The economic condition of the oil and gas industry is tighter than ever. Optimizing well productivity to maintain cost effectiveness is on every operator’s mind. Enhancing well productivity requires more-detailed analysis, more measurements, and even-more-advanced products and technologies, which all cost more money. Doing more and reducing cost can be a dilemma. This is the time when properly choosing sound technologies is critical to improve efficiency and increase success rates. Several well-stimulation products and techniques have been seen to benefit well productivity from recent field trials and implementations in carbonate reservoirs, including simpler acid fluid systems, integrated work flows, and coiled-tubing bottomhole assemblies.
Emulsified acid has been a preferred carbonate-stimulation fluid for 30 years. Clearly, mass transfer can be retarded significantly by the emulsion form for deeper live-acid penetration in matrix- and fracture-acidizing applications. The quality of the emulsified acid is sensitive to how it is mixed. In the laboratory, slowly adding acid into oil, mostly diesel, containing surfactant while blending is required to achieve good emulsion stability. Field practice is another story. The large amount of emulsified acid is normally batch-mixed in the service company yard or at the wellsite by circulation pumps and tanks. The mixing energy used in the laboratory is difficult to match in the field. Consequently, the consistency of the final emulsified acid pumped can vary from well to well. Recently, single-phased-acid systems have been developed to simplify the fluid preparation and to make the product performance more robust. Various single-phased acids are provided by service companies or even operators themselves using different formulations to achieve the effect of retardation.
Another important technology for successful acidizing, especially in horizontal wells or highly heterogeneous reservoirs, is diversion. Many diverters have been used. Evaluation of diversion during the treatment remains rudimentary. Pressure humps are used to infer diversion without good confirmation of how effective the diversion actually is. More-accurate information about fluid rheological and friction characteristics is needed to evaluate the downhole-pressure behavior and, hence, quantified diversion results. The term “diversion” is often used interchangeably with “placement.” Proper placement can be achieved by means other than diverting materials. Jetting is believed to place acid effectively in matrix stimulation without chemical diverters.
Each technology costs additional money at the early stage. When they are routinely used, fully integrated in work flows, and eventually linked with automation, true cost effectiveness of the stimulation treatments can be realized. The investment made today, therefore, will bear fruits in the future.