Acid Leakoff Mechanism And Conductivity Optimization In Acid Fracturing In Heterogeneous Carbonate Reservoir

Marine carbonate formations in China generally have complex geological structure, strong heterogeneity and discontinuous oil-bearing reservoir bodies, which causes difficulties in achieving effective reservoir-to-well connection. Acid fracturing is an indispensable technique to develop these reservoirs. The acid etched fracture, with a high-conductivity and long length, is an essential condition for deeply penetrating the reservoir, to connect more oil-rich reservoir bodies and improve the post production. To reach the condition, the following key factors need to be considered:acid fracturing fluids leak off in heterogeneous porous media and the conductivity of acid etched fractures. However, the leak off mechanism of acid is still unclear. Meanwhile, the optimization of acid etched fracture conductivity evaluation experiments is insufficiency.Therefore, researches on characteristic of acid leak off and acid etched fracture conductivity were investigated in heterogeneous carbonate reservoir from physical modeling and physical and chemical seepage modeling perspective. The acid leak off mechanism was revealed. Meanwhile, the optimization methodology of acid etched fracture conductivity was developed, and the strategy, for enhancing the acid fracturing efficiency in heterogeneous carbonate reservoir, was obtained. Following research works have been done in the dissertation:(1) Both acid leak off test under macroscopic scale and scanning electron microscope analysis under microscopic scale were utilized to study the acid leak off characteristics of porous and fractured reservoirs.(2) Based on laboratory results, considering the strong heterogeneity of marine carbonate formations and the complexity of leak off media, the two-scale wormhole propagation model was developed. On the basis of the wormhole dynamic propagation model, coupling acid leak off model, formed a new acid leak off model under reservoir condition in consideration of wormholes. The discretization equations were established for the model based on the operator splitting method and finite volume method, and then solved by combing linear overrelaxation iteration method with extrapolation technique.(3) Based upon the acid leak off model which takes wormholes into account, numerical simulation was performed to analyze wormhole propagation mechanism and acid leak off characteristics in porous reservoirs, vuggy reservoirs, fractured reservoirs and pore-fracture reservoirs respectively, showing the mechanism of acid leak off in complex porous media.(4) Using self-developed acid etched fracture conductivity evaluation system, the evaluation experiments, referring to acid-etching conductivity under reservoir condition and the potential estimation for improving conductivity by adjusting parameters of acid injection and technology, were conducted on core samples from biohermal limestone of JN gas field and limestone of TZ gas field, based on the acid etched fracture conductivity obtained from numerical simulation. Aiming at improving fracture conductivity,19 experiments were conducted systematically for limestone, including operation parameter optimization experiments, multistage alternative acid injection experiments and multistage alternative acid injection experiments combining closure acid fracturing. By means of laboratory experiments, the influence of acid injection parameters and acid injection technology on fracture morphology and conductivity were obtained, and optimal injection parameters and acid injection technology were achieved. Consequently, systematic experimental methods for optimizing acid etched fracture conductivity were established.Applying the acid leak off model and acid etch physical simulation test, taking TZ region Yingshan Formation limestone for exmple, the following two suggestions for improve the post acid-fracturing are obtained:to reduce leak off, gelled acid system is appropriate for porous reservoirs while cross-linked acid is suitable for pore-fracture reservoirs; to improve long-term fracture conductivity, the injection parameters obtained from the acid-etching experiment is the optimal parameter for acid injection and two stage alternative acid injection method was recommended. Overall,9 field acid fracturing tests were performed, achieving 78% success ratio.This thesis, combined with theoretical study, laboratory work and field tests, initiatorily described acid leak off mechanism in marine carbonate reservoirs in China. Systematic physical simulation experiments for complex carbonate reservoir acid etched fracture conductivity optimization were proposed and hence, providing better proposal for heterogeneous carbonate reservoir acid fracturing design in our country.