Annular Pressure While Drilling – What is it good for?
Managing equivalent circulating density (ECD) is one the key success factors in extended reach well design. ECD is influenced by both hole section length and flow rate amongst other factors which contribute to the annular pressure loss (APL). Check out my back of the napkin note below for a better list.
Many MWD tools include an annular pressure sensor, allowing the drilling engineer to measure ECD whilst drilling. You might think that an unexpected increase in measured ECD would indicate a problem downhole – and you may be right. With a pressure sensor in the BHA you have another set of eyes helping to develop a picture of how the hole behaves as drilling proceeds.
The annular pressure while drilling (APWD) data density, or the length of time between each APWD measurement being sent up hole, depends both on the MWD telemetry set-up and the number of other measurements being sent (with vast amounts of petrophysical data often competing for space). If you haven’t thought about it in advance, you may be left with a single APWD data point every 90 seconds or so, which is not going to give you much in the way of warning to changing downhole conditions. What is required is a proactive approach in responding to developing downhole conditions. Best practice requires that downhole tools have a measurement configuration which can be shifted towards higher density annular pressure data over critical hole sections, meaning proactive measures can be taken if ECD trends in an adverse direction.
In our training, we often discuss whether or not ECD is a good indicator of hole cleaning. By its nature, a single measurement source very close to the bit is only going to provide a gross approximation of what is really occurring in the wellbore (i.e. from the sensor to surface). Increases in ECD could be related to long sections of constricted annular clearance in high angle sections, indicative of poor hole cleaning; or they could also show that hole cleaning is working efficiently as cuttings are moving out of a high angle section and loading up the low angle part of the hole, increasing the effective mud weight.
When using wired drill pipe with APWD sensors spread out across the drill string, we will “see” sections of the string where ECD trends behave differently, indicating that something is going on and that we need to be aware and react accordingly whilst drilling or tripping. Unfortunately, not many have access to the kind of project that is necessary for the wired pipe value proposition to work.
In class, the conclusion we usually come to is that APWD is a useful tool for tracking changes to ECD. It gives us a warning that something is going on and primes us to be extra vigilant for what might happen next. We cannot be sure what our next steps will be based on ECD data alone, but we should always take note of any changes and correlate with other data in an attempt to work out what is going on.
By Stewart McGregor, Technical Development Director – Merlin ERD
