Seawater-based fracturing fluids provide greater flexibility when designing stimulation treatments for offshore environments by reducing delays in stimulation vessel scheduling and using an, essentially, unlimited supply of seawater, opposed to fresh water. Because seawater has a higher ionic strength than traditional fresh water, lower viscosity is observed from guar-based polymers. By comparison, the seawater can produce scaling precipitates when sulfate ions contact the calcium, barium, and strontium ions in the formation. The formed sulfate precipitates could lead to formation damage in the form of scaling, pore throats being clogged, and an overall reduction in production capacity. This paper presents an innovative polymer-based scale inhibitor (SI) to mitigate scaling issues from seawater with 4,000 mg/L sulfate when mixed with 200,000 mg/L total dissolved solids (TDS) formation water. From static bottle tests with seawater, 74% strontium sulfate inhibition was achieved at 300°F. To eliminate sulfate scaling issues, a nanofiltration (NF) membrane-based technique was used to filter seawater. The SI performance was tested by both static bottle tests at different mixing ratios and dynamic tube blocking tests at 300°F. Following static bottle tests, the key ion concentrations were determined using inductively coupled plasma (ICP) spectroscopy. Zero scaling was achieved by adding 300 mg/L SI. The fracturing fluid was prepared using a zirconate-crosslinked derivatized guar. The rheology of the fracturing fluid was measured with and without SI to investigate its influence on crosslinking and proppant carrying capacity at 300°F. High magnesium in the seawater consumes hydroxide ions and affects pH control. To obtain a stable fracturing fluid, the pH was optimized to minimize magnesium precipitation. Coreflooding experiments confirmed the addition of SI does not cause additional formation damage. Using NF-seawater in conjunction with proven SIs can provide effective treatment of seawater-based-fracturing fluids. The separation technique (NF) and chemical solution (SI) complement each other to neutralize scaling issues completely. This technique minimizes freshwater usage in fracturing, which can enable significant savings for offshore operations time and cost.

Water flooding using seawater is a major oil filed operation implemented in different carbonate and sandstone reservoirs to maintain reservoir pressure and enhance oil recovery. However, due to the high sulfate content in the injected seawater, significant calcium, barium and strontium sulfate scale deposition can occur and cause severe formation damage. Typically, scale inhibitors are applied in the field to prevent the formation of calcium sulfate scale where they act either as chelating agents to form a soluble complex, as threshold inhibitors, which block the development of the supercritical nuclei or as retarders of the growth of the calcium sulfate crystals. The objective of this study is to evaluate the effectiveness of different types of scale inhibitors to prevent the formation of calcium sulfate scale during the injection of seawater into high-salinity bearing carbonate reservoirs.Jar testing and SoftPitzer TM software were used to investigate the calcium sulfate precipitation due to the mixing of high calcium-content (37,000 mg/L) formation water and high sulfate-content (4,000 mg/L) seawater. This investigation was conducted on different mixtures of formation water and seawater at reservoir temperature of 155°F. Different types of scale inhibitors were tested to prevent the scaling of calcium sulfate in mixtures of seawater and formation water. The compatibility of these scale inhibitors with calcium ions present in formation water and also their effectiveness in preventing CaSO4 scaling were investigated using bottle testing. In addition, coreflood experiments were conducted to determine the adsorption behavior of these inhibitors in carbonate rocks.Results showed that the application of two scale inhibitors (Acrylic homopolymer-based) can successfully mitigate calcium sulfate scale formation in seawater/formation water mixtures up to 155°F. Each one of these two scale inhibitors has its own effective minimum inhibition concentration (MIC). In addition to these results, the paper provides insights into the adsorption behavior of these inhibitors in carbonate rocks and how this affects their performance.

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Acid treatments of carbonate formations are usually carried out using mineral acid (HCl), organic acids (formic and acetic), mixed acids (HCl-formic, HCl-acetic), or retarded acids. The major challenges when using these acids are their high corrosivity, fast reaction rate and health hazard. The improvement in corrosion inhibitors makes the use of a strong acid as high as 28 wt% HCl possible. The acid reaction rate can be controlled by increasing acid viscosity using gelling agent or emulsifying acid droplets, acid-in-diesel emulsion. While the issues of stimulation acids reaction and corrosion rates are relatively controlled, these acids health hazard rating of 3 by the National Fire Protection Association (NFPA) is a major concern. A health hazard rating of three is defined as an extreme danger where short exposure could cause serious injury. An acid replacement chemical that has no or minimum health hazard rating while still has the ability to dissolve carbonate rock would be a major forward step in stimulation technology. This paper presents the results of the study conducted on a synthetic stimulation acid (Syn-A) chemical, with health hazard rating of one and dissolving power similar to 15 wt% hydrochloric acid (HCl). An extensive experimental scheme including: thermal stability, dissolving power, acidity, compatibility, corrosion rate & inhibition and coreflooding on carbonate formation core plugs was conducted. The Syn-A was found to be thermally stable with similar dissolving power to 15 wt% HCl and lower corrosion rate. In addition, the Syn-A developed a breakthrough on core plugs with an average pore volume (PV) of 2.7 and approximately 3 folds increase in permeability.