Thursday, November 23, 2017

Matrix Acidizing chapter 2

By comparison, the reaction between HF and sandstones is much slower. Mud acidizing seeks to unblock existing pathways for production by dissolving wellbore damage and minerals filling the interstitial pore space, rather than by creating new pathways. The HF reacts mainly with the associated minerals of sandstones, rather than the quartz. The acid reactions caused by the associated minerals - clays, feldspars and micas can create precipitants that may cause plugging. Much of the design of a sandstone acid job is aimed at preventing this. 

The usual practice is to preflush the formation with HCl to dissolve associated carbonate minerals. If these were left to react with HF, they would produce calcium fluoride (CaF2) which precipitates easily. Then the HF-HCl mud acid is injected. Finally, the formation is overflushed with weak HCl, hydrocarbon or ammonium chloried (NH4Cl). This pushes reaction products far from the immediate wellbore zone so that if precipitation occurs, production is not too constricted when the well is brought back on line. 

 Another plugging danger is from fine particles, native to the sandstone, dislodged by the acid but not fully dissolved. To minimize this eventuality, Shell in 1974 proposed lower pumping rates - less likely to dislodge fines - and more important, a chemical system that did not contain HF explicity, instead creating it through a chain of reaction within the formation. In principle, this allows greater depth of penetration and longer reaction times for maximum dissolution of fines. 

As HF is spent, dissolving clays and other minerals, it is constantly replenished through hydrolisis from the remaining fluoboric acid. The slow rate of this conversion helps guarantee a retarded action and therefore deeper HF penetration. As a bonus, the fluoboric acid itself reacts with the clays and silt, forming borosilicates that appear to help bind the fines to large grains. Recent treatments with fluoboric acid for Ashland Nigeria have confirmed the power of this technique. 

After in all, sandstone acidizing poses a greater challenge than carbonate acidizing and certainly generates more than its fair share of controversy among both operators and service companies.


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