Application of abandoned mine drainage for reuse of Marcellus Shale flowback water: Wastewater and solid waste management.

摘要

Marcellus Shale play, underlying 70% of Pennsylvania, is the largest onshore shale gas reservoir in United States. Recent advancements in horizontal drilling and multi-stage hydraulic fracturing technologies enabled economical recovery of unconventional (shale) natural gas resource and greatly expanded natural gas production in the United States. Flowback water generated during shale gas extraction in Pennsylvania typically contains high concentrations of total dissolved solids (TDS), heavy metals (e.g., Ba and Sr), and naturally occurring radioactive materials (NORMs), which raises significant public concerns and environmental challenges related to wastewater management. Due to limited capacity for wastewater disposal by deep well injection in Pennsylvania, flowback water is generally reused for hydraulic fracturing. As only 10-30% of hydraulic fracturing fluid is recovered, large volume of make-up water is required to support hydraulic fracturing of new wells. Abandoned mine drainage (AMD) is an environmental legacy from coal mining industry and one of the most serious threats to water quality in Pennsylvania.;Application of AMD for reuse of Marcellus Shale flowback water has never been tried by the unconventional gas industry before. Key technical barriers include compatibility of the treated water with fracturing chemicals and management of radioactive solid waste generated from this practice. This study employs laboratory and pilot-scale systems to demonstrate the feasibility of this approach for flowback water reuse and to elucidate the underlying fundamental mechanisms as well as develop engineering solutions to implement this management strategy.;Laboratory studies evaluated the kinetics and equilibrium of precipitation reactions that occur when flowback water and AMD are mixed. Sulfate removal through mixing flowback water and AMD is governed by barite (BaSO4) precipitation and chemical equilibrium can be predicted thermodynamic models with Pitzer's equation for activity corrections. An empirical model was developed to predict the kinetics of barite precipitation. Celestite (SrSO4) precipitation requires over 10 hours to reach equilibrium and does not contribute significantly to the control of sulfate concentration in the finished water due to kinetic limitations in the treatment plant.;The feasibility of using microfiltration to separate particulate matter that is originally present in the wastewaters or that is created through mixing flowback water and AMD, was studied using both dead-end and cross-flow filtration systems. Early flowback water can cause severe membrane fouling due to the presence of stable submicron colloidal particles. Floc breakage is a key factor that may cause severe permeate flux decline during filtration of the flowback water that does not contain such colloidal particles.;A pilot-scale system was used to demonstrate the feasibility of co-treatment of flowback water and AMD. The finished water from this treatment process can be adjusted to meet the criteria for unrestricted use in hydraulic fracturing operations. The barite particles generated in this process have high radium content due to coprecipitation of radium with barium sulfate. The pilot-scale study revealed that sludge recycling could enable the use of Ra-enriched barite particles recovered from this process as a weighting agent in drilling mud formulation.;Impact of antiscalants on the fate of barium sulfate that may be formed in unconventional gas wells was also evaluated in this study. Antiscalants are unlikely to prevent formation of barite particles because of high supersaturation levels that are typical in unconventional gas extraction. When the fracturing fluid is rich in sulfate, barite particles will inevitably form in the subsurface and may be transported through the proppant pack during the flowback period. While most common antiscalants cannot act as threshold inhibitors for barite formation, they can enhance the mobility of barite particles through proppant pack by limiting the size of barite particle and providing steric repulsion at high ionic strength condition.;The key finings of this study indicate that it is feasible to utilize AMD as a make-up water source for flowback water reuse. The co-treatment process demonstrated in this study offers an alternatively approach for the management of flowback water generated in Pennsylvania.