The Study On Damage Evaluation System Of Tight Sandstone Gas Reservoir

Tight sandstone gas is the focus of unconventional natural gas exploration and development,which usually needs to implement stimulation to achieve economic value.But the tight sandstone gas reservoir has special micro-reservoir geological characteristics,which will easily lead to reservior damages in the process of drilling,completion and stimulation.As a result,the production of gas is not ideal.The reservoir damage restrict the efficient exploration and development of tight sandstone gas.The damage evaluation of tight sandstone gas reservoir has important significance to decision-making in gas exploration and development.Until now,there is no systematic method and standard for the damage evaluation of tight sandstone gas reservoir.The evaluation of the reservoir damage mainly refers to the national petroleum industry standard for conventional reservoirs,which has some deficiencies such as lack of representative,single of evaluation parameter.Therefore,basing on the geological characteristics of tight sandstone gas reservoirs and aiming at the existing problems in the process of damage evaluation,this dissertation establishes a core preparation and testing method,uses multiple methods and parameters to evaluate,establishes a damage evaluation system for the matrix and fracture of tight sandstone gas reservoirs,and provides a reliable basis for the optimization of working fluid system and the selection of formula for tight sandstone gas reservoir.After systematic theoretical research and laboratory experiment evaluation,this dissertation obtains the following main achievements:(1)This thesis comparatively studies the lithology,physical properties and pore structure characteristics of tight sandstone gas reservoirs.As different shape factor of the matrix pore and fracture pore,This thesis converts NMR T2 spectrum to pore size distribution by 1linear transformation method" and "nonlinear segmentation function conversion method".The micro-pore of dense sandstone is characterized by MICP,LPA and NMR to break the low test accuracy of the pore throat radius<0.01 ?m by traditional high pressure mercury.(2)Pointing at the existing problems in the damage evaluation of matrix and fracture core,such as permeability test error,salting out,difficult fluid saturation and single evaluation parameters.This dissertation also establishes some new methods of "vacuum imbibition and high pressure saturated" to solve the problem of super dense core saturation,acquiring of the initial water saturation by high speed centrifugation and cold evaporation to avoid salting out and high bound water,eliminating microscopic heterogeneity affecting on experimental results by inactiving sensitive mineral of tight sandstone by high temperature,improving the testing approach of the the dense matrix's permeability before and after damage.(3)Aiming at the permeability as the main damage evaluation parameter of tight sandstone gas reservoir,this study establishes permeability-pore throat radius-water film thickness as the damage evaluation parameters and evaluation system,constitutes a corresponding experimental method,and deepens the mechanism and theory of tight sandstone gas reservoir by fluid damage.The experimental method of reservoir matrix includes establishing the damage evaluation method of the drilling fluid dynamic loss,establishing the experimental evaluation method of matrix core "breakdown" by fracturing fluid,characterizing fluid self-absorption and release by T2 spectra and nuclear magnetic imaging,and establishing the evaluation method of selecting release water-block reagents with NMR and high speed centrifugation.By using these methods,the damage degree and mechanism of matrix permeability are investigated(4)The damage type and mechanism of fracture by working fluid system are very different from those of the matrix.This study establishes permeability-fracture width as the damage evaluation parameters and evaluation system,constitutes a supporting experimental method.The mechanism of fracture damage mechanism and protection measures preferred of working fluid system was carried out.The fracture damage evaluation system includes comparative studying of stress sensitive damage,establishing experimental evaluation method of natural fracture leakage damage,using finite element to simulate asperity deformation characteristics in fracture stress-sensitive,and changing of fracture width by hydraulic pressure.On one hand,the achievements of this dissertation can provide a new method to evaluate the damage of conventional natural gas reservoirs.On the other hand,it can be used to the selection of working fluid during the exploration and development of other unconventional natural gas such as shale gas and coalbed methane.