Iron Control In Fracturing And Acidizing Operations
Authors: Dill Walter , Smolarchuk Paul 
Published: March 2010

Investigative results from laboratory tests and field jobs show that iron presents a significant and complex problem in stimulation operations.

Non-acidic or weakly acidic fracturing fluids are not detrimental from the aspect of dissolving and reprecipitating iron compounds. However, fracturing fluids usually contain a significant cancentration of dissolved and entrained oxygen which makes the fluid incompatible with formation water that contains ferrous ions in solution.

The problem presented by an acidizing fluid differs from that presented by non-acidic or weakly acidic fracturing fluid. Acid dissolves iron compounds from equipment and flow lines as it is mixed and pumped to the formation. Acid may dissolve additional iron as it reacts with the formation. If acid does not contain an effective iron control system, dissolved iron precipitates. This precipitate may then accumulate as it is carried toward the wellbore during flowback. This accumulation of solids may plug natural and created permeability and have a detrimental effect on the recovery of the treating fluid and production.

Chemicals and techniques that help ensure the compatibility of fracturing fluids and formation water are described.

Iron control systems used in acidizing have properties that influence their effectiveness. The properties and effectiveness of the various systems are described so that a judicial selection of the most effective system can be made.

Introduction

Iron control problems are sometimes encountered when stimulating operations are conducted with weakly acidic or nonacidic fracturing fluids. The fracturing fluid ordinarily does not contain enough acid to dissolve iron and iron compounds from the well equipment or formation.

Fracturing fluids, because of blending operations and viscosity of gelled fracturing fluids, will contain dissolved and entrained oxygen, which makes the fracturing fluid incompatible with formation waters that contain ferrous iron, Fe(2).

The fracturing fluid mixes with the formation water. Oxygen in the fracturing fluid immediately oxidizes the ferrous iron, Fe (2), to ferric iron, Fe(3). The Fe(2) will remain in solution at pH levels up to about 7.5. The oxidation product, Fe(3), starts to precipitate at a pH of 2.5 and has completely precipitated when the pH reaches 3.5. The pH of the natural environment is usually greater than 3.5 and precipitation of Fe(3) occurs as the Fe(3) is formed by oxidation.

Acidizing operations are usually conducted with highly acidic solutions. Acid corrosion inhibitors inhibit the reaction of acid on the metal contacted; however, they do not inhibit the reaction of the acid with iron compounds (rust, mill scale, siderite, and other iron compounds) to an appreciable extent.

Acid dissolves these iron compounds as it is mixed and pumped through tubular goods to the formation. Iron content of the acid may possibly exceed 100 000 mg/I by the time it reaches the formation. This will depend on the condition of the pipe, the amount of pipe surface area contacted, concentration of acid used, and the temperature. Conditions may dictate a pre-clean job prior to the formation stimulation operation.

This acid, containing iron in solution, flows into the formation, and dissolves additional iron before it is spent.