| Permeability is one of the important indicators in the assessment of reservoir,the prediction of gas reservoir production has a decisive role.In the study of shale permeability,the density of features can reduce the accuracy of the conventional velocity experiment method.With the technology development of the gas pressure measurement,pressure transient techniques are widely used in laboratory unconventional reservoir permeability measurement.And traditional pressure pulse attenuation method ignores the stored gas compressibility and the influence of adsorption desorption characteristics,leads to deviation measurement results and the real value.The research object is taking from Chongqing county Shizhu under the Longmaxi formation and Wufeng formation of Lower Silurian.Through the electron microscope scanning,XRD test,the test of total organic carbon,laser Raman spectrum analysis experiment analyzed the characteristics of shale in the region;On the basis of the traditional pressure pulse method,introduced the related parameters of rock porosity and effective porosity,the Brace in the correction,the calculation methods for study at the same time analyses the stored gas compressibility and adsorption desorption characteristics on the result of permeability calculation;Improved experiment method,the pulse mode changed from one-way to two-way,through contrast experiment method before and after the improvement measurements of difference,to study the optimized method for seepage and the effect of permeability calculation process.In this paper,the main results are as follows:(1)Based on scanning electron microscopy to observe the sample internal microstructure,including all kinds of pore structure is given priority to with nanoscale,intergranular pore,inter-crystalline pore,particle inner pore and organic hole,etc.Sample of main minerals are brittle and two kinds of clay minerals content,its main components in quartz and illite,respectively.Test samples of the TOC content is between 2.5% ~ 3.5%,the overall average of 2.875%,belong to the better reservoir in the content of the rating.Based on the analysis of laser Raman spectrum detection,powder and block the maturity of the scope of 3.4613% ~ 3.7245% and 3.3759% ~ 3.7132% respectively.The region of the sample belong to high maturity,and indirect evidence has good potential.(2)The study of stress boundary condition of constant effect,in the change of permeability and pore pressure of the helium test curve,the slippage effect is the only factor;Methane seepage effect in measuring permeability curves is caused shale gas adsorption desorption expansion or contraction of the matrix and slippage effect of combined action result.Store gas compressibility of the main factors influencing the gas permeability is derived from the sample internal pore volume and the volume ratio of upstream and downstream container,with large container can effectively weaken the influence of gas compressibility.Adsorption desorption characteristics of the gas from two aspects to influence the calculation of permeability,rock sample matrix expansion when the adsorption volume,narrow makes the gas pipeline,the permeability decreases,and the sample resolution gas booster that substrate contraction and permeability;In sample adsorption desorption reaction occurs,will change within the system total gas content,leading to the final equilibrium pressure change,which affect the permeability.(3)Changing the traditional transient seepage experiment method,calculation formula can continue to use.Initial equilibrium pressure is different from the final balance of gas compressibility effects on permeability in bidirectional pulse attenuation can eliminate seepage experiments,experiments in constant balance of pore pressure makes the pressure difference.Upstream and downstream of the simultaneous adsorption desorption reaction,adsorption desorption reaction caused by the final equilibrium pressure and the balance of pseudo pressure difference is eliminated.And due to the upstream of the pulse sample substrate adsorption swelled with the downstream of the negative pulse sample matrix shrinkage effect on gas pipeline has been offset.Bidirectional pulse seepage experiments within the sample average pore pressure is constant,the sample internal real constant effective stress,so the final results are more accurate.The optimization of bidirectional pulse method compared with the traditional pulse filtration method is not only the factors can directly to avoid some unnecessary interference,but also can simplify the process of correction of calculation results,don't need any more additional isothermal adsorption,porosity determination,as well as experiments. |
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