There is a perception by some operators and regulators that water-based drilling fluids (WBDFs) are better environmental choices than nonaqueous drilling fluids (NADFs) and that enhanced WBDFs may someday deliver the performance of NADFs. However, NADFs are still unrivaled for providing formation stability, lubricity, temperature stability, solids tolerance, hydrate suppression, and protection from dangerous gases and for meeting other operational challenges when drilling today’s difficult wells.

When WBDFs consist of simple formulations useful in shallow-hole drilling situations, they may be environmentally preferable. But when drilling deeper, more difficult wells, the more complex WBDF concoctions formulated may not be as environmentally acceptable.

It is true that the continuous liquid phase or carrier fluid of NADF is the nonaqueous base fluid (NABF), while the continuous liquid phase of WBDF is water or brine. However, using TWMA’s thermomechanical desorption processing solutions – the TCC RotoMill or TCC RotoTruck – more than 99% of the NABF can be successfully recovered from the NADF cuttings waste stream. With the advent of TWMA’s at-site thermomechanical desorption capability, the solution to both operational and environmental challenges often is to drill with NADF and treat the drill cuttings’ outfall to remove and recover the NABF. By coupling NADF and at-site treatment, operators are able to recoup drilling waste treatment costs since they can recover and reuse undamaged NABF at site.

Not known or considered in detail by many operators or regulators is the potential of this emerging at-site cuttings treatment capability to enable optimal operational performance while simultaneously reducing the environmental footprint.

Volume comparison

Of concern is the extremely large volume of increasingly complex WBDF required compared to the much smaller volume of NADF to perform the same drilling operation.

As an example, consider the use of WBDF to drill 3,048 m (10,000 ft) of 8.5-in. hole – a gauge hole volume of 702 bbl. Solids removal efficiency for WBDF varies widely, but 70% is considered above average. This means that of the formation drilled, typical solids removal equipment on the rig may remove only seven-tenths or less. This is because even the most high-tech WBDFs are dispersive to many formation solids. The resultant 30% of fine-drilled solids that remain entrained in WBDF must be adjusted to an acceptable solids content – typically to a target equal to or less than 6% drilled solids by dilution.

The dilution volume required to dilute the 30% of drilled solids down to the 6% drilled solids tolerated in WBDF will generate 3,509 bbl of new WBDF dilution. This represents about 5 bbl of dilution generated for each barrel of gauge hole drilled. Most of this large volume becomes waste.

To meet the ever-increasing challenges of today’s highly complex well architecture, organic materials including hydrocarbons may be required and used to improve the lubricity of WBDF. With a treatment of 5% of an organic lubricant added and maintained in the drilling fluid, the volume of lubricant added to the 3,509 bbl of WBDF generated is 175 bbl. With a lack of practical and effective solutions to remove entrained lubricant from WBDF waste streams, this volume of organic materials will ultimately end up in the environment.

Also, in attempts to improve inhibition, various salts and other materials are common additives to WBDF. But similar to the situation with organic materials, current methods for removing salts from WBDF waste streams are ineffective or impractical. Therefore, even if clean liquid could be effectively recovered, it could still have high salt content, usually rendering it unacceptable for reuse. Offshore, salt content in WBDF waste discharge is probably of slight environmental consequence, but onshore it has significant implications for groundwater and soil contamination.

Due to their inhibiting capability, NADFs often achieve solids removal efficiencies equal to or greater than 90%, so only the 10% of solids not removed would require dilution to solids-content specifications. Drilling the same 702 bbl of formation with NADF necessitates only 1,170 bbl of dilution – or only 1.7 bbl of dilution per barrel of hole drilled.

For example, 702 bbl of cuttings multiplied by 0.1 equals 70.2 bbl of cuttings not removed by solids removal equipment; 70.2 bbl of cuttings not removed divided by 6% solids targeted for NADF would generate only 1,170 bbl of NADF dilution.

Approximately 702 bbl or more of NADF will accompany the wet NADF formation cuttings from the solids removal equipment (a one-to-one ratio of NADF to cuttings is common), and another 702 bbl of NADF will be required to fill the new hole drilled. So 1,404 bbl of NADF, or 2 bbl of NADF per barrel of formation drilled, are required to replace NADF lost with cuttings from the solids removal equipment and to fill the new hole drilled. This is more than enough to dilute unremovable drilled solids to specification.

Reuse solutions

Unlike much of the WBDF generated and then disposed of with cuttings, NABF lost on cuttings can now be essentially totally removed and recovered for reuse. Thermomechanical desorption processing technology can reduce NABF content on NADF cuttings outfall to less than 1%, and this low content of NABF is proven to have a negligible impact on the environment. This is even more true if NADFs are compounded using an ultra-low aromatic content NABF.

In the preceding comparative examples there would be 150 bbl less of organic material and usually much less salt from NADF cuttings waste after treatment than from inhibitive WBDF wastes.

Thermomechanical desorption processing technology can be positioned on onshore or offshore rigs to suit individual projects. Whether onshore, offshore, or in remote locations, this solution can take advantage of the drilling performance capabilities of NADF while simultaneously improving environmental performance and recovering NABF for reuse.

If a drilling operation would benefit from the use of NADF, the question from operational, environmental, and overall cost perspectives is, “Why not couple NADF and at-site thermomechanical desorption to improve drilling performance, reduce environmental footprint, and recover valuable NABF?”