Velocity – One of the major obstacles to overcome in horizontal and ERD drilling is hole cleaning. In vertical holes, cuttings have room to settle without causing problems. In horizontal wells, cuttings only have to settle 4+ inches to form a cuttings bed. Research and practical experience has shown that cuttings bed formation is analogous to sand dunes on an ocean beach. The bed moves in a wave fashion up the hole. If sufficient annular velocity is not present, the bed thickens until it reaches equilibrium, which usually occurs when turbulent flow conditions exist. With water, turbulent flow is easily obtained since viscosity is in the denominator of the turbulent flow equation, and water has a viscosity of 1. Fluids other than water require more pump output to approach turbulent flow conditions. Practical
experience in other industries has proven that materials can be transported in water slurries if the fluid velocity is between 3 and 7 ft/sec.
Rheological Properties – Ideally one would want a true thixotropic fluid to drill horizontally. In effect, two different conditions exist simultaneously. One occurs in the vertical portion of the hole where larger diameter casing may be set, or wash-outs in the open hole occur. In that case, a fluid having a high Yield Stress, or if the fluid is visco-elastic, the initial gel strength needs to be elevated. The opposite is needed in the horizontal section from a hole-cleaning point of view. Here one wants water for the reasons pointed out earlier. Compromises are the result. If casing is set into the zone of interest, or if formations up-hole are not a problem, then water may be possibly combined with occasional sweeps.
Surface Apparatus – Special attention must be placed on the solids removal equipment at the surface. Solids removal efficiency must be continually monitored so that unwanted particulate material is not retained in the fluid system that may negatively affect fluid weight or filter cake quality, resulting in lost circulation or increased torque and drag. The cost of problems that may occur due to not doing this correctly may far outweigh the cost of equipment needed.
Common Fluid Utilized – Table 1 is a comparison of drilling fluids utilized in horizontal wells which can vary depending upon the area in question, historical preference, environmental constraints, fluid company experience, and operator. Types have included gel and water, oil based fluids, polymers, and in some cases, sized-salt systems. Combinations of the above, as well as other fluids have been tried.
|Hole Size||Typical Flow Rates|
|17 ½”||1100 gpm minimum. Some rigs achieve 1250 – 1400 gpm.|
|12 ¼”||Aim for 1100 gpm (although 800 – 1000 gpm is typically achieved). If
1000 gpm is not achievable, ensure tripping procedures are in place for
poorly cleaned hole.
|8 ½”||Aim for 500 gpm.|