A Novel Calculation Method Of The Acid Leak-off In The Acid Fracturing

The acid fracturing has gradually become one of the key stimulation treatment techniques of exploration and development in carbonate reservoir. The carbonate formation is generally characterized with the natural fracture and developed cavity, strong heterogeneity. So, the fracture and cavity system is the main medium of the oil / gas storage and penetration. The matrix acidizing and the communicated natural fracture by acid fracturing are applied in this formation to realize the stimulation effects in the reservoir. Acid-wormholing in a carbonate formation is the result of several chemical and physical process, including acid difusion, reaction kinetics and dynamics of fluid flow in porous media. Its complex configuration looks like a worm, so it's often called as acid wormhole.Compared with the matrix acidizing, wormhole is formed inevitably during the process of high pumping rate acid fracturing treatment, which results in serious acid leak-off and short acid penetration distance. Thus this directly leads to a bad effect on the carbonate stimulation, and even brings the treatment into failure. Generally, the main reasons for the failure are the unclearness of acid leak-off mechanisms and the inaccurate calculation of the acid leak-off in the naturally fractured carbonates.So, the acid leak-off mechanism and the design of the controlling the leak-off method have been the concerned for the stimulation technician, and it plays a significant role in improving the acid fracturing effects. Therefore, the acid wormhole propagation mechanism study and the novel calculation model for the acid leak-off in the acid fracturing is more important.We have developed a set of acid leak-off experiment, based on the long core parallel plate acid-rock reaction model and the acid dynamic leak-off experiment, which is to analysis the initiation and propagation mechanisms of wormhole and the acid leak-off mechanisms during the process of the acid fracturing. Acid flow and reaction in a wormhole was studied by modeling the wormhole as a cylinder and numerically solving the mass balance equation that describes acid transport by convection and diffusion. The model can simulate the propagation process of wormhole, which changes with the model can simulate the propagation process of wormhole, which changes with pumping rate and time, it can also simulate the changes between the acid leak-off and time.It's illustrated in this paper as following:1, the acid leak-off in the acid fracturing treatment falls into three parts: 1) the leak-off in matrix, 2) the leak-off on the wormhole wall and 3) the increase of wormhole volume. This paper presents an acid fracturing mathematic model, including wormhole leak-off.2, the acid wormhole leak-off volume is concerned with the pumping rate, the acid viscosity,the acid concentration, acid injection time and the formation physical property. The acid wormhole leak-off can be decreased effectively and the available acid penetration distance is improved by optimizing the interest treatment parameter and adopting the corresponding procedure treatment.3 ^ the acid wormhole leak-off increases the acid leak-off volume, resulting in the short acid penetration distance; on the other hand, the form of the acid wormhole increases the porosity of the matrix, thus improving the matrix permeability. Meanwhile, the acid wormhole propagation effectively communicates the fracture and cavity system in carbonate reservoir along the direction of the wormhole length, which greatly promotes the drainage area.Based on the above basic theory, a acid fracturing design software, considering the acid wormhole leak-off has been programmed with the Visual Basic 6.0 software to guiding the acid fracturing treatment in the carbonate reservoir, and the model in this paper is modified and improved during the application on the field.