Challenge
A US-based fertilizer production facility injects wastewater into three underground storage wells. The chemical composition of the wastewater, coupled with high water temperatures from geothermal heat absorption, resulted in the formation of scale deposits in the well formation. These deposits would routinely limit downhole flow capacity to the extent that wastewater injection rates would fall below wastewater production rates.
As a result, monthly acidification of the well with hydrochloric acid was required to maintain downhole injection flow rates. This process required large quantities of acid to be stored onsite, posing a health and safety concern due to potential worker exposure to the hazardous chemical, and exposed the well infrastructure to potential corrosion issues from excessive contact with the acidic cleaning solution.
Solution
The Veolia team was approached in search of an alternative means to maintain optimal flow rates in the wastewater disposal well and minimize the frequency of acidification of the well. Veolia audited the system and analysed the wastewater for its chemical composition. By utilizing the chemical composition of the wastewater and knowing the temperature conditions the water would be exposed to as it was sent downhole, a model was developed to predict the solubility of inorganic scale-forming minerals.
The solubility model indicated a high likelihood of calcium carbonate scale formation, which was confirmed by a composition analysis of deposit samples collected from the system. Figure 1 shows the recommended limits relative to temperature, pH and treatment cost.
The Veolia team determined the optimal ScaleTrol deposit control treatment program and associated chemical dosage that would be capable of inhibiting scale formation in the well. The selected product was a ScaleTrol PDC product, consisting of a proprietary blend of calcium carbonate crystal modifier and dispersant technologies.
Result
After initiation, the ScaleTrol deposit control program substantially reduced loss of flow capacity. This significant reduction in loss of flow also decreased the frequency of acidification of the disposal well for deposit removal from a monthly to an annual occurrence.
Scale deposit coupons used to monitor the deposition rate of the wastewater corroborated the dramatic reduction in scale deposit formation (Figure 2). After 15 years of application history, the disposal wells continue to only require acidification on an annual basis, equating to a 92% reduction in deposit accumulation and acid use. The reduced acid cleaning frequency led to a substantial reduction in acid exposure risks for site personnel and reduced risk to disposal well infrastructure integrity.
Figure 2: Coupon installed in wastewater injection system for 24 weeks showing minimal deposition
*Trademark of Veolia; may be registered in one or more countries.