Several Basic Research Of Carbonate Reservoir During Acid-Fracturing

Acid fracturing is a main technical used in carbonate reservoir stimulation, in which the reservoir was fractured by acid fluid above the fracture pressure, and the new fracture wall were etched uneven by acid fluid, after the pressure of fluid in the new fracture was relief, the etched fracture will not close completely, finially form a fracture with conductivity.For the advantages of high conductivity, low net pressure, small fracture height growth, no screening or proppant backflow, acid fracturing is widely used in the carbonate reservoir.However, it is hard to predict these phenomenons, wormhole growth, the change of uneven etch volume and acid fracture conductivity. The conventional acid fracturing design assumes that the reservoir is continuity homogenous porous media and the acid flow pattern fulfils Darcy law. However, the carbonate reservoir is anisotropic void-cave reservoir. Acid fracturing transformation mainly through acid micro non-uniform dissolution and etching change reservoir poroperm characteristics, to achieve the purpose of increase field capacity, stable yield and recovery. So we need to combine microscopic heterogeneity and acid fracturing theory, to better research the mechanism of acid fracturing in carbonate reservoir and guide the field acid frcturing.Based on the reservoir analysis of the carbonate reservoir in S field, core digital minds and the visualization advantage of MTALAB, wormhole, acid fracture geometry shape and acid fracture conductivity which were influenced by the acid were studied on micro and macro perspective. Also, the limit concentration of the residual acid of this field was put forward. The main works are as follows:(1) Based on the corrosion experiment and reaction kinetics experiment, the best acid concentration was selected and the parameters of reaction kinetics of HC1, diversion acid and gelled acid were obtained;(2)The data of core scanning electron microscope was analyzed based on core digital minds; Porosity distribution was obtained by Kriging method; Porosity model and Darcy seepage model were built combining with acid-rock reaction kinetics and the wormhole growth pattern was studied also;(3)Fractal dimension range of the reservoir was calculated by box-counting method and a single-fractal fracture surface model of this reservoir was built based on Wererstrass function; The JRC relationship between facture fractal parameters and roughness was analyzed;(4)On the base of single fracture, double-fractal fracture surface model was built by introducing the coincide-degree JMC; Considering the filtration influenced by wormhole. double-fractal fracture surface acid-etching model was built based on the analysis of acid flowing pattern and reaction with the rock in the fracture; The influence of acid and limit concentration of the residual acid were studied also;(5)Considering the effect of non-Darcy seepage, acid fracture conductivity was simulated and compared with experimental data according to the closure characteristics of the acid fracture under closure stress;According to some basic theories study of the carbonate reservoir, some acquaintances were obtained:(1) The limit concentration of the residual acid in the acid fracturing stimulation should be tested and the conventional theory which the limit concentration of residual acid is 10% of fresh acid can't be used anymore;(2)Based on the reaction kinetics experiment of the S field, the sequence of reaction order of the acids studied is gelled acid>HCl>diversion acid. The sequence of the activation energy of the acid s studied is diversion acid>gelled acid>HCl.The sequence of the reaction rate of the acids studied is HCl>gelled acid>diversion acid;(3)Based on the core digital minds analysis of the S field, the SEM magnification times of this field is determined as 2000×, the range of the grey level threshold value is 55-80; Compared the wormhole growth pattern between uniform random porosity distribution and the local porosity distribution in this paper, the local porosity can describe the carbonate reservoir heterogeneity better. According to the simulation results, the sequence of ability of forming wormhole near the wellbore of the acids is:HCl>gelled acid>diversion acid and far away from the wellbore, the effect of reaction rate on wormhole breakthrough volume is small;(4)The range of the S field fractal dimension is 2.13-2.61 based on the box-counting method. From the analysis of the fracture surface we can see that the highest elevation and roughness were increasing with the fractal dimension increasing and the highest elevation and roughness were decreasing with the control coefficient increasing;(5)The simulation results of the N-S function shows that acid channel was easier to formed in strong heterogeneity fracture; The pseudo-3D analysis of filtration in the wormhole shows that there are more acid detained in the porosity with serious filtration;(6)The simulation results of the acid fractures formed by HCl, diversion acid and gelled acid shows that the corrosion rate of HC1 was the fastest and space formed by HCl is homogenous; Wormhole was formed under the effect of gelled acid; And diversion acid always concentrated in the wider zone;(7)The closure area was smaller while the closure stress, the control coefficient, the Young's Modulus and fractal dimension are growing larger; While the fracture width is larger, the non-Darcy seepage is more obvious and the fracture conductivity is larger; The larger the pressure differential between the core, the smaller the acid fracture conductivity is;(8)According to the acid fracture conductivity test of the S field, the uneven etching ability of gelled acid is outstanding. From the experimental data we can see that obvious acid channels were formed after the acid reaction and the fluid flow pattern satisfied the Darcy law; And the effect of closure stress on conductivity which were obtained by simulation coincided with the experimental results.Wormhole, acid fracture etching pattern and acid fracture conductivity which were generated in carbonate reservoir acid fracturing process were studied through theoretic and practical way on micro and macro perspectives which could be used to guide the acid fracturing design and construction.