Mud Acid Effect on Fines Migration Problem Using a Newly Developed Al/Zr-Based Clay Stabilizer (A): Mitigation Can Happen

Abstract

Sandstone reservoirs exhibit severe fines migration problems subsequent to conventional mud acid (HCl: HF) treatments. The acid treatment proved to be effective for a relatively short period of time, with a sudden production decline owing to either that mud acid spent rapidly in close proximity to the wellbore, causing clays to migrate into the acidized area and re-plug the formation; or that the mud acid system actually weakens the structure of clays in the formation matrix and dissolves them.  Treatments with COP’s over-flush did not prevent the release of migratory fines. An Al/Zr compound was used to address this problem. Stabilizer A was injected at 2 wt% as a preflush for HCl stage. It was also mixed with either HCl or mud acid. All core flood tests were conducted up to 300^oF. Laboratory studies were performed using Berea sandstone cores (6 in. length and 1.5 in. diameter). Different experiments were performed to determine the effect of 9:1 HCl: HF on core permeability in the presence of Stabilizer A. In these experiments, 5 wt % NH₄Cl was used. After that, the 2 wt% Stabilizer A was either injected before the 15 wt% HCl preflush, added to the preflush stage, or mixed with the 2 PV of 9:1( HCl:HF), then followed by 5 wt% NH₄Cl solution, and ended by fresh water injection. The pressure drop across the core was measured and samples of the core effluent were collected. Inductively, Coupled Plasma was used to analyze the core effluent samples to measure the concentrations of the key cations. Choline chloride was tested at the same conditions and the results were compared with those obtained using Stabilizer A. Lab results indicated that Stabilizer A worked effectively up to 300^oF. No fines or reduction in permeability was noted after the DI water injection following the mud acid.  The best scenario was injecting Stabilizer A with the main acid. Choline chloride could not protect the clays from the acid attacks, where the amount of Al leached from the core was much higher in the core effluent samples.

Keywords: HF acid, fines mitigation, clay stabilizers, fines migration, core flood,
berea sandstone