Part I1 -the field study, was presented as SPE 18223 at the national SPE meeting in Houston in 1988. It reported the results of a series of ionic analyses on returned fluids collected after a variety of acid treatments and the results of laboratory solubility tests on disaggregated core material. The field study had raised some key issues concerning the stimulation of the Sadlerochit formation. When and where was the precipitation of silica occurring downhole? Did this precipitation of silica result in significant formation damage? What was the relationship between the Si:AI ratios seen in the return samples analysis and those seen in standard acid solubility tests and acid core flow tests? Would an increase in the ratio of HCI:HF increase the Si:AI ratio and thus provide a more effective treatment? Was the Sadlerochit formation a source for the high ferric iron seen in the returned fluids?

To answer these questions an extensive laboratory study using Sadlerochit core plugs was initiated. SEM, thin sections, porosity, and permeability tests were done before and after acidizing a series of core plugs. The use of a high temperature, high pressure, ported flow cell provided the capability to monitor the acid treatment as it moved through each section of core material. Different HF acid systems (12% HCI:3% HF, 6% HCI:1.5% HF, 13.5% HCI:1.5% HF) were pumped through a set of core plugs and the effluents monitored under both continuous flow and flow after a shut-in time.

The results indicate that the Si:AI ratio decreases as the acid moves through the core. Lowering the HF concentration decreases the Si:AI ratio and the rate of change of this ratio as the acid reacts with the formation. Increasing the HCI:HF ratio had little effect on the Si:AI ratio of the spent acid. Si:AI ratios similar to those seen in the field study were seen only after a 14 hour shut-in at temperature with the Sadlerochit core material. No significant change in permeability was obtained after the silica was precipitated during the shut-in test. This suggests that although a large quantity of precipitated silica is left downhole, no significant loss in permeability occurs. This agrees with the overall production improvement seen in the HF acid treatments of the Sadlerochit formation.

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