• We introduced the concept of a Bayesian belief network for hole cleaning a few weeks back. When the hole cleaning belief goes below a certain threshold, it is a reminder that some action might need to be taken or care exercised as we restart drilling. To that end, one example user interface that was deployed (Slide 1) shows not just the hole cleaning efficiency belief, but also the reasons the hole cleaning belief is low. This is important as the rig crew is now provided guidance on what corrective actions to take. As a reminder, the main factors that bring down the hole cleaning belief are low circulation rates, static hole conditions, tight spots and sub-optimal bit hydraulics. We explored tight spots last week. Here we will explore the remaining three.

    Most of you are likely familiar with the concept of critical flow rate. It is generally defined as the minimum flow rate required to transport the cuttings to the surface. While physics-based models can be difficult to run in real-time, researchers such as Luo et al. (1994) have derived charts from physics-based models, so that they can be used at a rig site. We converted these charts into look-up tables and integrated them into the Bayesian network as one of the many nodes. This calculation of critical flow rate considers ROP, mud weight, YP, PV, hole angle and hole size. Please refer to paper SPE 27486 for more information on these charts. Slide 3 shows the trending of sufficient circulation belief.

    In addition to the flow rate, the bit hydraulics also needs to be considered for proper cuttings transport. The hydraulic impact force should exceed a threshold such that cuttings can be transported optimally. We have defined this threshold in our model to be a percentage of the hydraulic horsepower output from the surface, taking inclination into consideration. Drilling with a hydraulic impact force less than this threshold for a sustained period is an event considered detrimental to hole condition as the force of the fluid leaving the nozzles is expected to be too weak to push cuttings away from the bit.

    The network also considers periods of extended static hole conditions to be detrimental as they could potentially lead to cuttings settling in the hole. The “Static Hole” node keeps track of the amount of time in the recent past during which the drill string was static or out of hole. Check out Slide 6 to see how this belief moves during various rig activities.

    Click below for the slides on this topic: Hole Cleaning – Tracking Causes

    Next week, we will go into the effect of proactive measures on keeping the hole clean and close out our discussion on this topic. So, stay tuned.