For carbonate reservoirs, natural fractures and cavities are widely developed, which are the main mediums for storage and flowing of gas and/or oil. Both fracture length and conductivity are the critical factors controlling the stimulation effects in these reservoirs. Various limitations of hydraulic fracturing and great treatment risks have made acid fracturing more attractive and large numbers of field practices have verified its validity. The main purpose of acid fracturing is to furthest communicate natural fractures and cavities. Researches on laboratory mechanism and treatment design for deep penetration of acid fracturing have become hotspots of late years. An important aspect is to simulate the acid-etched fracture length and in-situ conductivity accurately. Assumed to be plug flow, the conventional acid flowing and reaction model is based on onedimensional (1D) flowing. Taking the flowing in the direction along fracture height and mass transfer into account, a set of novel 3D numerical model is developed in this paper.
To acquire the maximum net present value(NPV), 3D acid fracturing optimization design technique was presented. In conjunction with acid-etched conductivity model, 3D fracture propagation model and production prediction model, 3D analysis for acid fracturing process can be made. It has been applied on one gas well of one dolomite gas filed in the east of Sichuan and on the calcareous reservoir of Tahe oil field. A good matching is obtained with the actual stimulation ratio of 3.63 after acid fracturing and that designed of 3.21 in the gas field. Both the RPT* and the productivity of the acid-fracturing wells in Tahe oilfield have been improved evidently through the 3D optimization design of acid fracturing.
Most of acid fracturing modeling to date adopts 1D or 2D models. Associated with the change of average acid concentration in the direction along the fracture width (y direction), the pseudo mass transfer coefficient, kg firstly putted forward by Roberta and Guin, is introduced into these models. For 1D model, we can only get acid concentration along the fracture length (x direction) by integrating the basic equations.
Restructure Production Well: The well of which stable production exceeds 25 tons per day after one month since acid fracturing Restructure Production Ration(RPT): The ratio of Restructure Production Wells to all the acid-fracturing wells 2 Tony Settari[1] etc. developed 2D model in which the mass transfer directly was determined by effective diffusion coefficient De, acid- rock reaction rate constant k and the velocity field
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It is an important part of acid-fracturing simulation design to simulate the distribution of acid concentration for determining the effective acid-etched length. Without consideration of acid flowing in the direction across fracture height, the current models, widely used in acid-fracturing simulation, take the acid-rock reaction rate in this direction as constant[3–6]. If the formation is of serious vertical heterogeneity and the discrepancy of lognitudinal acid-rock reaction rate is outstanding, there will be a large error in simulation results. Romeroe[2] etc. have developed a 3D model, but the influence of mass transfer in the direction across fracture height was not taken into account.
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