A Comparison Between Seawater-Based and Freshwater-Based Fracturing Fluids
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Despite the lack of freshwater resources in the Arabian peninsula, fresh water is still used in unconventional-resource operations there. Seawater, however, is plentiful and could substitute for fresh water. The high salinity of seawater raises many chemical challenges in developing design criteria for fracturing fluids. This paper examines the chemistry of developing seawater-based fracturing fluids using two types of polymers as gelling agents and compares results to existing fresh-water-based-fracturing-fluid data under different conditions.
The oil and gas industry faces many challenges, including the availability of fresh water for making fracturing fluids, especially in the Arabian peninsula and other arid regions.
Using seawater to make fracturing fluid can help address several obstacles and reduce costs. However, using seawater to make fracturing fluids also poses several new challenges. The high salinity of seawater and its propensity for scaling, compared with fresh water, make it crucial to consider different factors and chemical properties that influence the process of developing fracturing fluid.
This paper presents issues that can arise when using seawater as a base to develop fracturing fluids with two different types of guar as the viscosifying agent—hydroxypropyl guar (HPG) and carboxymethylhydroxypropyl guar (CMHPG).
Ion Composition of Seawater
The high concentration of sulfate in seawater affects ion pairings in seawater. Sulfate can form ion pairings with strontium, calcium, and magnesium. Fifty percent of the sulfate present in seawater is free ions that can interact with polymer chains. Mixing, evaporation, and precipitation can change with ocean depth or location, which affects the salinity of seawater around the world and in stratification within a specific region. The major-ion composition in open waters, however, is almost always the same because the oceans are well-mixed by currents.
The seawater in the Arabian Gulf can have significant evaporation rates, leading to hypersaline conditions; hence, the Arabian Gulf’s major-ion content is very high. This is one of the major factors to consider when designing seawater-based fracturing fluids. Another challenge to consider is variation of salinity throughout the year in the Arabian Gulf.
Preparing a base fluid requires hydration of the polymer in water to increase viscosity. Guar gum and its derivatives, HPG and CMHPG, usually hydrate better in water under slightly acidic conditions. Additives such as bactericides, gel stabilizers, or breakers can supplement the base gel, depending on the fracturing-fluid design criteria. Buffering agents are added to keep the pH within the required range, and, finally, a crosslinker is added with the appropriate pH additive to stabilize the crosslinking in the targeted pH range to develop high viscosity.
Dissolved solids in water can adversely affect the fracturing-fluid properties such as polymer hydration and crosslinking. Scaling tendency is another issue. When certain ions (e.g., sulfate and calcium or magnesium) combine, they can form insoluble salts.
Hydration. Hydration is the process by which polymer chains absorb water to change their configuration from the compact state of coils into a more-extended and -relaxed state. Two types of guar were tested—HPG and CMHPG.
High salinity and the presence of sulfate in seawater affected the hydration process. HPG required only 5 minutes in fresh water to reach 100% hydration; however, it required 50 minutes in seawater to reach 100% hydration. In comparison, CMHPG in fresh water required 5 minutes to hydrate fully, while only 10 minutes was necessary in seawater to reach 100%.
The only concern for hydration is time required, with all other variables held constant; seawater required slightly more time to reach 100% hydration than fresh water.
Crosslinking. Crosslinkers were added to increase the viscosity of aqueous-based polymer fluids, linking two or more polymer chains and forming viscoelastic fluids that are able to suspend the proppant and transport it down the wellbore and into the fracture. Factors such as pH and temperature can affect the crosslinking behavior of zirconium-based crosslinkers.
HPG and CMHPG crosslinking efficiency and fluid stability were studied at 300°F in seawater- and freshwater-based fracturing fluids.
HPG and CMHPG in Seawater and Fresh Water. HPG was more stable in fresh water than in seawater. Seawater-based fracturing fluids were broken after approximately 30 minutes without an additional breaker additive, whereas freshwater-based fracturing fluid did not break under identical test conditions.
CMHPG also showed better stability in fresh water. Seawater-based CMHPG fluid was broken down after approximately 50 minutes without an additional fluid breaker, whereas the freshwater fluid did not break under identical test conditions.
The crosslinked CMHPG fluid in seawater provided more-long-term stability at well conditions than a comparable crosslinked HPG fluid in seawater. Furthermore, the time difference between HPG and CMHPG stability was approximately 20 minutes. As a result, CMHPG showed more stability than HPG in seawater.
