In a past newsletter we discussed the waste streams generated from horizontal well installations. But when we review projects with consultants, regulators and owners, we still encounter confusion regarding volumes and types of waste, containment strategies and disposal costs. With this sequel to our original article, we hope to clarify the topic of drilling waste.
One of the most frequent questions we receive from consultants and owners when discussing a horizontal drilling project is, “how much waste will be generated”.
Often this question arises at small site projects: drilling locations in highly urbanized locations where we have tiny construction footprints. Most of the time the area in which we have to set up the rig and support equipment is so small that a drilling fluid recycling/cleaning system can’t be utilized because there is simply no room. In those cases, we use a “one pass” drilling fluid system/method: the mud is mixed in a tank, pumped down the bore, exits at the bit, carries cuttings to the surface and then pumped directly to a waste roll off (more on roll offs later).
It may seem that this process would generate huge amounts of spent drilling fluid. But keep in mind, at these small sites the well materials are normally 2” or 3” diameter and the well lengths are relatively short. So we may be drilling a 5” diameter hole with a total length of 200’. This will generate a mud volume of about 600-800 gallons (assuming about three to five times the total bore which includes – drilling mud, development water and decon fluids), or about 80 to 100 cubic feet. That’s only three to four cubic yards of waste.
One fact to keep in mind with one-pass drilling fluid management – the waste slurry may contain up to 50% suspended solids, because this is a combined stream of solid soil cuttings and waste drilling fluid (a solution of water and colloidal minerals or organic long chained starches/sugars). The waste generated is directly proportional to the drilling fluid used and the rate of penetration, minus any mud loss in the bore. If drilling slows in rock or stiff soils, more waste may be generated.
The described methods to calculate waste volumes only provide estimates, the actual volumes vary considerably. Different geologies create varying amounts of waste – a caving borehole in sand can generate piles of solids, but a bore in tight clay may just be the volume of the bore itself. Sampling projects that require many trips in and out of the bore can multiple the waste quantity several fold.
Stepping up in rig size, larger rigs and greater diameter bores require a higher volume of drilling fluid to clean the borehole. It becomes uneconomical to use the “one pass system” with the volume of mud necessary and the expense of biodegradable fluid. For these reasons, we turn to a mud recycling system to mix, condition and store the drilling fluid. Rather than discarding the drilling mud after a single pass, it is pumped to the recycler, where most of the drill cuttings are removed from the fluid stream and the mud is reused.
On a small-rig project there is a balance between the cost of waste disposal, and the incremental extra cost for mobilizing and operating a recycler. On larger projects, the use of the recycler is assumed.
The waste soil cuttings are discharged from the mud recycling system into a roll off parked adjacent to the recycler. Although the goal is to divert very dry cuttings and recirculate nearly all of the drilling fluid, a variety of factors control how efficient this separation process can be. The result is that the discarded material may range from nearly dry sand, to a slurry with the consistency of a milkshake.
How do we contain the waste on site? The best container to use for both solids and liquids is a “water-tight” roll off bin, with a removable cover – IN GOOD CONDITION. Determine a location on site for the roll off, then call a supplier and tell them exactly what type of waste is going to be stored. Insist they provide a new container if possible and request a polyethylene liner. When the container(s) arrives, check for punctures in the roll off body, rusted out areas, faulty seam welds, and damaged or missing gaskets on doors and hatches. The last thing we want as a project team is a leaky roll off. While getting new, lined roll offs is a good plan to eliminate leaking, DTD recommends the precaution of placing the roll off on secondary containment. The bin supplier can generally supply secondary containment with the roll off itself, but it can also be constructed with sheet plastic and soil, timbers, straw bales or wattles, etc.
In most cases, high solids slurries can’t be hauled directly to a landfill; the material will require stabilization. There are many additives which can be mixed with the slurry to absorb excess moisture and “solidify” the material, including sawdust, ground corn cobs and super absorbent polymers. The polymer products are generally much more efficient in absorbing excess water than other options, reducing the total amount of waste to be disposed. Just make sure the additive is acceptable to the ultimate disposal location.
An alternative to solidification is to contract for the waste to be removed in slurry from, using a vacuum truck. Compare the cost of this option to that of solidification and disposal as a solid when developing project budgets – surprisingly the costs are often similar, and reduction of on-site waste handling is much more convenient.
Let’s discuss one more very important item. You can’t completely fill a roll off with cuttings or high solids slurry. It is impossible mix solidifying agents if the container is full to the brim. Also, the waste company can’t pick up and haul a full roll off. So when you are calculating the number of containers you need on site, remember a 20 yard box can only hold 10 yards of material.
Before you start the project, work with the driller and determine an estimated waste volume and a plan to not only contain the material, but also manage it in preparation for hauling from the site. Addressing this aspect of the job several weeks before the project starts insures that everyone has an understanding of how much waste will be generated, how that volume may potentially vary, how it will be stored and where the container is to be located on the site. DTD always discusses this topic in our preconstruction meetings – it is a significant issue and can be one of the defining factors in the ease or difficulty with which a project is completed.