Simulator Aids Design, Analysis of Acidizing Jobs With Fluoroboric Acid

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_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 205893, “Novel Simulator for Design and Analysis of Matrix Acidizing Jobs With Fluoroboric Acid in Sandstone Reservoirs,” by Mohammed Qamruzzaman, SPE, Mandeep Khan, and Dhirendra C. Roy, SPE, ONGC, et al. The paper has not been peer reviewed. _ Matrix acidizing with fluoroboric acid (HBF4) has gained special attention because of its deeper penetration of in-situ generated hydrofluoric (HF) acid and stabilization of formation fines by binding them to the pore surface. While numerous mathematical models exist in the literature for design and evaluation of conventional mud acid treatments, few attempts have been made in developing a laboratory-validated model that can do so for fluoroboric acid treatments. The complete paper presents a novel mathematical model that has been developed that takes into account the chemical kinetics and equilibrium aspects of important reactions and fluid flow inside the reservoir rock. Mathematical Modeling Because the complete paper contains numerous equations, it is essential for understanding the authors’ description of their mathematical model. The authors identify three benefits of their approach from a modeling point of view. First, it reduces the total number of chemical reactions taking place, which greatly simplifies computational complexity while maintaining reasonable accuracy. Second, the inclusion of separate chemical reactions for silica gel precipitation may require fixing the average value of stoichiometric coefficients for reactions between HF/hexafluorosilicic acid (H2SiF6) and slow- and fast-reacting minerals. This becomes challenging to do beforehand, especially when different types of clays are present with relatively equal proportions in the reservoir. Third, all resulting nondimensional numbers can be estimated easily by measuring effluent acid concentration during coreflooding. The following assumptions have been incorporated into the model: - The reservoir and core are assumed to be homogeneous and isotropic. - Flow is assumed to be one-dimensional. - All fast-reacting minerals are grouped together as a single fast-reacting mineral. - All slow-reacting minerals are grouped together as single slow-reacting mineral. - The reaction of HF with fast- and slow-reacting minerals is assumed to be first-order with respect to HF. - Changes in porosity are small and diffusion/gravity/thermal effects are neglected. Laboratory Studies Laboratory studies were performed on sandstone core samples from a prominent payzone in northeastern India to generate data for model validation. Core plugs of 1.5-in. diameter were taken from the same block to prevent mineralogical heterogeneities. Samples were characterized with X-ray diffraction (XRD) analysis and porosity measurements. This payzone was selected because it consists of a diverse range of minerals. Thus, a model that can be validated with the coreflooding from this sample could, in principle, apply to any other sandstone reservoir.