Fighting Scale—Removal and Prevention Imagine an oilfield menace that can smother a productive well within 24 hours. The buildup of scale inside wellbores does exactly that, causing millions of dollars in damage every year. New understanding of scale accumulation is allowing production engineers to predict when scale formation will occur, so that adverse operating conditions can be prevented with new inhibitor techniques. New tools are also available to blast scale away from casing and tubulars. Mike Crabtree Aberdeen, Scotland David Eslinger Tulsa, Oklahoma, USA Phil Fletcher Matt Miller Sugar Land, Texas, USA Ashley Johnson Rosharon, Texas George King BP Amoco Corporation Houston, Texas For help in preparation of this article, thanks to Andrew Few production problems strike fear into the oversaturated with scale components when two Acock, Houston, Texas, USA; Willem van Adrichem and hearts of engineers the way scale can. Scale is incompatible waters meet downhole. Whenever Warren Zemlak, Sugar Land, Texas; Mike Barrett, Steve Davies, Igor Diakonov and Jon Elphick, Cambridge, England; an assemblage of deposits that cake perfora- an oil or gas well produces water, or water injec- Leo Burdylo, Sugar Land, Texas; Ingrid Huldal and Raymond tions, casing, production tubing, valves, pumps tion is used to enhance recovery, there is the pos- Jasinski, Aberdeen, Scotland; and Scott Jacobsen, Houston, Texas. and downhole completion equipment, thereby sibility that scale will form. In some areas, such Blaster Services, Bridge Blaster, CoilCADE, CoilLIMIT, Jet clogging the wellbore and preventing fluid flow. as the North Sea and Canada, where entire Advisor, Jet Blaster, NODAL, ScaleFRAC, Sterling Beads and StimCADE are marks of Schlumberger. Hipp Tripper is a Scale, just like the scale found in home plumbing regions are prone to scale, it is recognized as one mark of Baker Oil Tools; Hydroblast is a mark of Halliburton; or tea kettles, can be deposited all along water of the top production problems. and RotoJet is a mark of BJ-NOWSCO. paths from injectors through the reservoir to Scale can develop in the formation pores near 1. Brown M: “Full Scale Attack,” REview, 30 The BP Technology Magazine (October-December 1998): 30-32. surface equipment. Most scale found in oil the wellbore—reducing formation porosity and fields forms either by direct precipitation from permeability. It can block flow by clogging perfo- the water that occurs naturally in reservoir rocks, rations or forming a thick lining in production tub- or as a result of produced water becoming ing (above). It can also coat and damage downhole completion equipment, such as safety 30 Oilfield Review > Scale in production tubing. Calcium carbonate Sources of Scale sandstone reservoirs usually contains an abun- scale growth in production tubing here obstructs In oilfield scale, water is of primary importance, dance of divalent calcium [Ca+2] and magnesium more than 40% of the flowing area of the tubing since scale will occur only if water is produced. [Mg+2] cations. Sandstone formation fluids often and prevents access to lower sections by well- +2 +2 remediation tools. Water is a good solvent for many materials and can contain barium [Ba ] and strontium [Sr ] cations. carry large quantities of scaling minerals. All natu- In reservoir fluids total dissolved solids can reach valves and gas-lift mandrels. The effects of scale ral waters contain dissolved components acquired 400,000 mg/L [3.34 ppg]. The precise composition can be dramatic and immediate: in one North Sea through contact with mineral phases in the natural has a complex dependence on mineral diagenesis well in the Miller field, engineers were shocked to environment. This gives rise to complex fluids, rich and other types of alteration encountered as for- see production fall from 30,000 B/D [4770 m3/d] in ions, some of which are at the saturation limit mation fluids flow and mix over geological time. to zero in just 24 hours.1 The costs can be enor- for certain mineral phases. Seawater tends to Scale begins to form when the state of any mous also. Curing scale problems costs the indus- become rich in ions that are by-products of marine natural fluid is perturbed such that the solubility try hundred of millions of dollars per year in lost life and water evaporation. Ground water and limit for one or more components is exceeded. production. Until recently, ways to treat the water in the near-surface environment are often Mineral solubilities themselves have a compli- problem were limited and sometimes ineffective. dilute and chemically different from deep subsur- cated dependence on temperature and pressure. When scale forms, a fast, effective removal tech- face water associated with gas and oil. Typically, an increase in temperature increases nique is needed. Scale-removal methods involve Deep subsurface water becomes enriched in the water solubility of a mineral. More ions are both chemical and mechanical