NOTICE CONCERNING COPYRIGHT RESTRICTIONS

This document may contain copyrighted materials. These materials have been made available for use in research, teaching, and private study, but may not be used for any commercial purpose. Users may not otherwise copy, reproduce, retransmit, distribute, publish, commercially exploit or otherwise transfer any material.

The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted material.

Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specific conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research." If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement.

This institution reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law.

GRC Transactions, Vol. 30, 2006

Recent Developments: Drilling Fluids for Geothermal Applications

John D. Tuttle Sinclair Well Products & Services [email protected]

Keywords fluids technology have continued to develop and add value to Drilling, drilling fluids, lubricity, torque, drag, geothermal drill- the drilling operation. Currently, with energy costs increasing ing, mud, drilling, potassium, rheology, filtrate control, fluid dramatically and the global demand for energy expanding at loss, TORKease, SHALease, Polythin HT, thinner, deflocculant, near-exponential rates, cost-effective specialty additives are lost circulation, stuck pipe, inhibition, formation damage becoming more readily available for addressing the unique requirements of geothermal drilling activities.

ABSTRACT Drilling Fluids, Viscosity and Rheology Control— Polymer Viscosifiers The scope of this paper is to introduce some of the current technical trends in geothermal drilling fluids, with special focus Viscosity control in geothermal drilling fluids includes the on those systems and additives which can be used in some concepts of increasing and decreasing viscosity and yield point, of the more extreme geothermal drilling environments (500- apparent viscosity, and control of high temperature gelation. 600°+F, 260-316°C). Benefits to Geothermal Operators have Viscosity control is a crucial characteristic of geothermal drill- included increased penetration rates, improved hole stability, ing fluids to promote good hole cleaning and reduce potential reduced drilling ‘trouble-time’, enhanced control of lost circu- problems such as surge and swab pressures, excessive buildup lation, and reduced drilling mud and overall well costs. of wallcake, differential sticking tendencies, lost circulation, and other potential problems. For years, the primary recom- Geothermal Drilling History, mended viscosifier for geothermal drilling was API grade Drilling Fluids Systems Bentonite (Sodium Montmorillonite). Although API Bentonite is still used extensively to provide Most early (circa 1970’s) geothermal drilling fluids systems desired particle size distribution and rheological properties (as were holdovers from oil and gas drilling; high pH, high solids well as to build ‘cheap’ muds for fighting extensive or remote lignite systems, KCl systems[1,2], lime-morrex, modified lig- lost circulation problems), various temperature-stable, poly- nosulfonate systems, unproven polymer and exotic dispersed mer-based viscosifiers have been developed in recent years as systems, and others. Drilling fluids additives were generally not cost-effective replacements for Bentonite. These polymer- suited for controlling properties at elevated temperatures over based additives are especially useful for high viscosity sweeps, extended periods of time, nor were they specifically designed to enhance existing drilling fluids’ carrying capacity and Yield for the various contaminates and operational requirements Point, and for use in creating low solids geothermal drilling of geothermal conditions. With the introduction of technical and workover fluids. A secondary benefit of the use of the improvements in the mid to late 1980’s, the development and polymer viscosifiers is the encapsulation of drill cuttings, thus modification of specially prepared polymer additives, and the leading to improved separation efficiency by rigsite solids renewed interest in geothermal drilling activities, geothermal removal equipment. drilling fluids additives became more efficient, as well as more readily available and cost-effective. Drilling Fluids, High Temperature Thinning The past 20 years of geothermal resource development and Rheology Control and drilling has continued to be marked by inconsistency and a general slump; however, technical improvements in solids Viscosity control is a crucial characteristic of geothermal removal equipment, hydraulics and bit design, and drilling drilling fluids, to promote good hole cleaning and reduce po-

