Going Beyond Traditional Mud Recycling Systems

Good solids control programs are often ignored in today’s mud rotary drilling process because basic principals are misunderstood or lack of training amongst rig operators. Today’s contractor uses traditional shale shaker technology and/or hydrocyclone methods for drilling fluid recovery and solids waste disposal, but few have added a decanter centrifuge using amphoteric-style chemistry to create a true closed-loop waste management system.    

In an effort to demonstrate the importance of chemistry in solid/liquid separation technology, KEMTRON Technologies completed a field trial that demonstrated the benefits of chemically-enhanced mechanical separation. This article highlights KEMTRON’s success in formulating a hybrid drilling fluid that exhibited formation-appropriate drilling properties, while providing ultra-fine solids removal from the drilling fluid. This ultimately led to improved rate of penetration and a 100 percent closed-loop waste management system.

Drilling Through Reactive Clays

For several years, KEMTRON has worked with various customers throughout Barnett Shale Region using a variety of horizontal directional drilling (HDD) rigs. Soil conditions in and around the area are predominantly limestone, gravel, montmorillonite clay and mudstone. The Barnett Shale play has plagued drilling contractors trying to maintain low mud weight and viscosity. As mud weight and viscosity increase, drillers experience slowed bit penetration rates, mud loss/frac-out, pipe torque/drag and bit balling. This taxes conventional solids control equipment and the capabilities of even the best decanter centrifuges.

Prior to the study, KEMTRON collected samples from other projects within the vicinity. Each sample was subjected to a Particle Size Distribution (PSD) Analysis. The results of this analysis painted a bleak picture: The mean particle size was 5.6 microns, the median particle size was 3.7 microns, and mode particle size was 5.4 microns. Ultimately, 85 percent of all solids shown were less than 10 microns and 100 percent were less than 50 microns.

Given the nature of conventional drilling fluid reclamation systems, even with the support of high-efficiency desilter hydrocyclones, the best cut point that can be practically achieved would be 25 microns. Given the rig’s situation, this is just 15 percent of all solids present (Assuming that the entire solids/liquid phase from the hydrocylone underflow was discarded from the drilling mud circulating system). Instead, the underflow from the hydrocylone’s liquid phase contains fine solids that pass thru the pore openings of the screens. The result is silt-size particles that continue to be re-circulated and ground up into ultra-fine/colloidal size particles that can only be removed by hauling off-site or chemically enhanced solid/liquid separation using a decanter centrifuge.   

Jobsite Description

Specific to the jobsite, a 330×500 rig was to drill a 26-in. diameter borehole with 4,700 ft of lateral for 16 in. of pullback pipe located beneath Interstate 20 and a residential community in Benbrook, Texas. Additionally, there were three shorter bores to be completed. Each of the shorter bores was completed with a 100×120 drilling rig and used KEMTRON’s Tango 500T as the primary mud recycling system. Along with the primary mud system, a triplex pump package, polymer hydration/dosing system, and KEMTRON trailer-mounted KT-1448 decanter centrifuge were located approximately 200 ft behind the rig in a linear arrangement.

Based on the PSD analysis previously conducted, minimal solids removal would have occurred using traditional linear motion shaker and hydrocyclone technology. This would have allowed a constant build-up of solids, ultimately resulting in whole mud dumping, or haul-off, and preparation of new drilling fluid. As such, KEMTRON’s KT-1448 centrifuge was installed to draw fluids from the clean tank in the mud recycling system and further remove ultra-fine /colloidal sized particles. Though the sole use of the decanter centrifuge would have provided better results than conventional solids control equipment, its efficiency would have been significantly hampered due to the nature of the reactive clays. To achieve optimum solid/liquid separation, KEMTRON utilized two specialized dewatering polymers to flocculate the colloidal solids prior to the centrifuge.  

Jobsite Dewatering

Amphoteric chemistry takes advantage of both cationic and anionic functional sites. When used properly in drilling fluid, the results can be significant, including shale inhibition, well-bore lubricity, and improved liquid/solids separation efficiency from both the hydrocyclones and the decanter centrifuge. The primary goal of a closed-loop dewatering system is to control mud weight throughout the bore and eliminate spoiled drilling fluid disposal.

The first step is to make the reactive clays, “less reactive.” This is known as clay inhibition. The inhibition of clays is achieved with the addition of KEMTRON’s CLAY-KATCH, which causes cationic exchange with the potential reactive sites found between reactive clay layers. By satisfying these reactive sites with CLAY-KATCH, the clays become hydrophobic (i.e. water repulsive) and water is unable to attach to the clays. The second step is to protect the inhibited clays from physical break-down. An anionic emulsion-grade friction-reducing polymer, named KEM-VIS HD, makes up the anionic group along the clay sites to form protective hydration layers (i.e. steric barriers) to prevent further hydration and degradation of the shale particles. In effect, CLAY-KATCH creates a bridge between each clay layer, where KEM-VIS HD creates a protective wrap that encapsulates the bridged clay layers therefore being resistant to the centrifugal shear forces. The primary flocculant used was KAN-FLOC E50 and was introduced into the centrifuge’s PC pump suction to allow sufficient time for a chemical reaction to occur before being subjected to the centrifuge

As the pipe was pulled into the hole, KEMTRON began to draw from the primary mud system and restored the mud back to clear water. This water was fed into the clean tank and intermittently back to the primary mud system to dilute the down-hole mud, as well as make-down KAN-FLOC E50 working solutions. As a result, KEMTRON maintained the primary mud system tank and earthen pit volumes for 5.5 hours. This was accomplished through the selective use of the onsite frac tanks used to collect treated volume on an as-needed basis. Following this period, KEMTRON began to eliminate volume exclusively into the dirty water tanks.

Dewatering Economics

Based on the rig’s experience, the typical daily costs are summarized in Table 1. During the course of the trial performed by KEMTRON, the daily operating costs ranged from $200 to $900, with an average operating cost of $350 per day. This equated to an average daily savings of approximately $1,060 (See Table 2).
Even more important than the cost-savings highlighted above, was the fact that the rig was able to reuse the cleaned drilling fluid on their next job, creating even greater savings and environmental benefit. Based on this particular case study, the closed-loop treatment system reclaimed approximately 70 percent of the originally prepared drilling fluid.

Taking the time to match the right technology with the right chemistry and process, four bores were completed with no spoiled fluids being disposed. In addition, the customer was fully prepared to start the next job without preparation of a new batch of drilling mud. Though there are initial capital expenses required to create a closed-loop or zero haul off job, the savings generated will generally pay for those costs within the first year of operation, even in the most inhospitable formations.

Michael Rai Anderson, P.E., is president of KEMTRON Technologies.