Industry operations are shifting toward high-temperature (HT) downhole settings, expensive tubular metallurgy, and extended reach wells, while health, safety, and environmental requirements become stricter. Consequently, conventional stimulation treatments, such as applications using hydrochloric acid-based fluids, will no longer meet the industry’s needs as operational environments evolve. A new stimulation fluid developed by AkzoNobel, Dissolvine StimWell, is based on glutamic acid diacetic acid (GLDA) and has been successfully applied in the field.
GLDA has high thermal stability and low corrosion potential, and is an effective stimulation fluid without adverse environmental impact. A vertical gas well in a deep, sour carbonate reservoir was successfully stimulated using GLDA. Previous matrix stimulation treatments with conventional acids in this HT gas well did not sustain the performance, and a fracture treatment was considered for the well. However, the case proved that matrix acidizing with the GLDA-based fluid was the best stimulation method for the well in terms of cost-effectiveness and regulatory issues.
Stimulation Purpose
The object of stimulation is to remove the production zone damage caused by the drilling and completion processes in sandstone reservoirs, and to create channels or wormholes in carbonate reservoirs. Although an industry workhorse for decades, hydrochloric acid (HCl) often produces subpar stimulation results, especially at high temperatures, because of its fast reaction near the wellbore, low acid penetration, and high corrosivity. Many problems may occur during sandstone acidizing with HCl/hydrofluoric mud acid, such as decomposition of clays in HCl acids, precipitation caused by the presence of fluoride, silica gel filming, and colloidal silica gel precipitation. As a result, mud acid may cause significant damage to sandstone reservoirs, especially for those with a high content of calcite or clays such as illite. The desirable alternative stimulation fluid would be suitable for all of these conditions, be globally applicable, and have an acceptable environmental profile.
Fluids based on GLDA meet those requirements. The fluids are effective in improving permeability in carbonate and sandstone formations. Many additives such as iron control agents that are required in other fluids are not needed in GLDA-based stimulation fluids.
High Temperature Stability and Versatility
The thermal stability of a stimulation fluid is of utmost importance because of the increasingly high temperatures of the reservoirs being drilled. GLDA is very stable and effective up to 400°F. Furthermore, unlike conventional fluids, the effectiveness of GLDA-based fluid improves with temperature. At higher temperatures, greater improvement in permeability is achieved with a lower volume of GLDA.
Low Corrosion
Compared with other acidizing fluids, GLDA has a remarkably low corrosion potential for both low-carbon steel and chrome-based tubulars. In most cases, this helps to eliminate the need for corrosion inhibitors in the acidizing program. If inhibitors are needed, the quantity required is significantly reduced. Elimination or reduction of corrosion inhibitors will not only lead to substantial cost reduction in the acidizing process, but it also will help to avoid formation damage induced by the corrosion inhibitors themselves and alleviate or eliminate the negative environmental effect of the inhibitors.
Fig. 1 shows the results of different types of corrosive stimulation fluids used on Cr-13 tubulars at 300°F without a corrosion inhibitor. GLDA is shown to be the least corrosive stimulation fluid compared with other conventional fluids. It also can maintain corrosion rate below the accepted rate of 0.03 lbm/ft2 without needing a corrosion inhibitor, even at 300°F. Fig. 2 shows that even on low-carbon steel, GLDA is the least corrosive fluid and that with an addition of 0.001 vol% of an organic acid-based inhibitor, the corrosion rate can be bought well below the accepted rate, even at 300°F.
Minimizing or Eliminating Additives
In addition to minimizing or eliminating the need for corrosion inhibitors, GLDA removes the need for other additives such as iron control and anti-sludge agents. The presence of iron can cause numerous problems in an acidizing job, such as precipitation and formation of iron hydroxides. Iron control agents are typically added to the acidizing fluid to tackle this problem. GLDA is able to control iron by itself and keep it from precipitating or forming other undesirable products during acidizing, thereby eliminating the need to add an iron control agent and reducing the overall level of additives required. And while it controls iron, GLDA simultaneously can dissolve carbonates.
Table 1 shows that the GLDA can work as a stand-alone stimulation fluid even when the iron content is high, whereas a treatment with HCl caused a more than 40% reduction in permeability under the same conditions because of iron hydroxide precipitation.
Environment and Safety
Along with the effectiveness and convenience of using GLDA-based fluid, it has an environmentally friendly profile. The main component of GLDA is derived from natural and sustainable raw materials. GLDA-based fluid is biodegradable under freshwater and seawater conditions.
Another major advantage of GLDA is that it is not a hazardous chemical, and thus can be handled and transported without special labeling and extraordinary safety precautions. It also is nontoxic to aquatic and human life.
Case Study
In a recent case study, the productivity of a tight, sour gas well had decreased after a workover. Repeated attempts with various conventional matrix treatments failed to achieve persistent performance. The well had been completed with chrome-nickel-based corrosion resistance alloys. Selection of the stimulation fluid proved challenging because of the high temperature (325°F), corrosive gases (19% hydrogen sulfide and 9% carbon dioxide), and the presence of chrome-based internals, along with low-carbon steel tubulars.
A customized matrix program with GLDA was designed to cope with the challenging reservoir condition and strict environmental legislation. The treatment included pre-flush, main flush and post-flush stages. The aim of the pre- and post–flush states was to prevent water blockage and improve flowback performance. The main fluid contained GLDA, with minimum concentration (approximately 0.8 vol%) of a corrosion inhibitor for added protection from the elevated temperature and high level of corrosive gases present. The treatment was applied by bullheading. The flowback and production were resumed after 6 hours of soaking time.
The GLDA treatment resulted in a significant increase in gas production, as shown in Table 2. After the treatment, the gas production of the well improved by 110% of the initial production, which was above the targeted level. The treatment efficiency with GLDA was excellent. Not only did the result exceed the target level, the production rate nearly equaled the rate expected had the well been fractured. Extensive analysis of flowback after the treatment indicated that GLDA dissolved large amounts of calcite and dolomite while effectively maintaining the integrity of the well tubular and internals.
Conclusion
The newly developed GLDA-based stimulation fluid is effective in improving well productivity and maintaining well integrity in carbonate reservoirs. Because of its environmentally friendly profile and nonhazardous properties, GLDA comfortably met the strict regulations and environmental guidelines established by the regulatory bodies and the operator. GLDA proved easy to handle in the field, leading to much safer working conditions—especially compared with conventional acid treatments. Logistical demands were lower, as only two components were needed for the main treatment and a much lower volume needed to be pumped, compared with conventional acid treatments.
Additional Applications
The value of GLDA as a stimulation fluid is not limited to carbonate reservoirs. Sandstone formations also benefit from the unique properties of the product. GLDA has superior performance in improving the permeability of various types of sandstone formations. GLDA-based fluids can also be used to remove various scales, such as carbonate, oxide, and sulfide. GLDA can be used as a pickling fluid to remove mill scales without adversely affecting tubular integrity. In addition, the use of GLDA-based fluids can serve as an alternative to fracturing treatments where economic, environmental, and regulatory factors pose obstacles to the use of the technology.