Several oil producer wells in a sandstone reservoir have experienced productivity decline due to obstruction material. Bailer samples were collected at different depths to identify the nature of the obstruction material and recommend an optimized remedial treatment to restore productivity of damaged wells. The samples were found to be heterogeneous and contained different types of scales including calcite and iron containing compounds. Various forms of iron sulfides, iron hydroxides were present in the collected samples at different concentrations. Sand, elemental sulfur and trace amounts of iron oxides were also present in some of the samples.
Mechanisms of scales formation were investigated in this study to recommend prevention measures. As an example, greigite (Fe3S4), present as one of the major scales (49 wt. %) in one of the samples, may form due to the reaction of iron sulfide (FeS) with excess sulfur, both detected in the sample. Formation of iron sulfide was attributed to SRB's which dictates the need to have effective biocide treatments.
Solubility tests of collected scales in different dissolver systems were examined as a function of temperature and soaking time. Among other examined dissolver systems, (HCl) acid was found to be the most effective. Effects of acid additives on the solubility of scales were examined in order to optimize the acid recipe. The formation of interest is sandstone containing up to 2 wt. % authigenic clays. The use of HCl acid may induce fines migration and sanding problems. Based on lab results, a novel two-stage remedial treatment is recommended to restore productivity of damaged wells. The first stage involves treating the wellbore with HCl acid including the required additives to remove the scaling material from the wellbore area. The second stage includes incorporating organic acid to stimulate the formation matrix.
This paper presents a study detailing the design of a novel two-stage treatment able to remove scales and overcome some of the anticipated challenges in stimulation of sandstone formations. The paper also addresses results of extensive lab work conducted to optimize the treatment recipes.
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