A model and interpretation method of pressure decline for acid fracturing

A model is presented for the pressure decline of acid fracturing wells after shut-in, which based on the material balance principle, and comprehensively considered the influences of compressibility and thermal expansion of fluids (wasted acid and supercritical carbon dioxide gas produced by acid-rock reaction), and artificial fracture width change for wasted acid keeping on etching fracture face after shut-in. The effects of formation temperature and reaction heat are also taken into account in this model. Meanwhile, we assume that the length and height of hydraulic fracture do not change any more after shut-in, while merely the fracture width increases caused by continuous acid-rock reaction. An automatic fitting method based on genetic algorithm was introduced to solve this model accurately, and then a simulator was developed for acid fracturing pressure decline. The simulator can calculate the geometry parameters and other relevant parameters of artificial fracture, and the results match well with test datum. This model and algorithm can provide reliable and credible parameters for optimizing acid fracturing design.