Recently, this author read an article about a horizontal directional drilling contractor who fracked out under a creek. The drilling fluid was referred to as natural gas drilling lubricants and, allegedly, the inadvertent return of 50 gallons of bentonite caused several water wells in the surrounding area to become cloudy. It is easy to see how this sort of thing can get blown out of proportion and even confused with a totally unrelated hotbed issue such as natural gas well fracturing. While there are unseen conditions in which frack-outs can be virtually unavoidable, a large percentage of frack-outs that occur during the horizontal directional drilling process are preventable. This article will delve into the causes of frack-outs, preventive measures to reduce the chances of frack-outs/inadvertent returns, and possible ways to seal off frack-outs.
Frack-outs/inadvertent returns occur when the pressure within the borehole (annular pressure) exceeds what the formation can withstand. There are a multitude of causes for high annular pressures, and one of the most common causes is high viscosities in drilling fluid returns. Viscosity is the resistance of a fluid to flow. Therefore, the higher the viscosity in drilling fluid returns, the higher the resistance to flow, and the more mud pump pressure it takes to push material out of the borehole. The days of mindlessly mixing a super-thick muck of just high-yield bentonite and water and calling it drilling fluid are gone thanks to a better understanding of the ill effects of high annular pressures, along with a better awareness of the benefits in matching drilling fluids to the various soil conditions encountered. Today, we know that the ideal drilling fluid will perform all of the required functions, such as gel-strength to suspend and transport cuttings to the surface, fluid-loss/filtration control to maintain an open borehole, and controlling reactive clays, at the lowest possible viscosity. Failure of a drilling fluid to perform the required functions can lead to high annular pressures and frack-outs, along with many other problems such as stuck pipe or stretched product line.
If the gel strength in a drilling fluid is insufficient to suspend and transport the cuttings out of the bore path, the cuttings create blockage in the borehole and cause the drilling fluid to over-pressure the formation. A lack of filtration/fluid-loss control can cause coarse soils to collapse and restrict annular flow, while failing to control sticky reactive clays will yield the same results.
Inadequate pumping rates, out running the drilling fluid, can also lead to excessively high annular pressures. The pullback speed of a back-reamer is largely determined by the amount of drilling fluid that can be delivered to that back-reamer and pumping requirements change depending on soil conditions. Coarse soils, a.k.a. non-reactive soils, such as sand require a ratio of between two to three times the amount of drilling fluid versus the volume of soil being drilled, while fine (reactive) soils such as clay require pumping rates of three to five times the amount of drilling fluid soil. To calculate drilling fluid pumping requirements, simply take the square of the hole diameter and divide by by 24.5. That number will be the hole displacement in gallons per foot of hole. The equation looks like this:
Volume of hole = diameter2 ÷ 24.5 = gallons/foot
Take, for example an 8-inch backream and 200-foot bore.
(8 x 8) ÷ 24.5 = 2.61 gallons/foot
2.61 gallons/foot x 200 feet = 522 gallons (based on 1:1 ratio)
Remember to adjust the ratio requirements for different soils. Sands would require two to three times the volume of the hole, and clays would need three to five times the volume of the hole.
In order to ensure that adequate volume is being pumped, contractors should always have someone constantly monitoring the return flow, especially at the exit pit when back-reaming, and watch for problems. If the returns start flowing back at the consistency of peanut butter and the pump is running at maximum volume, the person monitoring the return flow can contact the driller so that the back-ream speed can be reduced enough to provide adequate drilling fluid to provide a thinner and more flowable return flow. This is also a good way to catch unexpected changes in soil conditions and make adjustments to the drilling fluid mix before problems arise.
Improper reamer selection also can cause excessively high annular pressures, which can lead to inadvertent returns. A reamer should not restrict pumping volume or annular flow. Spiral or fluted reamers work great in coarse unconsolidated soils. Avoid using these types of reamers in clay, though, because these reamers have far too much surface area for clay to stick to and ball up around, blocking return flow from getting to the exit side of the bore. Back-reaming in clay/reactive soil conditions requires a back-reamer with minimal surface area, and a reamer that can provide good chopping action to break up large clumps of clay into something that can easily be carried out with the drilling fluids returns.
Drilling technique can also play a role in contributing to high annular pressures and frack-outs. Failure to pull back and rotate the bit through areas where push-steering corrections were performed can result in a reduction in annular space, which can block cuttings from exiting the bore path and over-pressure the formation. Monitoring return flow on the pilot hole, and pulling back while rotating the bit and pumping to restore flow when problems do arise, can prevent problems such as frack-outs and even damage to surface structures such as buildings or roads.
Although the odds of sealing off frack-outs are low, this author has had some success by identifying the cause, correcting it, increasing filtration control by adding poly anionic cellulose (PAC) polymer, and using loss circulation material. After adding loss circulation material and pumping it into place, especially when using a super-absorbent polymer type of loss circulation material, it is a good idea to stop circulation and let the loss circulation material swell and seal the zone off before circulation is resumed.
Frack-outs on horizontal directional drilling projects can delay the completion of these projects and can also be very expensive. Preventive measures can go a long way in avoiding the headaches associated with fracking out. Properly matching the drilling fluid, down-hole tooling and pumping volume to soil conditions, along with making a concerted effort to maintain and monitor return flow will greatly reduce the chances of inadvertent returns and up the odds for success.