Study On Fractal Acid-etched Morphology And Reaction Kinetics Of Carbonate Rocks

In recent years,China has discovered large oil and gas reservoirs with carbonate rocks in the Tarim Basin,Sichuan Basin and other places.Some reservoirs are deeply buried,poor in physical properties and high in temperature.Consequently,it is necessary to implement the measure of reservoir stimulations.Acid fracturing has proven to be a valid and widely-used measurement for increasing production in carbonate reservoirs.The physical and chemical reactions of active acdds and rock minerals would alter the mineral composition,pore structure and surface wettability,and gradually change the surface morphologies as well as the kinetics characteristics of acid-rock reaction.The current researches on the acid etched morphology mainly focused on the macro scale.Meanwhile,the study of carbonate rock and acid reaction has not objectively reflected the non-homogeneous and non-catalytic reaction of liquid and solid on the surface of acid and rock,which failed to descript the characterization of acid etched morphology.Furthermore,the acid rock reaction area is usually treated as a certain value during data processing of reaction kinetics experiments.And the heterogeneity of minerals distribution in formations is also rarely reflected in acid fracturing model.In general,the study of carbonate rock and acid reaction has not objectively reflected the non-homogeneous and non-catalytic reaction of liquid and solid on the surface of rock.In response to the above deficiencies,the laws of multi-scale etched morphology and reaction kinetics have been preliminarily revealed through systematic researches.The main research contents and cognitions obtained include the following four aspects:(1)Based on chemical reaction kinetics,hydrodynamics and fractal theory,etc.the creation of acid etched surface and two types of reaction control model are proposed by comparing the relation and difference of mass transfer control and surface reaction control at first.And the advantages and disadvantages of various acid rock reaction experiments are analyzed to provide a theoretical basis and reference for multi-scale etched researches next.(2)Considering the effects of various factors such as acid types(styles and concentrations),reaction environment(injection velocities and reaction temperatures),rock mineral compositions and so on,the multi-scale orthogonal etching experiments are designed and conducted.For the micro scale etching experiments,the morphology and images of rock dissolution before or after the reactions are captured by scanning electron microscope.The morphological changes of the feature points are recorded.The fractal dimension is obtained by the fractal Differential Box-counting Method,and the law of acid etching is quantitatively analyzed eventually.For the macro scale etching experiments,rotating disk experiments are adopted,and the data matrix of rock samples are acquired by 3D laser scanner.On the one hand,the real contacted areas of acid-etched rock samples after reaction are calculated.On the other hand,the rock surface morphologies are gotten,and the key control factors of macroscopic etched morphology are summarized.(3)The true values of the reaction areas are acquired via computer programming,where the triangle area is calculated through the spatial relationships between any three adjacent data points,and then it is summed to get the modified acid-etched area at last.Therefore,the face and volume ratio is modified,so do the kinetic parameters after correction and the macroscopic reaction rate.(4)Geological model regarding the distribution of dolomite and limestone is established,and then the pseudo three-dimensional fracture propagation model as well as the acid flow reaction model is proposed,in which the dynamic equation could describe the influence of fracture width on the acid rock reaction of fracture geometrical characters and it contains the modified kinetic parameters.Combined with the assumption of setting initial conditions and boundary conditions,the numerical solution is obtained.And under the influence of rock mineral distribution and trend,the macro morphologies angle in reservoir scale are obtained.