315 Tuttle tential problems such as surge and swab pressures, excessive complex polysaccharide system utilizing low concentrations buildup of wallcake, differential sticking, lost circulation, and of Bentonite, SSMA and synthetic interpolymer deflocculants, other potential problems. and low molecular sulfonic acrylamide copolymers provide Viscosity and gellation control become difficult and critical control of HTHP fluid loss while providing shale stability at on geothermal wells encountering BHTs in excess of 350°F BHT in excess of 600°F. (177°C), due to thermal degradation (failure) of conventional Recent concerns for formation stability and minimizing mud thinners. Recent drilling fluids research and development mud damage has focused on mechanical and chemical meth- efforts to address this problem have resulted in the develop- ods; specifically, significant developments have been made ment of a new generation of blended polymeric thinners and on additives that can provide enhanced wellbore protection deflocculants[3]; proprietary blends of low molecular weight through the use of active electrolytes. Potassium and Alu- co-polymers and modified starch derivatives, can be used in minum cations have long been known to provide shale and low concentrations to not only control the circulating viscos- clay stability (especially in illites and mixed layer clays) based ity and yield point of a geothermal drilling fluid, but is also on the nature of their ionic structure[4], and potassium and highly effective in counteracting the gellation tendencies ob- aluminum based mud additives[7,8] are used regularly in O&G served with most Bentonite drilling fluids subjected to elevated drilling and workover operations to provide formation stabil- temperatures over time. Funnel viscosities following bit trips ity and to enhance well productivity. The molecular size of are often only slightly higher than circulating viscosities, when potassium (K+) and aluminum (Al+3) is such that they fit very utilizing these innovative polymer thinners to control viscosity well within the crystal lattice structure of hydratable clays and high temperature gelation. and, once in place, are relatively stable in the clay structure. This chemical property of potassium and aluminum provides wellbore stability by ‘locking up’ the clay structure, thus not Drilling Fluids, Lubricity allowing water to interact with the clays and promote swelling Lubricity is a significant issue affecting geothermal wells, and sloughing; this inhibition quality results in improved hole especially during the implementation of or conditions, reduced clay swelling, shale control, fewer wiper redrilling programs, or during workover operations in existing trips to maintain hole conditions, and other direct benefits. holes. In the past, diesel and various ‘snake oils’ were utilized Potassium and Aluminum based additives that have ap- to attempt to free stuck pipe and reduce excessive torque and plication in geothermal drilling operations are becoming more drag levels, with limited effectiveness due to the products’ available and acceptable, including the use of Caustic Potash general instability at high temperatures and/or incompatibil- (KOH, as a pH adjuster), proprietary Aluminum Silicate ity with other geothermal drilling fluids additives. Very few blends (effective shale stabilizer and inhibitor), and others. additives have been time-proven (with sufficient laboratory The successful enhancement of geothermal drilling operations and field success) to provide reliable lubricity in geothermal through the use of cationic protection is well documented, drilling operations. and the trend of using these specialty additives is expected to TORKease[5] is an environmentally friendly, non-oil and continue as geothermal drilling projects encounter more critical non-damaging lubricant that has been in use since the mid- drilling environments. 1970s, and reintroduced to the geothermal industry with 3Al Si O (OH) + 2K+ → 2KAl Si O (OH) + 3H O great success. A modified potassium (K+) based lubricant, 2 2 5 4 3 3 10 2 2 + TORKease exhibits excellent temperature stability (>650°F, Kaolinite (Stable) + 2K → Illite (Hydratable) +2H+3H2O 343°C), and has proven itself in geothermal drilling and work- over operations worldwide. TORKease is especially effective in Potassium Additives Can Actually Destabilize Some Clays reducing rotary torque levels in deviated geothermal wells. This (Kaolinites) additive can be used in small quantities on an as-needed basis to reduce specific torque and drag problems, or carried in an appropriate concentration in the geothermal fluid as a preven- Drilling Fluids, Lost Circulation tative additive. TORKease is compatible with all water based geothermal drilling fluids (including foam/air systems). Geothermal drilling operations often encounter significant whole mud losses. Indeed, many desirable producing zones on Drilling Fluids, Fluid Loss, Formation Stability geothermal projects are vugular loss zones that will allow the and Inhibition unrestricted flow of water and/or steam. These zones will also allow the invasion of whole drilling mud, generally observed Recent R&D efforts have generated significant improve- as lost circulation during the drilling process. Whole mud ments in the inhibitive qualities of geothermal drilling fluids. losses can range from seepage losses to total loss of entire mud When drilling projects encounter geologic formations that systems. For this reason, effectively drilling and protecting are characterized by hole deterioration, swelling, and general any potential production zones from mud damage (during lost instability, it is often the responsibility of the drilling fluid to circulation) becomes a critical aspect of successfully drilling provide adequate inhibition and hole stability. and completing geothermal wells. Various geothermal mud systems are being formulated to Historically, lost circulation has been combated with provide effective fluid loss and shale stability. For example, a conventional LCM (lost circulation materials) additives and

