Energized Acid Fracturing in Deep Carbonate Reservoirs. CO2 versus N2

Abstract

Abstract Flowing back initiation for cleaning up process after a fracturing treatment in low-pressure reservoirs are becoming a challenging operation. One of the alternatives is to conduct a post-frac CT intervention using nitrogen to kickoff the flow. This operation is known to be expensive as well as time-consuming. The use of foamed fluids for fracturing operations in low-pressure gas reservoirs would help in facilitating flow- back operations and also minimizing the volume of water pumped to the formation. Nitrogen (N2) and Carbon Dioxide (CO2) are the most common gases used in foam fluids. N2 is an inert gas, and most frequently used. CO2 is more soluble in water than N2, so more CO2 is required to saturate the liquid and create the foam. But CO2 has significant advantages over N2: greater hydrostatic pressure resulting in lower treating pressure; it has more expansion during flow back that aids in total fluid recovery; the solubilized portion of carbon dioxide reduces the interfacial tension of the fracturing fluid. A study to compare the production performance from two nearby horizontal wells conducted where both were stimulated with Foam fracturing using a different gas type. This paper describes the comparsion of acid fracturing treatment using CO2 and N2. The two wells drilled from the same pad; the horizontal parts section drilled in the opposite direction, both wells completed with OH MSF (Open Hole Multi-Stage Fracturing) completion designed for four (4) separate acid fracturing treatments in each well. The stimulation designs for both wells were prepared in such a way to have a similar fluid type, pumping sequence, and volume of reactive fluids. The major difference was the gaseous phase used to foam the fluid: CO2 was used in Well A and N2 was used in Well B. Post Fracturing flow- back parameters were measured and short pressure build-up analysis results compared between both wells. Fracturing execution showed that CO2 can provide much higher foam quality than N2 (40% vs 12%) at the same total liquid pumping rates. Post-production showed that the well treated with CO2 gas was capable to produce at higher PI (Productivity Index) compared to the N2 well. Even after normalization to count the reservoir quality, well treated with CO2 showed better productivity compared to the N2 well.