Successful Application of Single-Phase Retarded Acid System in Tight and Conventional Limestone Formations – Case Studies

Abstract Conventional HCl acid systems have been successfully employed to stimulate limestone formations, remove near wellbore damage and optimize production from a reservoir. However, these HCl based stimulation fluids pose multiple challenges due to high reactivity of HCl. These challenges increase exponentially with the increase in reservoir temperature. HCl based acid systems cause formation face dissolution, have high PVBT ratio and limited penetration. Emulsified acid systems reduce the reactivity of HCl but require special mixing equipment, have pumping rate limitation due to viscosity and are extremely hazardous to the environment. To overcome these challenges, single-phase retarded acid system has been developed. This paper discusses in detail the successful application of single-phase retarded acid system to stimulate tight and conventional limestone formations in Kurdistan region of Iraq. An extensive lab testing was performed on the single-phase retarded acid (SPRA)system employed in this case study prior to field deployment. The lab testing included core flooding, corrosion testing, retardation test, static dissolution test, sludge & emulsion test and compatibility testing. SPRA systems comprise of an engineered acid retarder which significantly lowers the reactivity of HCl. The retardation test of 15% SPRA employed in this case study suggests a retardation factor of 11 at a temperature of 180 F in comparison to conventional 15% HCl system. Moreover, core flood testing performed at 275 F provides a PVBT of 0.45 in comparison to 5.2 PVBT forconventional15% HCl system. Furthermore, the 3D high frequency Macro-CT scan of core plugs flooded with SPRA reveal deep penetrating dominant wormholes generated by the acid. After successful lab testing SPRA was deployed on applicable cases. In case study A & B, SPRA was employed to overcome the low transmissibility of fluids from tight matrix to natural fractures of limestone formations by generating deep penetrating dominant wormholes. In case study C, SPRA was employed to successfully stimulate conventional limestone formation with free pipe across the pay zone, avoiding formation face dissolution and future cross flows behind the casing. In case study A, production increased from 10 BOPD to 500 BOPD. In case study B, production increased from 0 to 10,500 BOPD. In case study C, production increased from 2000 BOPD to 7500 BOPD. The increment in WHFP was also very promising and was more than 200% for each of the case studies. This paper illustrates the lab testing and successful field application of single-phase retarded acid system in tight and conventional limestone formations. SPRA stimulation system has been successfully deployed for the first time in KRI. The techniques and fluid systems employed in the case studies presented in this paper can be referred to when dealing with similar challenges.