Effects of sulphide scales (PbS, ZnS & FeS) on BaSO4 crystal growth and dissolution

Sulphide scales (iron sulphide, FeS, zinc sulphide, ZnS, and lead sulphide, PbS) are occurring more commonly in many water producers and these deposits and colloids may lead to various production problems. In addition to being a problem, the presence of these sulphides also affects barium sulphate deposition. The resulting crystal distortions of the barite may play an important role in either enhancing or retarding the barite scale inhibition itself. This study investigates the effects that sulphide scales have on the crystal lattice of barite during its formation. These effects vary based on the type of sulphide scale which co-precipitates with the barite scale. FeS was found to have the most profound effect on the barite crystal by causing severe crystal distortions and it also adheres more strongly to the outer layers of the barite. It is also found that the FeS/barite scales are also the easiest of the mixed scales to inhibit. The ZnS and PbS scales did not cause significant crystal distortion of barite compared to the FeS and they were also the hardest mixed scales to inhibit. The effects of generic scale inhibitors (DETPMP and PPCA) on the barite crystal in the presence of sulphide scales were also studied and several interesting findings are noted. The experiments described were carried out using the static bottle tests and crystal morphology was monitored using Environmental Scanning Electron Microscope (ESEM) techniques. Better understanding of the effects of FeS, ZnS, and PbS on harder mineral scales such as barite will help in the design and application of scale inhibitors for mixed scaling systems which occur quite regularly in the field as well as in designing downhole squeeze treatments. © 2010, Society of Petroleum Engineers.