approaches, each ions through alteration of sedimentary minerals. dissolved at higher temperatures (below). with its own niche—depending on the location of The water in carbonate and calcite-cemented Similarly, decreasing pressure tends to decrease the scale and its physical properties. Some mineral scales, such as calcium carbon- Mineral solubilities versus temperature ate [CaCO3], can be dissolved with acids, while most others cannot. Sometimes tar-like or waxy coatings of hydrocarbons protect scale from 1000 100 chemical dissolvers. Accumulated solid layers of Halite impermeable scale can line production tubing, 10 Gypsum sometimes completely blocking it, and are less lbm/bbl 1 Strontium Anhydrite easily removed. Here, mechanical techniques or 0.1 sulfate chemical treatments are traditionally used to cut Solubility, Solubility, 0.01 through the scale blockages. Nevertheless, com- Calcite Barite 0.001 mon hard scales, such as barium sulfate [BaSO4], 0.0001 Siderite are extremely resistant to both chemical and 77 120 170 210 260 300 mechanical removal. Before recent developments Temperature, °F in scale-removal technology, operators with hard- scale problems in their production tubing were Minerals solubilities versus pressure often forced to shut down production, move in workover rigs to pull the damaged tubing out of 0.004 the well, and either treat for scale at the surface 0.0035 14,000 psi or replace the tubing. 0.003 lbm/bbl In this article, we review the physical causes 4, 0.0025 7000 psi SO of scale buildup during oil production. Knowing a 0.002 1500 psi the conditions that lead to scaling and when and 0.0015 14.5 psi where it occurs helps in understanding how to 0.001 remove scale and in designing intervention treat- Dissolved B 0.0005 ments to restore long-term well productivity. 0 70 120 170 220 270 Then, we survey the chemical and mechanical Temperature, °F techniques used in scale removal—including the latest developments in jetting techniques—and Minerals solubilities versus salinity examine the strengths and limitations of each approach. Finally, we look at advances in water 5 x 10-3 treatments and new inhibitors that help control 25°C the delicate chemical balance to prevent scale 4 x 10-3 150°C precipitation from recurring. 3 x 10-3 > Mineral solubilities have a complex dependency 2 x 10-3 on many variables including temperature (top), 250°C pressure (center) and salinity (bottom). 1 x 10-3 Strontium Sulfate solubility, mol/L Strontium Sulfate solubility, 01 23456 Seawater NaCl mol/L Autumn 1999 31 solubilities and, as a rule-of-thumb, the solu- An additional complexity is the solubility of grow by ions adsorbing onto imperfections on the bility of most minerals decreases by a factor carbonate minerals in the presence of acid gases crystal surfaces—extending the crystal size. The of two for every 7000-psi [48-MPa] decrease such as carbon dioxide [CO2] and hydrogen sulfide energy for seed crystal growth is driven by a in pressure. [H2S]. Carbonate solubility increases as fluid acid- reduction in the surface free energy of the crys- Not all minerals conform to the typical tem- ity increases, and CO2 or H2S at high pressure tal, which decreases rapidly with increasing perature trend; for example, calcium carbonate supply significant acidity. Consequently, forma- radius after a critical radius is exceeded. This shows the inverse trend of increasing water sol- tion waters, in contact with both carbonate rock implies that large crystals favor continuing crys- ubility with decreasing temperature. The solu- and acid gases, can be rich in dissolved carbon- tal growth, and also implies that small seed crys- bility of barium sulfate increases by a factor of ate. This trend has a complex nonlinear depen- tals may redissolve.2 Thus, given a large enough two in the temperature range 25°C to 100°C dence on brine composition, temperature and the degree of supersaturation, the formation of any [77° to 212°F] and then decreases by the same pressure of the gas above the liquid phase; this seed crystal will encourage an increase in the magnitude as temperatures approach 200°C gas pressure effect is orders of magnitude greater growth of scale deposits. The seed crystal, in [392°F]. This trend is itself influenced by the than the normal effect of pressure on the solubil- effect, is a catalyst for scale formation. background brine salinity. ity of a mineral. Generally, as pressure falls, CO2 Crystal growth also tends to initiate on a pre- leaves the water phase causing the pH to rise— existing fluid-boundary surface, a process called leading to calcite scale formation. heterogeneous nucleation. Heterogeneous nucle- Homogeneous nucleation ation sites include surface defects such as pipe Forming Scale surface roughness or perforations in production Supersaturation Although the driving force for scale formation liners, or even joints and seams in tubing and may be a temperature or pressure change, out- pipelines.