Fine scale sandstone acidizing coreflood simulation
Contrary to the traditional understanding of matrix acidizing of sandstone that the acid front propagates in the formation with a piston-like style, some wormhole like structures were observed in lab tests under certain conditions. Most current models treat the rock as a homogeneous porous medium to describe the matrix acidizing in sandstone. The most sophisticated design models divide the formation into a series of layers with constant properties (minerals, permeability, etc.) in each layer. However, sandstones invariably have small – scale heterogeneities in minerals and flow properties that may cause the effects of injected acids to differ greatly from what is predicted by a model based on a homogeneous formation. A fine-scale model of the sandstone core acid flooding is developed based on mass balance and the chemical reactions between acids and minerals that occur during sandstone acidizing. This mathematical model is numerically solved to predict the permeability response and demonstrate the distributions of acids, precipitates, flow velocity and porosity in the core during acidizing. Cores are divided into 8000 grid blocks to simulate the fine-scale structure of sandstone. Using standard geostatistical techniques at the beginning of simulations can generate heterogeneous porosity or/and minerals. The permeability response to acidizing is predicted using a model in which not only the porosity, but also the minerals, tortuosity, and statistical parameters of the particle size are considered. Application of the new model to typical acidizing conditions shows that acid tends to channel through a heterogeneous sandstone, with the most efficient acidizing occurring when the rock has a layered structure. A layered structure is simulated by assuming a correlated permeability field in the main flow direction, as occurs in sandstones having horizontal laminations. The model shows that acid can stimulate the matrix permeability two to three times farther into the rock than would be predicted with a standard acidizing model, which takes the rock as homogeneous porous medium.