Optimizing Acid Fracture Design in Calcite Formations: Guidelines Using a Fully Integrated Model

The performance of acid-fractured wells depends on the conductivity distribution along the acid-penetration length, which is a function of reservoir properties, treatment design, and execution. The previously published models for fractured-well performance assume constant fracture conductivity, which cannot be achieved in acid-fracturing operations. This work proposes a design optimization method where acid-fracture and reservoir models are integrated. Fracture-conductivity distribution along the fracture surface is considered in the optimization process. In the new integrated model, the acid transport and reaction are joined to the fracture-propagation and heat-transfer models. The dissolution patterns along fracture surfaces are generated, and this is converted to a conductivity distribution. To predict the fractured-well productivity, the reservoir model is built using the input reservoir properties, as well as the calculated acid-fracture geometry and conductivity distribution. The acid-fracturing parameters that lead to the optimal fracture productivity are determined with this integrated model. This method shows that there is an optimal productivity that can be obtained for a given acid treatment volume and reservoir properties. Design parameters such as flow rate, viscosity, acid concentration, acid treatment volume, and pad and overflush volumes can be selected to achieve optimal well performance. Reservoir permeability has an important impact on how acid-fracture jobs should be designed. At low-reservoir permeability, a more evenly distributed conductivity and a long acid-penetration length are preferred. This can be accomplished by injecting a retarded acid system at moderate- to high-flow rate. However, excessive fracture-height growth should be prevented by carefully designing the injection rate and viscosity. For high-permeability reservoirs, higher conductivity along a shorter acid-penetration length is targeted. This can be obtained by selecting a more-reactive acid system such as straight acid and injecting at moderate rates, or by lowering the injection rate of retarded acids. A minimum amount of pad should be used in this case. Still, the flow rate should be highly sufficient to keep the fracture open during acid injection. We also show how acid concentration and fluid stages can be designed to optimize productivity and presents a procedure for selecting the acid treatment volume. A theoretical model, which integrates acid fracturing and reservoir flow, is used to implement the guidelines on optimizing acidfracture design parameters. In this paper we provide a scientific approach to determine acid treatment volume that yields optimal outcomes for acid fracturing.