The objective of this paper is to study and mitigate the calcium sulfate (CaSO4) precipitation in proppant fracturing of unconventional gas wells in a carbonate formation. With sulfate content in the source water ranging from 500 to 2,000 ppm, and with up to 200,000 ppm total dissolved solids in the flowback water, calcium sulfate scale precipitation hindered fracture productivity and was considered an extreme challenge.
Flowback water analysis revealed an abundance of calcium ions, mainly because of rock dissolution while having insignificant formation water presence in the source rock. Based on scaling tendency simulation results, an experimental study was conducted at the reservoir downhole temperature of 280 °F, to evaluate the flowback water compatibility with the source water used for fracturing fluids, simulating the contact of frac water with the formation. The sulfate content varied up to 2,000 ppm in the fracturing fluids. This paper addresses: 1) the scale tendency of sulfate-containing fracturing water with flowback (formation) water interaction; 2) examples of different scale inhibitors’ efficiency at different concentrations; 3) the fracturing fluids stability with scale inhibitors; 4) field trial performance for the selected scale inhibitor in an unconventional gas well; and 5) extensive flowback analysis and monitoring program during well clean out.
Based on the static scaling tendency analysis in the laboratory, several findings were concluded. Acids and spacer stages must be prepared using fresh water; all high pH gelled fluids should be prepared using relatively low (≤ 500 ppm) sulfate (SO4)-containing water. An optimized multistage propped fracturing stimulation was conducted, and led to improved well performance with significant production increase compared to offset wells. Implementation of the recommended measures, including usage of low sulfate-containing water wells, addition of approved scale inhibitors, monitoring of water quality, etc., have showed no scales formation during well intervention and production operations, which was also confirmed by the flowback water analysis.
A journey of learned lessons within a laboratory study-field test-laboratory study loop, serves as a solid guideline for all future propped fracturing jobs which involve a challenging water source and high TDS formation water composition. The present work exhibited an effective methodology to prevent potential formation damage induced by fracturing treatments.