Effects of Seawater Ion Composition on Crosslinking Capability. The primary purpose of this study is to understand how the individual salt ions found in seawater can affect the performance of the fracturing fluid. These ions can alter the crosslinking capabilities of guar-derivative fracturing fluids through the formation of solid precipitates, as well as chemical interference with crosslinking sites. Through rheological testing, a synthetic seawater-based fracturing fluid was built and tested.
Ion Effects on Crosslinking Capability. In conventional applications, a borate-crosslinking technique works strictly under alkaline conditions. Such an environment can be accomplished through the use of various pH buffers. In the case of seawater-based fracturing fluids, high alkalinity and the presence of hydroxide ions from the buffers allow magnesium salts to form insoluble precipitates. The presence of such a precipitate can cause uncontrollable pH and, thus, failed crosslinking. Calcium salts present in seawater are taken into consideration because there are some scaling issues related to the precipitation of calcium carbonate. Seawater also contains a large amount of sulfate salts, which are capable of precipitating with strontium or barium ions found in formation waters. Barium sulfate is a particularly troublesome scale, which also has inhibiting effects on gel crosslinking.
Deionized (DI) water and synthetic seawater (SSW) with added magnesium were used to construct HPG fracturing-fluid systems; the same case held for calcium and sulfate.
Fluid samples were unable to stabilize with HPG, both in DI water and in SSW samples containing individual ions such as magnesium, calcium, and sulfate salt only. The ions appeared to have constricted the crosslinking stability of the gels despite high gel loading.
DI water containing individual ions as well as SSW samples were used to construct CMHPG fracturing-fluid systems. The fluid systems were subjected to rheological testing under high temperatures to observe crosslinking and fluid stability.
In the CMHPG study, DI-water analyses showed different behavior compared with SSW samples containing individual ions. In the case of magnesium, both studies showed precipitation, and fluid samples were unable to stabilize at high temperatures. In the case of calcium, in the SSW sample of CMHPG, the fluid system indicated crosslinking stability of at least 500 cp for less than 20 minutes at high-temperature applications, whereas DI-water samples showed stability for more than 1 hour under the same conditions. The same behavior was observed for the sulfate; the fluid was stable for more than 1 hour in DI water, whereas stability did not last for 20 minutes when using SSW.
Rheological-testing procedures clearly indicate that the high concentration of magnesium, calcium, and sulfate ions in seawater depresses hydration of gelling agents and restricts crosslinking viscosity. In the study, where DI water and SSW contained the individual ions by themselves, no fluid system was able to crosslink successfully in the HPG fluid system.
Scaling Tendency. Scale is a precipitation of mixed minerals in water resulting in the crystalline deposition of salts.
Fresh water used for fracturing-fluid development does not pose scaling problems as much as seawater does. With seawater, scaling is a significant issue that should be considered. Seawater contains a large amount of sulfate ions that interact with strontium and barium in connate waters to form scale.
Because of the high content of sulfate in seawater and high barium and calcium concentration in connate water, the scaling tendency of the resulting mixed aqueous fluid at high temperatures is predicted to be high, particularly for barite scale.
Results obtained from acidizing-design software showed that barium sulfate is the major scale. Neat connate water had a lower scaling tendency than a 50:50 formation-water/seawater mixture. This can result in significant unintended scale formation when using seawater-based fracturing fluids.
Several types of scale inhibitors have been used in the field to inhibit and minimize scaling. Tests using a scale loop instrument showed a compatible seawater fracturing fluid when using the proper type and concentration of scale inhibitor to control the sulfate scale. The pH must be balanced for the borate equilibrium to have proper borate-ion concentration in the presence of scale inhibitors.
Conclusions and Recommendations
After conducting tests on seawater and fresh water, the following were determined:
- The high salinity and total dissolved solids of seawater, especially in the Arabian Gulf, create issues with fluid stability at high temperatures as well as scale formation in the development of seawater-based fracturing fluid.
- Base gel viscosity was not affected whether HPG or CMHPG polymers were used to make seawater-based fracturing fluid.
- Full hydration is slightly delayed when using both polymers with seawater.
- The crosslinking mechanism is affected when seawater is used, and the fluid is less stable than fresh water with HPG or CMHPG.
- Optimizing the use and type of crosslinker is recommended to help avoid or reduce crosslinking disturbance.
- Seawater-based fluid can meet stability criteria, with CMHPG showing more stability than HPG.
A Comparison Between Seawater-Based and Freshwater-Based Fracturing Fluids
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