Optimization of Acid Fracturing Process for Carbonate Reservoirs in Daniudi Gas Field

The Daniudi gas field is located in the Ordos Basin's northern section of the Yishan slope. The intertidal-subtidal depositional environment dominates the lithology of the Ma55 sub-member, resulting in a stable, thick-layered dark gray-gray-black limestone and lime dolomite. The stratum is stable laterally as well as dolomite, with an average thickness of 26.8 m. Fractures, dissolution expansion pores, and inter-crystalline dissolved pores are the primary reservoir space kinds, with a minor number of karst caves and fractures generated as well. The main distribution ranges for porosity and permeability are 1-8 percent and 0.01-1 mD, respectively. Low porosity, tightness, and ultra-low permeability are common characteristics, and a single well typically has no natural productivity. Production stimulation technologies like pre-fluid acid fracturing, compound sand addition acid fracturing, and multi-stage injection + temporary plugging volumetric acid fracturing have been gradually optimized using the horizontal well development method, and breakthroughs in the development of tight and low-permeability carbonate rock reservoirs have been made. However, the conditions of different types of reservoirs are quite different, and the acid fracturing process is not matched and imperfect, resulting in large differences in the productivity of different horizontal wells after fracturing, as well as a high proportion of low-yield wells, which cannot meet the needs of cost-effective and effective development of this type of gas reservoir. In light of the aforementioned issues, a series of laboratory tests have been carried out to explore the stimulation effects of acid fracturing on different types of reservoirs and to optimize the acid fracturing process in the Daniudi gas field. The results show that the rock mechanical performances and the acid etching conductivities of the rock specimens are related to the types of reservoirs. The rock mechanical properties can be deteriorated after acidizing, but different types of reservoirs have different degrees of deterioration. According to the results of acid etching conductivity of different types of reservoirs, conductivities obtained by high and low viscosity and cross-link-gelled acid (two stage injection) processes are higher than those of high viscosity systems. The experimental results of process suitability suggest adopting high and low viscidity acid systems for pore type and fracture-dissolved pore type reservoirs, and cross-linked acid systems for fracture-pore type reservoirs. The findings of this study can help form a better understanding of the performance of different types of reservoirs under the various acidified conditions that can be used for the optimization of acid fracturing processes in carbonate formations.