Observations and Complexities in Experimental Measurement of Acid Fracture Conductivity for a Deep Carbonate Field in Kazakhstan

Abstract

Acid fracture conductivity experiments provide information valuable to effective design of acid fracture stimulation treatments in carbonates. They are complex tests, and a number of procedural details must be taken into consideration in order to upscale results to an adequate representation of well-scale acid fracture behavior for well productivity predictions. This paper focuses on a study performed on analog, quarried limestone core samples and a small number of reservoir core samples from the Unit 2 formation of Kazakhstan's Tengiz field to understand the impact of different acid fluid systems and procedural steps on acid fracture conductivity. The physical structure of the etched channels is observed to be impacted by the nature of the fluid and has a strong impact on the conductivity. The quality and uniformity amongst the core samples has a critical impact on the measurements and is assessed. The residence time of the acid in the acid-etched experimental channel is small, and the equivalency of acid volumes injected at the experimental scale to the large acid volumes injected into a well-scale fracture channel is considered. Special consideration is given to the procedure of applying stress to and measuring conductivity of the experimental acid fracture channel which may result in large-scale mechanical failures of the core sample, preventing a high-quality measurement of conductivity following injection of a closed fracture acidizing stage. This body of work discusses the design options and challenges which play a role in defining the testing strategy for an acid fracture conductivity study. Results demonstrate that with selection of appropriate fluid systems, acid fracture conductivity can be retained up to a closure stress of 6,000-7,000 psi in the Tengiz Unit 2 reservoir. A modified stress ramp-up procedure to improve closed fracture acidizing conductivity testing results obtained through the testing program is presented.

Effect of Acid Type on Acid-Etched Fracture Surface Morphology and Conductivity along the Fracture

“…10 limestone and found that the VES acid performed the best among the acid systems, including emulsified acid, sing-phase retarded acid, self-diverting acid, and the straight acid. 11 Zeeshan Tariq developed a comparative study on fracture conductivity in Silurian dolomite, Indiana limestone, and Austin chalk using glutamic diacetic acid (GLDA) and blank HCl (namely, straight acid) and found that HCl acid generated high conductivity in the three rock types and GLDA generated sufficient fracture conductivity for different rocks except for dolomite. 12 The above research results show that acid types have a direct impact on the acid-etched fracture pattern and conductivity, but the findings are obtained based on specific experimental conditions, such as fixed fresh acid concentration and temperature.…”

“…Pournik investigated the influence of acid types on fracture conductivity and found that the VES acid generated the greatest degree of etching and the highest conductivity at low closure stress compared with gelled, in situ gelled, and emulsified acids . Mcduff performed the experiments to investigate the impact of different acid systems on the conductivity of Indiana limestone and found that the VES acid performed the best among the acid systems, including emulsified acid, sing-phase retarded acid, self-diverting acid, and the straight acid . Zeeshan Tariq developed a comparative study on fracture conductivity in Silurian dolomite, Indiana limestone, and Austin chalk using glutamic diacetic acid (GLDA) and blank HCl (namely, straight acid) and found that HCl acid generated high conductivity in the three rock types and GLDA generated sufficient fracture conductivity for different rocks except for dolomite …”