Acid stimulation of carbonate formations using hydrochloric (HCl) acid-based systems to improve hydrocarbon production or water injection has been a common practice since the early days of the oil and gas industry. The challenge of achieving full zonal coverage with the stimulation fluid still persists. This task is even more difficult in highly slanted layered, naturally fractured reservoirs exhibiting high permeability and porosity contrast.
During acid stimulation treatments, the fluid has a natural tendency to take the path of least resistance, penetrating layers with the highest porosity and permeability while little or no acid is injected into lower-quality zones. Excessive fluid loss to these high-quality thief zones results in a nonhomogenous distribution of stimulation fluids across the targeted interval. These higher-quality zones are essentially over stimulated, allowing them to contribute the most to post-treatment production. This leaves neighboring lower-quality zones unstimulated, preventing them from reaching their maximum potential.
A novel fiber-laden polymer-free self-diverting acid system was introduced in Saudi Aramco to improve zonal coverage across the entire interval of interest in vertical and highly slanted wells during stimulation treatments. This diversion system combines fiber and a self-diverting acid, which utilizes a nondamaging viscoelastic surfactant that gels as the acid spends. The combination of the self-diverting acid and fiber enhances the diversion process by combining the aspects of both particulate and viscosity-based diversion techniques. The fluid system has a distinct advantage in that it does not contribute to reservoir damage as the viscoelastic surfactant will break down upon contact with hydrocarbons, and the fiber will dissolve with time and temperature.
The fiber-laden self-diverting acid system has been applied in new developments and producing gas wells in the Khuff reservoir of Saudi Aramco with impressive results. The diversion and acid reaction signatures were evident in the acquired treating pressure plots during acidizing with the fiber-laden polymer-free self-diverting acid system. The initial results indicated substantial pressure responses and excellent gas production performance.