Lost circulation (loss of drilling fluids being returned to the rig or exit pit) is a result of the fluid escaping to the formation being drilled. Losses may occur as a result of a natural fracture, a void, or in a high porosity/high permeability formation. Lost circulation can also be created when boring in a section of formation that is not able to withstand the pressure being exerted by the circulating fluid and cuttings (for example, slurry).

When boring in formations with large fractures, voids, extremely fragile formations or with minimal formation cover, lost circulation can be difficult to avoid. Under these circumstances, the operator should be prepared to treat for losses from the outset. At times, lost circulation in horizontal directional drilling (HDD) operations can occur when the bore path is blocked by cuttings that are not flowing. This is usually caused by improper fluid selection and/or reaming too fast for the size of hole being cut.

The losses go beyond thousands of barrels of drilling fluid and include numerous hours or even days, and significant sums of money that are wasted by not dealing with lost circulation problems immediately. Losses worsen over time and correction becomes more difficult - perhaps even impossible. Loss situations should be treated with urgency and operators should have both plans and materials on hand to address these situations.

The majority of the time, however, lost circulation is avoidable. Minimising the pressure created by circulating fluid is critical to preventing and correcting lost circulation events, and circulating pressure is a factor that can be controlled. Maintaining fluid viscosity to provide the needed gel strengths for proper bore path cleaning and borehole support is a valid concern. Unfortunately, quite often lost circulation is caused by excessively high viscosities, resulting in high circulating pressures which break down the formation. This can lead to unnecessary costs related to production time and reduces job profitability. Circulating pressures are routinely monitored in the vertical drilling industry; however in HDD where the slurry thickness is generally a lot greater, the circulation pressure is barely considered. Why is this the case?

High viscosity versus low viscosity

The use of bentonite as a viscosifier is definitely advantageous. Bentonite usually provides the backbone of the drilling fluid and is the first line of defence against lost circulation problems. As formation permeability increases, however, so does strain on the wallcake-building ability of the bentonite. Adding more bentonite to provide more wallcake-building bentonite platelets is an option, but the additional bentonite increases the mud weight. This in turn increases the circulating pressure. There is a better alternative.

Adding QUIK-TROL® LV polymer to the circulating fluid can tighten the wallcake and reduce fluid loss to the formation without increasing the solids content. While QUIK-TROL LV polymer (the LV stands for low viscosity) has the same function as its water-well cousin, QUIK-TROL filtration control additive, the LV version does not significantly increase drilling fluid viscosity. Maintaining a relatively low viscosity helps keep the circulating pressure under control and minimises fluid losses.

Selecting lost circulation treatments

Preventative treatment is by far the most efficient and cost-effective approach to dealing with the potential for lost circulation. However, lost circulation is typically only dealt with after it has become a problem. Addressing lost circulation as soon as it is observed is critical to success in curing the problem. Adding lost circulation material (LCM) promptly can minimise the chances of escalating to larger problems. Preventing the erosion of smaller particles within the matrix of the formation can lead to much higher success ratios because the LCM has a greater chance of sealing smaller openings, rather than enlarged, eroded openings.

In formations where permeability increases to void conditions (for example, coarse gravels and cobbles or fractured rock) a more aggressive approach to maintaining circulation is required. Effective lost circulation materials include N-SEAL™ inert mineral fibre and/or DIAMOND SEAL® swellable crystal polymer. N-SEAL fibre can be mixed into a bentonite drilling fluid with no appreciable change in the fluid properties, but it can quickly block off formation pore throats. When circulated downhole, the N-SEAL material is drawn to the loss zones. The mineral fibres cannot enter the voids in the same way as the bentonite drilling fluid and will begin to build up, thereby slowing the loss of fluid from the bore and allowing bentonite to form a thin, slippery wall cake.

DIAMOND SEAL swellable polymer behaves in a similar manner. When wetted, the polymer bead swells to many times its original size. Added directly to a joint of pipe, a DIAMOND SEAL pill can be pumped down the rods and, like N-SEAL fibre, will be drawn to the loss zones. Once lodged in a fracture or void, the DIAMOND SEAL polymer will swell and slow the fluid loss, allowing the bentonite to build a wall cake. In extreme circumstances, both N-SEAL and DIAMOND SEAL treatments can be used together effectively to take advantage of their unique physical characteristics.

Optimise product selection and drilling practices

The selection of lost circulation treatment products should be based on several factors: environmental impact; appropriate sizing to pass through HDD pumps, valves and bit orifices; correct particle size distribution to ensure the range of formation pore throat sizes is addressed; sequence and timing of multi-product treatments; and determining the right combination of products for a specific formation.

Drilling techniques and tool selection also play major roles in avoiding lost circulation problems. For example, when engaging the rig’s fluid pump, slowly increasing its pressure to the desired operating flow rate helps prevent formation breakdown and subsequent losses. This is particularly important when the bore has been static for some time, allowing cuttings to settle around the reamer, drill string and potentially the product lines. A gentle start can save time and trouble later, much like gentle acceleration from a stop when driving on ice helps preserve traction. Once the traction is lost, it is difficult to regain, as is the integrity of a formation.

Selecting a reamer that allows minimal resistance to flow of the drilling fluids and cuttings can reduce lost circulation problems. If a reamer behaves as a plug in the bore path (such as a non-fluted barrel reamer in clay ground), common sense dictates that more back-pressure will be created and the chances of breaking down the formation, losing circulation, and having to deal with a ‘frac-out’ will increase. Controlling reaming speed can prevent the development of an excessively thick, cuttings-laden slurry that cannot flow easily from the bore path. Frac-outs are quite detrimental to profits and can be very time consuming. They can cause traffic problems and damage public relations for the drilling company and the HDD industry.

Conclusions

Preventing lost circulation is the best option, but if it is suspected or observed, it should be dealt with promptly. Otherwise it will likely become more difficult and expensive to fix. The selection of proper drilling fluid bentonite, additives, reamers, and good drilling practices can help the operator achieve success and profitability.

For more information on the use and selection of lost circulation materials or any questions associated with fluid selection for directional drilling, contact Robert Petrie of Baroid Industrial Drilling Products on 0414 557 917 or Robert.Petrie@halliburton.com