Proposal

Barite is a common weighting agent used for oil well applications. It is commonly used to provide adequate weight to control downhole pressures in drilling slurries. Since it is used for control of pressure, it is a common source of formation damage, especially in deep, hot formations where drilling fluid weights exceed 17 ppg. Although alternatives do exist for barite, (e.g., formate drilling fluids) they are significantly more expensive, and not available on a rapid turnaround basis as in emergency situations required by well control operations. Once damage has occurred, remediation is a very difficult proposition. Barite is not soluble in typical oilfield inorganic acids used in stimulation. The most common methods of remediation are fracture stimulation and/or deep perforations. The use of these methods can remediate the problem, but at high cost. Also, since the well productivity cannot be evaluated prior to stimulation operations, a poor quality well cannot be identified until a significant investment has been made.

The most common chemicals currently in use to address this problem are "barite dissolvers". These agents are all strong metal ion chelators. Aminopolycarboxylic acids (e.g., EDTA) and similar reagents have been used with low success rates.1 Recently, hydroxylaminopolycarboxylic acids were introduced, but to the best of the author's knowledge was not extensively used in the field.2

This study investigates a new barite dissolver and compares its performance with other commercial dissolvers. The new material is a proprietary dissolver supplied in a liquid form. This study investigates the dissolving capacity, and evaluates the effectiveness of the dissolver by conducting coreflood tests. These tests were conducted using sandstone cores obtained from a deep gas well (300°F). The effectiveness of the dissolver was followed by measuring the concentration of barium in the core effluent. Unlike other barite dissolvers, the new chemical is shown to be very effective in restoring the permeability of damaged cores. The new chemical does not require long soaking times, and does not induce fines migration due to chemical means.

Introduction

Barite, BaSO4, is very difficult scale to remove. This is because it has very low solubility in water (2.5 mg/L)3 and its low solubility product Ksp = 10–9.99 at 25°C.4 In comparison, gypsum has a solubility of 2,080 mg/L in water at the same temperature.5

There are two cases where barite can cause problems in downhole environments. The first case occurs in oil fields where seawater is used for injection. Seawater has high sulfate content, whereas the formation contains high levels of barium. Barium sulfate will precipitate once these two waters mix together and the solubility product of barium sulfate is exceeded.5,6 In the second case, the case of interest in the present study, barite is used as a weighting material in drilling mud. Barite in this case will be present in the filter cake, and will cause formation damage.

Barite can be removed from well tubulars by mechanical or chemical means. Chemicals means are not effective to remove scale present in the welbore. This is because the scale accumulates downhole, and adheres to well tubing. Chemical solvents or chelating agents are not effective in this case because the scale has very low surface area. Mechanical means including jetting or even drilling can be used to remove the scale from the wellbore. Chemical means can be used if barite invades the formation or precipitates in the formation due to mixing of incompatible waters. Traditional mechanical means cannot be used to remove barite once it is deposit or precipitate in the formation. In should be mentioned that acid and/or hydraulic fracturing can be used bypass the damaged zones, however this technique is expensive and cannot be applied in every well. Chemical means is best suited for dissolve barite that is present in the formation.

A thorough literature survey, especially the patent literature, reveals that chemical means rely on aminopolycarboxlyic acids (chelating agents).1–15 EDTA and DTPA (Fig. 1) are typical barium sulfate dissolvers, however similar compounds were also proposed.12,13,16 DTPA is one of the most effective dissolvers for barite.1,11 It has an octodentate ligand, forms a strong 1:1 chelate in solutions at pH greater than 12. It binds to metal center with five carboxylate oxygen atoms and three nitrogen atoms to form BaDTPA3- complex.4 The stability constant of the latter is 8.78.

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