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March 2017 – Coal Combustion Residual Dewatering Goes Sideways

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Coal Combustion Residual
Dewatering Goes Sideways

Recently, Directed Technologies Drilling (DTD) field crews installed a horizontal dewatering well in a CCR impoundment utilizing horizontal directional drilling (HDD) methods. The landfill covers about nine acres and contains bottom ash, fly ash and miscellaneous waste associated with a building demolition. The purpose of the well is to remove leachate from the impoundment through the placement of the horizontal well at the deepest portion of the landfill.

DTD mobilized a 60,000 lb. push/pull capacity drill rig along with support equipment including a 5,000 gallon drilling fluid recycling system to the site. Based on geotechnical reports

(hollow stem auger/split spoon sampling) that indicated the CCR material to be sand and silty-sand with intermittent gravel zones, a conventional “duckbill” jetting assembly was chosen for the initial pilot bit. Once the equipment was placed on the drilling pad and kick off safety meetings were conducted, drilling commenced.

The plan was to install the well in a continuous bore hole (entry-exit) by drilling a 6” diameter pilot hole along an entry curve to the base of the landfill, running about 550’ at a slight upward pitch along the bottom of the landfill (just above the compacted soil liner) and then steering the drill upward to exit the CCR waste.

The steering/locating system utilized for the pilot bore drilling was a down-hole wireline system used in conjunction with a surface coil (Tru-Tracker). This particular tool set was used because it provides for greater accuracy than a standard walkover location method (details of locating tooling can be reviewed in our April 2014 issue of “Inflection Points”). Improved accuracy was desired because the bore path for this project was near the base of the landfill and penetrating the clay liner may have resulted in uncontrolled release of leachate into the ground water.

However, the drilling operation differed slightly from the initial plan. Although the pilot hole was advanced successfully through the entry curve and along the target depth near the bottom of the landfill, several challenges were met throughout the remainder of the pilot-hole drilling. First we encountered plastic bags filled with waste materials; the shredded plastic plugged the drilling fluid transfer pump which slowed the drilling operation. Secondly, as the bit was steered upward from the end of screen elevation to the surface, a hard consolidated layer of material was encountered; the duckbill bit assembly was unable to penetrate this hard zone. The crew pulled the bit from the hole and changed to a more aggressive bent sub assembly with a tri-cone bit. Unfortunately even the
bent sub/tri-cone would not drill through the cemented material, instead it skated along the undersurface of the hard zone. The down-hole tooling was again removed from the borehole and a down-hole mud motor with a tri-cone bit was placed on the drill string.

The mud motor assembly allowed the operator to drill through the cemented zone. However, once through the hard material, the stiff heavy mud motor could not be steered to the desired curve in the soft ash material above the cemented zone. Additionally, a magnetic anomaly was encountered in the landfill by the steering tool which caused an azimuth discrepancy between the down-hole probe and the surface Tru-tracker system. At this time the project team – DTD, site owner and the consultant – called a halt to the operation to re-plan the well bore.

A decision was made to move the rig from the entry to the exit, and re-drill the bore “backwards”, attempting to drill through the hard zone again with the mud motor, then intercept the existing borehole to exit at the original entry location. The process worked according to plan and we successfully completed the 6″ diameter pilot bore.
The pilot hole was reamed to a final diameter of 14” in two stages by pulling a hole opener back to the rig. Again during the reaming operations the plastic waste bags caused some issues because they wrapped around the teeth on the bit, decreasing its effective cutting action and increasing the pull forces. After the final reaming pass, the well materials were pulled into the open borehole. The well screen and casing are composed of longitudinally-slotted, 8” diameter, DR 14 fusible PVC. Note that no filter pack was installed and the screen was not pre-packed. The final installed well parameters are:

  • Total well length: 643′
  • Entry riser length: 30′
  • Screen length: 550′
  • Screen depth bgs: 29′ – 80′ varies due to landfill surface topography
  • Exit riser length: 63′
  • Well screen and casing: 8″ diameter, SDR 14 fusible PVC
  • Well screen: 0.040″ Slot width, ~24 slots/foot

The drilling mud utilized for this well installation was a biodegradable polymer. Well development consisted of jetting and flushing with water with an enzyme additive designed to break the starch based polymer drilling fluid. Following the jetting and flushing, a submersible pump was placed in the well and operated at multiple intervals along the screen section. Total flushing and jetting was carried out for two hours and the pumping occurred for about twenty hours.

After development was completed, the DTD crew grouted the exit side annulus of the bore, leaving the entry side to be completed by the owner.

Results from the owner indicate the well continues to produce approximately 20 gpm and has lowered the leachate levels within the impoundment over 20 feet; without the presence of CCR material in the outflow water.

Lessons Learned

  • Horizontal directional drilling can be utilized to install wells in CCR waste.
  • Coal ash material may not remain in an unconsolidated form.
  • Other material can be found in CCR landfills. At this site we encountered plastic bags and identified a magnetic anomaly.
  • Filter pack and/or prepacked screens may not be required for pumping wells in CCR waste.