and friction pressure. This is not usually a problem, but in a losses situation, like any other, the cement always takes the path of minimum resistance. (Which is typically the losses zone). Hence, preventing its way up in the annulus. This can be simulated (predicted) by software, and there are several alternatives to ‘prevent’ or ‘mitigate’ the losses, yet we’re not always successful. Another option is Reverse Cementing, where the effect of friction pressure (and some hydrostatic) in the annulus can be removed, allowing us to fill it with cement from the top.
In conventional cementing, density and rheology need to be higher in displacing fluids to ensure cement coverage (channelling prevention). In reverse cementing, on the other hand, higher density and rheology in displacing fluids would likely have an adverse effect.
I Kept the pumping pressure in mind and was grateful for support from colleagues. I decided to propose that the cement slurry should only be 20% denser than the fluid in the well, and just slightly higher cement slurry rheology than the mud. (Previous jobs used conventional systems that were 90% denser than the mud – 1.9 sg). The job was performed operationally as planned, even without being able to simulate the job correctly and with a ‘hand’ estimated effect on ECD.
The outcome, as seen with the sonic log, was not perfect. It was much better in terms of TOC than previous jobs done conventionally, most of them with losses during cementing.
Honestly, despite the results, I was not that happy, because the engineering was not that sound to my work standards. Sincerely, I felt a bit in the dark to adequately answer questions like; what to do to improve the results next time?
From my previous posts, you know that the thing I enjoy more about simulation is the capacity to truly affect the outcome. However, with no proper reverse cementing computer model, I feared that only educated trial and error, supported by cement log evaluation, was possible.
The next time, with a combination of products and software, we proposed the job conventionally. To be honest, the first time loses still happened but we almost reached the TOC. Nevertheless, the ‘conventional cementing solution’ sustained. In later attempts, the results improved, leaving the reverse cementing case only as a viable (not the preferred) alternative.
So, What is your experience with reverse cementing? Can you share?