ABSTRACT

The results of laboratory experiments conducted to investigate the effect of matrix acidization on long berea sandstone cores (6"-12") and their theoretical analysis are presented. Both damaged and undamaged core samples were treated with different concentrations of HCl/HF acids (4%HCl/1.5%HF and 12%HCl/3%HF). One undamaged core sample was treated with 6%HF acid. The effect of spent acid on permeability was investigated. The treated cores were cut into four sections each and the mineralogy of each section was analyzed using an X-Ray Diffraction (XRD).

A simplified model for matrix acidization induced formation damage was developed and facilitated to analyze the laboratory test data and to determine the relative affects of adverse processes in matrix acidization. The model developed takes into account the change in concentrations of both acid and minerals, thereby predicting the change in permeability.

The results indicated that higher concentration of acid causes more formation damage due to the precipitation of silicates and/or iron oxides. In addition, when the spent acid resides in the core for a longer time it causes a severe formation damage. It is shown that the experimental and simulation results agree well within the accuracy of the simplified model.

INTRODUCTION

Although matrix acidization is extensively used as a well stimulation technique in the petroleum industry for the removal of near-wellbore damage, frequently, it can cause formation damage at later stages away from the near-wellbore region. Formation damage during matrix acidization may occur due to inappropriately designed acid jobs, precipitation of byproducts, etc1. Major factors governing matrix acidization induced formation damage are pH of the formation fluids, concentration of acid, type of acid, mineralogy of the formation and residence time. High concentration of HF acid can cause pore collapse near the wellbore due to the complete dissolution of cementing materials.

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