Laboratory Evaluation of Biosealers
Authors: Nasr-El-Din H.A. , Fadhel B.A. , Al-Juaid S.K. , Mohamed S.K. 
Published: December 2007

Abstract

Description of the Paper.
Solubility and diameter of biosealers were measured as a function of ionic strength and pH. Solubility tests were conducted using HCl (15 and 28 wt%) and organic acids (10 wt% formic acid, 10 wt% acetic acid and 10 wt% citric acid). To mimic the effect of spent acids on the solubility of biosealers, solubility was also measured in CaCl2 (10 wt%), MgCl2 (10 wt%) and NaCl (10 wt%) brines. These tests were conducted at 75 and 19°F, with soaking times of up to several days.

Application.
Diversion and efficient sweep are needed during placement of various treatment fluids. Biosealers, which were recently introduced to the oil industry, can be used to direct injected fluids into low permeability zones or plugged perforations. The performance of biosealers depends on their solubility and diameter. It is important to maintain certain characteristics of the biosealers to ensure efficient diversion throughout the treatments.

Results, Observations, Conclusions.
At 7°F, the weight and diameter of the biosealers monotonically increased with respect to time when placed in distilled water. However, the biosealers continuously dissolved when placed in 15 or 28 wt% HCl solutions. The rate of dissolution of these balls was independent of acid concentration from 15 to 28 wt% HCl. In calcium and magnesium brines, the weight of the biosealers showed an initial increase (swelling) before the biosealers started to dissolve. Unlike room temperature results, the biosealers continuously dissolved at high temperatures in all fluids examined. The rate of dissolution was found to be a function of acid type and ionic strength. Lower dissolution rates were noted when the biosealers were soaked in organic acids. Surfactants (anionic, cationic or nonionic) and mutual solvent reduced swelling of biosealers in distilled water. Corrosion inhibitors, mutual solvent and surfactants (especially the anionic surfactant) reduced the rate of degradation of biosealers in HCl.

Significance of Subject Matter:
Biosealers are used to divert various fluids during acidizing and other chemical treatments. The results obtained from this study showed that the weight and diameter of biosealer depend on pH, soaking time, temperature, and ionic strength. These factors should be carefully considered when using biosealers to divert acids and other chemicals in the field.

Introduction
Ball sealers have been used in the oil and gas industry for more than four decades.1 They are used in cased wells to divert treating fluids (hydraulic frac fluids and acids) into low permeability zones and plugged perforations. In a typical treatment, these balls are pumped with the treating fluids. They are supposed to flow with these fluids into the perforations that correspond to high permeability zones. They seat on these perforations and force the treating fluids to enter the low permeability zones. These balls will remain in the perforations during the treatment. However, at the end of the treatment, they will either flow back to the surface with the spent acid or the frac fluids where special devices are used to catch these balls before any choke, or sink to the bottom of the rat hole.
Ball sealers are available in different sizes and densities. Selection of the size and density of ball sealers depends on several factors, some of the main ones are: perforation size, perforation density, the length of the target zone, and the density of the treating fluids. Obviously, the effectiveness of ball sealers depends on the seating efficiency of these balls. Ball sealers can be lighter (floater) or heavier (sinker) than the treating fluids. The advantages and disadvantages of each type of ball sealers are discussed by Bilden et al.1 Several research groups addressed methods and factors to enhance the performance of these balls.2–4