Research Of Permeability Model And Measurement Methods In Shale

Permeability is one of the most critical reservoir parameters for shale gas reservoirs characterization and well-performance evaluation,and is critical for applications in unconventional gas recovery.Laboratory test is crucial for studying permeability.However,an accurate measurement of shale permeability is challenging due to the low/ultra-low permeability and testing results are vulnerable to the influencing factors such as gas adsorption,temperature,and experiment configurations.Researchers and engineers of shale gas extraction project are still working on it.This problem is perplexing the researchers and engineers of shale gas extraction project all the time.There are mainly three obstacles in measuring shale permeability.First,the pressure-pulse decay often shows quite different characteristics during the early time and the later time.Most approaches for estimating the permeability proposed in literature are required to use the later time pressure-pulse decay measurements.However,the later time data are often selected subjectively,lacking of a universal criterion.Second,the estimation of permeability is complicated due to possible existence of gas adsorption.Third,laboratory measurements for permeability anisotropy are very limited.Based on these deficiencies,this paper first presented a modified pressure pulse decay method.Then a one chamber pressure pulse decay（OC-PPD）method is developed.The canister degassing tests is utilized to determine permeability anisotropy.At last,permeability models with consideration of the temperature effects are built to predict gas exploration by injection CO₂.The main conclusions are listed as follows:（1）Permeability estimation from traditional pressure-pulse decay method is complicated by two facts:1)the decay curve often deviates from the single exponential behavior in the early-time period;2)possible existence of gas adsorption.We first present a thorough analysis of pressure-pulse propagation process to reveal the mechanism behind the early-time and later-time behaviors of pressure decay curve.Inspired by the findings from these analyses,a new scaled pressure is proposed which can:1)be easily used to distinguish the early-time and later-time data;2)make the decay curves of all cases into a single 1:1 straight line for later time.A new data proceeding method,which calculates permeability using the new scaled pressure with later-time data,is then developed.A MATLAB code is developed based on the proposed method.The new method could not only remove the effects of the adsorption on the permeability estimation but also identify the apparent porosity as well as proper adsorption model and parameters.The proposed method is verified by comparing with true values and calculated values through numerical simulations that cover variations in typical rock properties（porosity,permeability,slippage,and adsorption）and the experiment configurations.It is found that the new method is accurate and reliable for all test cases whereas the Brace’s and Cui’s approaches may cause permeability error in some cases.（2）A modified and improved one Chamber pressure pulse decay method（OC-PPD）for simultaneous measurement of rock adsorption and permeability has been developed.The analytical solution of the OC-PPD method has been obtained,and permeability estimation which considering gas adsorption effects has been built.A set of adsorption-permeability experiments were conducted on shale samples.The test results show that the effects of adsorption on permeability are influenced by two factors:1)adsorption-induced storage which causes an incremental in apparent porosity,leading to a significant error in permeability measurement if true porosity is use;2)adsorption-induced swelling which potentially closes the existing natural fractures and reduces the intrinsic permeability.The adsorption storage effects are more significant at low pressure and are influenced by the experiment configurations（the adsorption storage effect exhibits a maximum effect on permeability when volume ratio is 0.75）.At last,the empirical functions of permeability and adsorption-induced swelling,the amount of adsorbing gas are established.（3）A method for simultaneously determining axial and transverse permeability of tight reservoir cores by canister degassing test is presented.Analytical solution of continuity equation in anisotropic core sample with canister degassing tests is obtained.Axil and transverse permeability of the core are estimated by matching gas production data with analytical solution.The proposed method is verified by comparing with true values and calculated values which are obtained by using data generated from TOUGH+simulations.The method is applied to real data measured in canister degassing tests（CDT）involving two different directional shale core samples.Then the experimental results are compared to the results from pulse pressure decay（PPD）method.The research results show that the testing results of the two methods have the same trend.At last,the influence factors and limitations of the proposed method are discussed.The parametric study shows that the transverse and axial permeability affect the gas-flow transient time significantly,but have no effect on the cumulative gas volume,while the effects of initial gas pressure are on the contrary.For other parameters,such as porosity,rock dimensions,and desorption,can affect both the gas-flow transient time and the amount of gas produced.（4）A series of gas flow tests using He and CO₂ are conducted on shale samples through the modified PPD method.The influencing mechanisms of pore pressure and temperature on shale permeability are investigated.The research results show that the effects of pore pressure on shale permeability is altered as a combined action of the effective stress,the matrix swelling and the gas slippage.While the effects of temperature on shale permeability is altered as a combined action of the thermal expansion of grains,the gas slippage,thermal motion of gas molecular and the matrix swelling induced by adsorption.Permeability models with consideration of the temperature effects are built to predict gas exploration by injection CO₂.The proposed permeability models are validated by comparing the experimental results and analytic results,which gives satisfactory agreements between each other.