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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 directional drilling 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.