Introduction To Hole Cleaning
Drilling fluid hydraulics and hole cleaning is an integral part of the practical processes required for planning and drilling a well.
Optimization of cuttings transport depends on so many factors- hole angle, cutting size, drill string rotation, drill pipe eccentricity, optimization of bit hydraulics, ROP, hole cleaning pills, cuttings transport ratio, cuttings bed properties, etc.
The key to a successful hole cleaning relies upon integrating optimum drilling fluid properties with best drilling practices.
The use of hydraulics and hole cleaning, therefore, goes beyond calculating pump pressures and jet nozzle selection, although these functions are critical to optimum bit performance.
The primary use of drilling hydraulics and efficient, effective wellbore cleaning should be considered proactive to design, engineer and drill a well, with careful attention applied to hydraulics and hole cleaning aspects, to reduce or eliminate potential problems before they occur.
The flow rate is one of the main factors affecting hole cleaning and when we talk about flow rate and hole cleaning. More flow rate is not always the way to improve hole cleaning because might cause erosion or hole washout, therefore enlarging the hole causing less annular velocity for hole cleaning and decreasing cuttings transport.
Below a useful table showing the recommended flow rate for highly deviated and horizontal wells:
Note: It is important to calculate the minimum flow rates required for hole and wellbore conditions, i.e. hole angle, casing setting depth, mud properties, anticipated ROP’s etc. If not achievable, then ensure that drillstring engineering, BHA design, drilling & tripping procedures are in place to best manage cuttings beds that may form.
Circulate the hole clean prior to tripping. A single bottom up is not sufficient below a recommended bottoms up factor.
Symptoms of Inadequate Hole Cleaning
- Small volume of cutting at the shakers as compared to the volume of the hole drilled.
- Large amount of fill after trips.
- Increasing drag.
- Increasing Torque.
- Reground (small and rounded) cuttings.
- Increased ECD ans measured by the PWD tool.
Poor hydraulics and wellbore cleaning may cause the following drilling problems.
- Inadequate cuttings evacuation from the bit.
- Lower than expected drilling performance.
- Grinding of cuttings into fines at the bit.
- Mud and solid control maintenance difficulties.
- Increased ECD.
- Increased mechanical torque and drag – especially for deviated holes.
- Overpull in the wellbore due to fines cuttings beds that often result in unnecessary backreaming.
- Stuck pipe.
- Side-track of well.
It is important to note that vertical wells are the easiest to clean where cutting beds will not form but where cuttings concentration and slip velocity considerations are perhaps the most important considerations when evaluating wellbore cuttings transport efficiencies.
Higher angle wells
In angles wells e.g. > 30degrees, hole cleaning and cuttings build up in the annulus is highly dependent upon hydraulic conditions. This is especially true in more deviated or horizontal wellbores.
Temperature/Pressure Effects on Mud Rheology
Temperature and pressure play an important role in mud rheology and density because they vary with depth.
The most significant effect is generally seen near the bit where the temperature is greater. This is also the location where most of the swab/surge pressure is developed, and swab/surge calculations may be affected significantly.
Perhaps more significant than effects on pressure loss is the effect on the true yield value in the mud. Significant reduction in the yield value downhole, although the surface value is satisfactory for cuttings removal, could create hole cleaning problems.
Mudflow rate and Mud Properties are perhaps considered the two major variables to ensure effective and efficient wellbore solids removal.