Effects of Acids on Gravels and Proppants

Abstract

Cheung, S.K., SPE, Chevron Oil Field Research Co. Summary. The effects of acids on the integrity of gravels and proppants should be considered in acid treatments. This paper reports on the influence of acid type, acid concentration, and contact duration on the acid solubility of five sands and bauxitic materials. The effects of the acids on the mechanical strength and the size distribution of the solids are determined. We found that intermediate-density and low-density bauxites (IDB and LDB) are very soluble in HF acid and that sintered bauxite is weakened by HF acid. Introduction HCl and HF acids are pumped downhole to remove near-wellbore damage in many types of wells. The effects of acids on formation material have been studied extensively; however, the effects of acids on gravels and proppants should also be considered. Concerns about acid effects on formation materials differ from those about effects on gravels and proppants. For formation material, one must ensure that the acid can remove or bypass skin damage and does not cause further damage. For both gravels and proppants, one must ensure that the acid will not dissolve the solids significantly. The effects of the acids on size distribution and mechanical strength can also be important. API has prepared guidelines for testing gravel-packing materials and hydraulic fracture proppants. The acid-solubility testing procedure in the API guideline recommends acid contact of 1 hour at room temperature for gravels. This, however, is intended for quality-control purposes rather than for simulation of field operations. In actual field practices, the accumulated acid contact times are usually longer than 1 hour. In jobs with mechanical problems, acid contact time can be longer than planned. Furthermore, if an acid job with a 2-hour acid stage is performed annually, in 5 years the accumulated acid contact time is 10 hours. To simulate field operations, we determined acid solubilities at longer time intervals. Procedures and Tested Materials In this paper, we report the solubility of five different materials in HCl and HF acids. The effects of acid on the mechanical strength, bulk volume, and particle-size distribution of these solids were determined. All solids were sieved between 20- and 40-U.S.-mesh screens and then tested according to the procedures described in the Appendix. All five solids tested can be used as either fracture proppant or gravel-packing material. One difference is that the particle-size specification is tighter when the solids are used in gravel packs. The sands used were Ottawa and Heart of Texas sands. There are three types of bauxitic materials. Sintered bauxite in this study is dark gray and typically contains 87% aluminum oxide. Most of its aluminum oxide is in the form of corundum (Al2O3). IDB is usually dark brown and contains about 75% aluminum oxide, which has almost equal portions of corundum and mullite (Al6Si2O13). LDB has a bulk density almost equal to that of sands, is pale gray, and contains about 70% mullite and 30% cristobalite (SiO2). Test Results Dissolution in Acid. The acid solubilities in 15% HCl for all the solids tested for up to 24 hours were less than 2% (Tables 1 and 2). The resulting solids remained consolidated, and fines were not generated. In HF, all the solids tested have some solubility at 150F [66C]. Test results (Figs. 1 and 2) show that dissolution in HF generally increases with the concentration of HF and the length of acid exposure. Our study shows that the order of dissolution in HF, in increasing dissolution rates, is sands, sintered bauxite, IDB, and LDB. Sintered bauxite seems to dissolve quite rapidly during the first 6 hours of HF acid contact; then the dissolution rate tapers off. IDB and LDB dissolved alarmingly quickly in HF. After 24 hours, the HF acid solubility of IDB was 58% and that of LDB reached 97%. This means that for wells gravel packed with these two bauxitic materials, HF acid jobs may cause a loss of sand control. For wells propped with them, the HF acid job closes the fractures and causes a loss of fracture conductivity. Size Distribution. One concern about treating gravel packs and hydraulic fractures with acid is the reduction of particle size. For gravel packs, gravel-size reduction mayreduce the height of the gravel-packed interval and leave some zones without sand control,create voids within perforations and cause perforation collapse,change gravel-size distribution and decrease sand control efficiency, and/orgenerate fines and plug the gravel pack. For hydraulic fractures, proppant-size reduction may decrease fracture conductivity and generate fines. The loss of fracture conductivity near the wellbore can cause a dramatic reduction in well productivity. The particle-size distributions of 20/40-mesh solids treated with HF for 24 hours are listed in Table 3. For both sands, more than 90% of the resulting solids were within the 20/40-mesh size range after the HF acid exposure. Microscopic examination of HF-tested sintered bauxite showed pits in the particles (Fig. 3). For IDB and LDB, large amounts of fines were generated by the HF acid treatment. One interesting result of this study is that the particle-size distribution of sintered bauxite is not significantly affected by HF acid treatment. More than 99% of the HF-treated sintered bauxite remained within the 20/40-mesh range after 24 hours in HF (Table 4). Another interesting result is that the bulk volume of sintered bauxite is not affected by the HF acid treatment. After 24 hours in HF, the mass was reduced by 18.8% (acid solubility); the bulk density was also reduced by 17.3%. In terms of the definition of density (density equals mass divided by volume), the bulk volume of sintered bauxite remained essentially unchanged by HF. Because the particle-size distribution and bulk density are not affected, shallow wells gravel packed with sintered bauxite can be treated with moderate amounts of HF acid. Mechanical Strength. In hydraulic fracturing, the mechanical strength of the proppant is very important because proppants are sometimes used in deep zones where overburden pressure is large. If the proppants are crushed by this load, the fracture might close and lose its conductivity. SPEPE P. 201^

Interactions Between Mud Acid and Sand Proppants

“…Hydrochloric acid has minimal solubility on proppants, unlike mud acid mixtures (Cheung 1988). So, the contact time with these acids should be minimized or their usage should be altogether avoided.…”

“…So, the contact time with these acids should be minimized or their usage should be altogether avoided. Mud acids should be used cautiously in deep wells because they drastically affect the mechanical properties of bauxite (Cheung 1988). Sand gravel is also not affected by HCl, however HBF4 treatment can cause permeability damage (Chan and Parmley 1992).…”