Optimized Acidizing Stimulation Technology Achieves Production Increase in Ultra-High Temperature Carbonate Reservoirs
Authors: Dong Yifan , Lei Yu , Jin Ting , Wang Bo , Wang Johnson , Zhang Yong 
Published: September 2023

Abstract
Hydrochloric acid is commonly used for carbonatite acidizing. However, it fails to penetrate deep into the reservoir especially in ultra-high temperature reservoirs to create deep etched fractures due to its fast reactivity and strongly dissolve the rock surfaces. Delayed hydrochloric acid systems such as gelled, crosslinked, and emulsified acid system have been employed but still fall short of meeting the requirements. This paper presents the utilization of an ultra-high temperature suitable, super delayed non-hydrochloric acid system for effective acidizing treatments.
Comparative laboratory tests were conducted between the super delayed acid system and existing delayed systems (gelled, crosslinked, and emulsified hydrochloric acid systems) at a formation temperature of 360 °F. These tests included core solubility and reactivity experiments, core-flood studies, corrosion rate measurements, and solubility tests using the same acid volume to dissolve the core until the reaction was complete. Core-flood studies utilized the same acid volume and injection rate to create wormholes, followed by CT scans to examine the internal structure. These tests aimed to determine the acid volume, pumping schedule, loading of corrosion inhibitors, and shut-in time required for complete acid reaction.
Existing delayed acid systems exhibited limited penetration into the core, with most of the live acid reacting immediately at the core inlet, resulting in inefficient wormhole creation. In contrast, the super delayed acid system demonstrated significantly lower reactivity, which no more than one third of the existing delayed systems. The slow reactivity of the super delayed acid allowed it to flow deeper and react in a controlled manner, facilitating the dissolution of damage and enabling the propagation of effective wormholes. The reduced reactivity of the super delayed system also led to a 50% decrease in treatment acid volume compared to the existing delayed system, as more super delayed acid was utilized to create dominant wormholes rather than being lost in the creation of smaller, branched wormholes. Additionally, the super delayed acid system exhibited lower corrosion rates, requiring only a small amount (no more than 15% of the existing delayed system) of corrosion inhibitor. In conclusion, the super delayed acid system proved more efficient than existing delayed acid systems for stimulating wells in reservoirs with a formation temperature of 360 °F. The acidizing pumping design specific to the super delayed acid system was successfully executed, the well opening for flow after a three-day shut-in period, The treated well achieved over twice the production increase compared to offset wells treated with existing delayed acid systems.
The creation of deep penetrated wormholes is a crucial aspect of acidizing stimulation in carbonate reservoirs, particularly in ultra-high temperature reservoirs where conventional methods are inadequate. The super delayed acid system described in this paper successfully addressed this challenge in a 360 °F carbonate reservoir, resulting in a doubling of production. Furthermore, the super delayed acid system is applicable in temperatures up to 500 °F, providing the petroleum industry with a successful solution for acidizing stimulation in ultra-high temperature carbonate reservoirs.