Removal of Barite-Scale and Barite-Weighted Water- or Oil-Based-Drilling-Fluid Residue in a Single Stage

Abstract

Summary Barite is one of the most common weighting materials used in drilling fluids for deep oil and gas wells. Consequently, the main source of solids forming the filter cake is “barite particles,” the weighting material used in drilling fluids. Barite is insoluble in water and acids such as hydrochloric acid (HCl) and formic, citric, and acetic acids, and barite is moderately soluble in chelating agents such as ethylenediaminetetraacetic acid (EDTA). The present study introduces a new formulation to dissolve barite scale and barite filter cake using converters and catalysts. Barite can be converted to barium carbonate (BaCO3) in a high-pH medium (pH = 12) using a combination of potassium hydroxide (KOH) and potassium carbonate (K2CO3) solutions. Subsequently, HCl or low-pH chelating agents can be used to dissolve the BaCO3. Another solution is to use the EDTA chelating agent at pH of 12 and K2CO3 or KOH as a catalyst/converter in a single step. The removal formulation also contains a polymer breaker (oxidizers or enzymes). The three components of the new formulation are compatible with each other and stable up to 300°F. Solubility experiments were conducted using industrial-grade barite (particle size of 30 to 60 µm). The solubility experiments were conducted at 300°F for 24 hours. Varying concentrations of the catalyst were added to determine the optimal concentration. The developed formulation was tested for the removal of filter cake formed by barite drilling fluid using a high-pressure/high-temperature (HP/HT) cell. Filter-cake removal was conducted for filter cakes formed by both water- and oil-based drilling fluids. The results of this study show that the barite-removal efficiency of the new formulation is 87% for water-based mud (WBM) and 83% for oil-based mud (OBM). The test results show that the solubility of barite particles in 0.6 M EDTA is 62 wt% in 24 hours at 300°F. Adding K2CO3 or KOH catalyst to the 0.6-M-EDTA solution increases the solubility of barite to 90 wt% in 24 hours at 300°F. Thus, barite scale can be removed efficiently using high-pH formulations (pH = 12) to avoid the safety issues associated with HCl. Because the EDTA chelating agent is compatible with the polymer breaker (oxidizer), the filter cake can be removed in a single stage. The concentration of the components of the formulations used in this study is as follows: 10 wt% oxidizer, 10 wt% K2CO3 or KOH concentration (catalyst/converter), and 0.6 M EDTA. The developed formulations achieved more than 80% filter-cake removal in both oil-based and water-based drilling fluids. For OBM, a water-wetting surfactant, a mutual solvent, and an emulsifier were added to the formulation to remove the oil and to make the surface of the filter cake water-wet. In this study, two solutions are proposed to remove the barite filter cake and barite scale from oil and gas wells at different conditions. The first one is using HCl after converting the barium sulfate (BaSO4) to BaCO3 by use of a high-pH medium such as KOH and K2CO3. Although HCl can easily remove the resulting BaCO3, the generated barium chloride (BaCl2) is a safety and health concern. The second method is to create a high-pH medium (pH = 12) using the removal fluid itself, which uses the EDTA chelating agent in addition to using K2CO3 or KOH as converters.