Depending on who explains it to you, there can be several possible ‘key elements’ to get a successful cement job in place. Some people would emphasize the design. Others would prioritize the execution aspects instead. In my case, I like to think that a successful cement job can only be achieved by a combination of proper design and a controlled job execution.
In this post, I am going to try to make a case, from basic concepts, to explain my point of view as a cementing expert with over 20 years of experience. But, like I always say, the only important thing those years gave me, is the capacity to listen to the well and to the people I work with. So, if any of you have comments or feedback, please feel free to drop some words at the end of the article.
Every cementing engineer knows well, that when working out the hydraulic program, the information entered is made of: actual data. For example, the casing size, depths, directional survey. We also include assumed data, including variable information. (Like hole excess, casing IDs – nominal or averaged). Then we add derived data from models (like formation pressures, temperature and the fluid’s rheology).
All these inputs collectively, e.g., as result of their individual accuracy (definition: ‘accuracy reflects how close a measurement is to a known or accepted value’), will add a level of uncertainty to the simulation output.
In other words; how close are the actual job results to the predicted output? Obviously, for us, it is just going to be a single output, the simulation report. However, what we don’t see is the wide spread of possible outputs derived from individual value’s standard deviation, for the data derived from models (definition: “standard deviation is a measure that is used to quantify the amount of variation or dispersion of a set of data values”) or certainty, for the variable information.
This is the first key element of a successful cement job:
The accuracy of the input data.
The greater the dispersion of the input values, the wider the gap of possible outputs, or in common words: ‘garbage in garbage out’. For us cementers, our efforts should be focused on getting our data set right, and for well construction managers to make available representative and valid data.
This means ‘cost’, but you should know by now that data collection, logging (4-arm caliper, formation evaluation, etc.), and data processing and reservoir engineering, is similar to buying an old car. You invest at the beginning, with repairs, new parts, etc. If you do things right, your cost should reduce in time to a minimum, depending on the model, year, etc.
Additional variability is added during your execution, when variables like fluids rheology (mud, spacer and cement), fluids density (cement, spacer), fluid volumes, etc. differ from the values the hydraulic model is based on. This then is related to job supervision.
… Has to do with planning and preparation for the job, equipment maintenance, the quality and availability of materials, people’s competency and the involvement of rig personnel (pre-job meetings). This is the second key element to a successful cement job.
Well, I know in these terms, this seems like a worthless pursuit of job success. And that is because we know that at least one key element: accuracy of the input data, in a greater or minor degree, will always be questionable. So, what do we do?.
I already posted the answer to this one: post-job analysis, and you can see why I think it is so powerful here: The real power behind Computer Simulations. Now it should be evident to you, that post-job analysis is a way to calibrate and re-calibrate our model.
Yes, it is correct, for those of you asking, we are not going to get the right value for all the variables. We are going to find, if we do it right, which variables are affecting the output more, and work on them to make the next job better. (This is if we are coming from a failure, or to reproduce it, if we are coming from a success). The crucial 3rd element is…
… Post job simulation or job results analysis.
I hope this post is helpful in removing the generalized concept, that cementing is a given once you have done the ‘hard’ part: drilling the well. Proper cementing requires adaptable engineering to the true requirements of the well. Are you ready to see those?