Foamed Acid Fracturing Enables Efficient Stimulation of Low-Pressure Carbonate Reservoirs

Abstract

Abstract Ordinary acid fracturing treatments cannot deliver consistent production results in low-pressure carbonate reservoirs. The reservoir pressure is not sufficient to flow back the large volume of treating fluids from the formation after the treatment, thus minimizing the benefits of performed acid fracturing. The use of foamed acid fracturing fluids will provide additional energy that will help to enhance flowback and push treating fluids out from the reservoir during post-fracturing flowback operation. There are two types of gaseous phase that are commonly used to foam the fluids for stimulation treatments: nitrogen (N2) and carbon dioxide (CO2). N2 is an inert gas; it is widely available and therefore the most frequently used. CO2 is more soluble in water than N2; therefore, more CO2 is required to saturate the liquid and create the foam. CO2 has more expansion during flowback, which aids in total fluid recovery. Additionally, the solubilized portion of CO2 reduces the interfacial tension of the fracturing fluid. A deep high-temperature carbonate reservoir typically requires acid fracturing treatment to produce at economic gas rates. When reservoir pressure declines over time, foamed acid fracturing treatment becomes the preferred stimulation option. Both types of the gaseous phase show good success. The multiple case studies suggest that foamed acid fracturing resulted in easier flowback initiation and better well productivity compared to regular acid fracturing. Moreover, CO2-based foams provided better results compared to N2 foams, especially in horizontal wells completed with multiple acid fracturing stages within the same reservoir. The specific fracturing fluid was deployed to use CO2 foam in the wells with high bottomhole temperatures up to 300°F. The innovative CO2 foam chemistry enables formulating non-crosslinked gels that deliver viscosity equal to or better than the industry-standard foams of low-pH guar crosslinked fracturing fluids. This fluid delivers those results at significantly lower polymer loadings and with a reduced number of additives, thus improving the operational aspect and increasing well productivity. Another noticed benefit of foamed acid fracturing with CO2 is the easier achievement of higher foam quality at bottomhole conditions. N2 is pumped in its gaseous phase and requires specific pumping units with limited pressure and rate capacity. In contrast, CO2 is pumped in its liquid phase through the common fracturing pumping units; therefore, a significantly higher pumping rate of the gaseous phase can be achieved with minimum additional equipment.