To-date, more than 60% of the world production comes from complex-structure carbonate reservoirs; also, a considerable part of the Volga-Ural petroleum play reserves is in carbonates. Carbonate reservoirs are characterized by a wide range of project recovery factor values, from 0.15 to 0.50. A common technique to improve well performance is acid fracturing. This method can only be used in acid-soluble formation, such as carbonates. A fracturing fluid is injected into the formation under a pressure higher than formation breakdown pressure, which produces a build-up in wellbore pressure leading to fracturing of the rock. Acid is then injected to react with the rock and to etch the surface of the induced fracture. Etching results from non-uniform dissolution of the rock, which, in its turn, is controlled by a number of factors, including variations in permeability and porosity, complex mineral composition (presence of both limestone and dolomites), turbulent flows in the acid-etched fracture. The rough surface of the created fracture is thus the main mechanism to maintain the fracture open during the well life, while in the alternative proppant fracturing, proppant is used to prevent fracture from closing. In acid fracturing, a large number of parameters are used to determine the efficiency of the induced fracture in terms of conductivity, including the amount of the dissolved rock, the etch pattern, the proppant occasionally used to maintain the fracture open. A 3D acid fracturing simulator is an essential tool to predict and to evaluate the acid-etched fracture performance and to safeguard against geological risks.
This paper discusses studies to improve the efficiency of mathematic modeling of acid etching and the use of model studies results for development of tools for acid fracturing engineering in PJSC TATNEFT.
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