316 Tuttle cement. Excessive whole mud losses have often promoted the been utilized to drill and redrill/workover geothermal pros- desire for many Operators to drill blind, or with air/foam, to pects for many years. Additives required for this application minimize formation damage, reduce mud costs, and to improve include a synthetic polymer viscosifier or high temperature drilling rates. foamer, a pH adjuster (generally Caustic Soda or Caustic Various potential lost circulation tools have been developed Potash is recommended), a corrosion inhibitor/oxygen scav- over the years[6], with limited success; thus geothermal opera- enger, and an appropriate high temperature lubricant such tors have generally returned to using conventional LCM and as TORKease. modified cement plugs for massive geothermal lost circulation problems. In determining the selection of an appropriate lost circula- Conclusions tion additive(s), it is important to first determine the eventual R&D efforts continue to develop new geothermal drill- use of the zone. If it is an unproductive loss zone, then it ing fluids that can provide temperature stability, maximum can be sealed forever, if it is a desired producing zone, then flexibility, and cost-effectiveness. Operators can now design temporary sealing may be preferred. For non-producing loss a ‘project-specific’ geothermal drilling fluids program, utiliz- zones, readily accessible and cost-effective conventional LCM ing viable geothermal specialty additives to satisfy the unique and cement may be appropriate treatment methods. Often, requirements of each project. due to the massive losses involved, either air/foam drilling or Worldwide geothermal development will increase with repetitive cementing may be required to effectively seal these the recent emphasis on renewable energy sources, and in light loss zones. of recent instability in the world energy market. Technical For sealing potential producing zones, a non-damaging lost research will continue to develop new drilling fluids additives circulation material is preferred. Cottonseed (hulls or pellets) with superior temperature stability and characteristics. At and sawdust have been shown in a laboratory environment to the present time, temperature-stable additives are available to be almost totally degradable by temperature over time; how- satisfy the needs of geothermal drilling projects on a global ever, recent field information has indicated that both additives basis. Cost savings of 15% to 50%, as well as a significant are being recovered from geothermal wells in their native or reduction in drilling times and trouble costs, may be realized partially degraded state, thus implying that the degradation by utilizing a methodical approach to the selection and proper process has not been complete and that the additive may application of geothermal drilling fluids. have been creating blockage of the producing interval over an extended period of time. This development may prompt the operator to preferentially drill producing intervals with References air/foam, when possible, using as little solids (Bentonite, etc.) 1. Mondshine, T.C.: “A New Potassium Based Mud System”, Journal and LCM additives as possible. of Technology (Sept. 1973) 2. Listi, R.: “Potassium Lignite : Related Problems and their Solutions”, (May, 1999) Drilling Fluids, Clay Free Drilling Fluids Option 3. Sinclair Well Products and Services: Technical Literature, POLYTHIN Bentonite-free drilling fluids have gained in acceptance in HT, NARLOTHIN, Sawdust, Cottonseed Hulls, SINPAC HT, K- Lignite, Gilsonite, API Bentonite, Caustic Potash (KOH) recent years. The primary advantage of drilling with a ‘Ben- tonite-free’ system is the minimization of damage resulting 4. Wagh, AS; Ramkumar, N; McDaniel, RL: “Aluminum Phosphate Cements Help with Deep, High-Temperature Wells”, O&G Journal from invasion of fines and bentonite into the producing zone, (May 2006) as well as the maintenance of minimum mud densities and reduced chances of lost circulation problems. 5. DSC Incorporated: Technical Literature, TORKease In general, if geologic information suggests that adequate 6. Geothermal Hot Line: “Rigid Foam Appears to be Good Lost Cir- formation stability can be maintained without the formation culation Material” (July, 1985) of a firm mechanical wallcake, then a ‘Bentonite-free’ system 7. : Technical Literature, Alplex can be designed that will provide other desired fluid properties 8. Capacho, C; , MR; Osorio, J; Kenny, P: “Replacing Potassium (rheology control, lubricity, alkalinity, filtrate control, etc.). with Aluminum Complex Overcomes Wellbore Instability Problems in Indeed, ‘Bentonite-free’ air/foam geothermal systems have Kaolinitic Shales in South America”, AADE-04-DF-HO-17 (2004)

317 318