Since the mid-1990s the use of HCl/HF at a weight ratio of 9 to 1 has been used extensively in field operations to minimize precipitation during sandstone acidizing. Although it was perceived as applying to all secondary reactions, it was originally directed at fluosilicates. Field case histories have been reported to support this recommendation. However, little evidence is available to justify the use of 9 wt% HCl - 1 wt% HF (9–1 mud acid) at the 300°F temperature exhibited in the Jauf reservoir. Thus, a laboratory study incorporating HF systems with various HCl to HF ratios was performed to quantify the amount of precipitation in Jauf reservoir cores at 300°F, and the impact on permeability. The focus of this paper is on the silicon (Si) and aluminum (Al) concentrations in core flow test effluents that were corrected for post-core test precipitation of reaction products in collection tubes. This technique accounts for the Si and Al in the liquid and solid phases of core flow effluents.
The literature teaches that the Si/Al molar ratio of a core test effluent can be used as a diagnostic tool to quantify hydrated silica formation. It is assumed that hydrated silica precipitation increases as the Si/Al ratio decreases from the theoretical value as a result of a greater completion of the secondary and tertiary reactions of HF with silt and clays. The results of this study on short Jauf reservoir and Berea cores at 300°F indicate that cores treated with 9 wt% HCl - 1 wt% HF yield a lower Si/Al ratio than observed with 12 wt% HCl - 3 wt% HF. However, cores treated with 4 wt% HCl - 1 wt% HF yield the lowest Si/Al ratio. The Si/Al ratio is not the only diagnostic tool required to determine the amount of hydrated silica precipitation i.e. factors including the mud acid flow pattern (matrix vs. channel) have a significant impact on the Si/Al ratio.
Core flow test results are presented for acetic acid preflush followed by various mud acid systems. All cores were stimulated, although hydrated silica formed in all core tests and aluminum fluoride precipitated in core tests using 4–1 mud acid formulations at 300°F.
Introduction
Sandstone matrix acidizing has been applied in oil, gas and injection wells for decades to remove formation damage in the critical area surrounding the wellbore.1,2 The chemical reactions that occur between HF acid systems and the aluminosilicate minerals in sandstone reservoirs have been investigated in detail. It is obvious that the sandstone matrix process is significantly more complex than carbonate acidizing. Unlike carbonate acidizing where the formation damage is simply bypassed via the creation of wormholes, during sandstone acidizing the damage that is plugging the pores normally must be dissolved. Subsequent reactions must not occur that create formation damage i.e. precipitation of calcium fluoride (CaF2) or potassium fluosilicate.3–5 Formation of insoluble reaction products during carbonate acidizing using hydrochloric acid (HCl) is rare, but it occurs in every sandstone acid treatment incorporating an HF acid system i.e. hydrated silica.6–7 The focus of this paper is on the formation of hydrated silica as a function of the HCl to HF ratio in mud acid systems, and its impact on formation damage.
Hydrated silica, Si(OH)4, is formed during the secondary and tertiary reactions of HF with feldspar and clay as shown in Equations 1 and 2, respectively.4–9
Secondary Reaction:
Equation 1
Silica gel, aluminum fluoride complexes (AlFx(3-x)+, and potassium (K) cations are formed during the secondary reaction (Equation 1) via the reaction of pentafluosilicic acid with feldspar/clay. This reaction goes to completion in most sandstone acidizing treatments before well flowback occurs.4,6,7
Secondary Reaction:Equation 1
Silica gel, aluminum fluoride complexes (AlFx(3-x)+, and potassium (K) cations are formed during the secondary reaction (Equation 1) via the reaction of pentafluosilicic acid with feldspar/clay. This reaction goes to completion in most sandstone acidizing treatments before well flowback occurs.4,6,7
