This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 222126, “Effect of Fracture and Vug Network on the Dolomite Carbonate Acid-Stimulation Process,” by Cunqi Jia, SPE, Saif Alkaabi, SPE, and Jinchuan Hu, SPE, The University of Texas, et al. The paper has not been peer reviewed.
Acid stimulation in carbonate rocks represents a classic example of reactive flow in subsurface porous media and is an extremely effective improved-oil-recovery technique with broad applications. The acid solution is injected into the formation under the formation fracture pressure to increase formation permeability while bypassing damaged domains around the wellbore. This work extends an integrated two-scale continuum model that contemplates mass, momentum, and energy changes to study the acid-stimulation process in complex carbonate acid-stimulation systems with the development of fracture and vug networks.
The improved two-scale continuum model, consisting of the core-scale model and pore-scale model, is introduced to simulate the acid-stimulation process in dolomite rock. When dealing with the development of fracture and vug networks, the improved two-scale continuum model does not require significant modification. This is primarily because of the application of the universal momentum-conservation equation for porous media—the Navier-Stokes-Darcy equation—within the model. This equation can be used to describe the momentum transport of acid fluid throughout the entire region and the entire acid-stimulation process, which can avoid the use of different control equations in the porous media region and the free-flow region and avoid the determination of complex interface conditions between the porous-media region and the free-flow region through experimental and numerical simulation methods.
The complete paper includes subsections in which chemical-reaction models, two-scale continuum models, and boundaries and initial conditions are detailed, including relevant equations.
This section of the complete paper applies the two-scale continuum model to study the acid-stimulation process in complex carbonate acid-stimulation systems.
Benchmark Case Analysis.
The authors consider a schematic diagram of benchmark cases with fracture and vug network developments as shown in Fig. 1. The fracture and vug networks each consist of nine uniformly distributed individual fractures and vugs. Each vug maintains a square shape with a side length of 1 cm, and each fracture measures 1 cm in length. The interiors of the vugs and fractures are set to be fully developed with a porosity of 0.999. When fractures and vugs are not developed within the target core sample, the core average porosity and core average permeability are 0.15 and 2.5 md, respectively. The development of a fracture network in the target core sample results in core porosity and permeability values of 0.1702 and 2.8950 md, respectively. When a fracture network is well-developed but remains isolated, its contribution to the core storage capacity and fluid-flow capacity is limited. In contrast, the presence of a vug network within the target core sample enhances the storage capacity for fluids. The extensive development of vugs results in an increase in the core-sample porosity to 0.3421. The contribution to fluid-flow capacity remains limited, however, with the core-sample permeability being numerically measured at 3.3546 md.
