A new approach to controlling barite scaling in unconventional systems

Barite scale formation in unconventional systems can severely reduce production rates and recovery. Barite in drilling mud (DM) is highly susceptible to dissolution (up to 20% of the total barite) when it interacts with a 15% hydrochloric acid spearhead leading to significant barite scaling as solution pH rises to circumneutral levels. In order to maintain the low pH spearhead that is favored by most operators, a new acid spearhead formulation was developed to stabilize the barite in place, closer to the borehole where open volume is large. This approach is dramatically different than typical scale mitigation practices in which precipitation from solution is targeted for control; but with unsatisfactory results. By using sulfuric acid instead of hydrochloric acid (equivalent acidity), the amount of Ba and SO₄ leached from DM was reduced by > 850-fold. This major reduction in barite dissolution translates into a significant reduction in the dispersal of these mineral-forming aqueous species in the hydraulically fractured shale matrix. As a result, reprecipitation of barite scale in the fracture network is minimized. Because some unconventional systems have high concentrations of calcite that dissolves upon interaction with the acid spearhead, gypsum can precipitate due to excess Ca²⁺ derived from the calcite and its interaction with the additional SO₄^2- introduced from the new acid spearhead formulation. To mitigate this potential scale formation, Na-citrate was added to chelate Ca²⁺ and inhibit gypsum formation. When only H₂SO₄ was used, Na-citrate was not able to completely inhibit gypsum formation in high carbonate systems. However, by adjusting the H₂SO₄/HCl ratios and including Na-citrate, both barite and gypsum scale in the stimulated rock volume can be mitigated. These findings suggest that an incremental new best-practice is to stabilize the Ba/SO₄ in place rather than attempt to control precipitation from solution.