Narrow Results

As one of the key status of gas in shale reservoir, adsorption gas accounts for considerable percentage of total gas amount. Due to the complexity and nanostructure of shale gas reservoir, it is very challenging to represent adsorption gas through traditional methods. However, the integration of the fractal theory and molecular dynamics (MD) simulation may provide a new perspective of understanding such nanostructure and the micro-phenomenon happening in it. The key purpose of this paper is to investigate the adsorption phenomenon in shale kerogen. By using MD simulation and grand canonical Monte Carlo (GCMC) algorithm, the adsorption of methane in 2, 5 and 10nm slit-like pores is simulated for different temperature and pressure status. According to the results, the average gas density in smaller pores is higher than that in bigger pores, and multilayer adsorption presents on some areas of pore surfaces. Then, the simulation results are analyzed using the multilayer fractal adsorption model. The analysis indicates that the number of adsorption layer increases with pressure increase: four-layer adsorption presents in 10nm pores while three-layer adsorption shows up in 2nm and 5nm pores due to pore volume limit. Fractal dimension of pore wall surface generated in this study is in the range of 2.31–2.63. Moreover, high temperature could decrease the adsorption behavior in reservoir condition.

Gas flow mechanisms in shale are urgent to clarify due to the complicated pore structure and low permeability. Core flow experiments were conducted under reservoir net confining stress with samples from the Longmaxi Shale to investigate the characteristics of nonlinear gas flow. Meanwhile, microstructure analyses and gas adsorption experiments are implemented. Experimental results indicate that non-Darcy flow in shale is remarkable and it has a close relationship with pore pressure. It is found that type of gas has a significant influence on permeability measurement and methane is chosen in this work to study the shale gas flow. Gas slippage effect and minimum threshold pressure gradient weaken with the increasing backpressure. It is demonstrated that gas flow regime would be either slip flow or transition flow with certain pore pressure and permeability. Experimental data computations and microstructure analyses confirm that hydraulic radius of flow tubes in shale are mostly less than 100 nm, indicating that there is no micron scale pore or throat which mainly contributes to flow. The results are significant for the study of gas flow in shale, and are beneficial for laboratory investigation of shale permeability.

This work provides a general analysis method for the crack detection in shales. The acoustic emission (AE) testing with data recovery is proposed for determining the crack modes and positions in the test and analysis process of the shale fracturing experiment. A fracturing and in situ AE monitoring system is constructed to collect the experimental data in at least six channels for the crack detection, and the source positions output from AE testing represent the positions of cracks. Due to some uncontrollable reasons, such as the poor coupling between sensors and sample and the sudden failure of the sensors, some parts of experimental data are missing during the experiments. Therefore, a data recovery neural network algorithm based on wavenet model is introduced to reconstruct the missing parts of experimental data in the waveforms. Since the accuracy of data recovery is not satisfying based on the collected experimental data, the interpolation of experimental data is performed to refine the data which can obviously improve the accuracy of data recovery. After all the required experimental data have been recovered, the crack mode for each crack can be determined based on the moment tensor analysis. This analysis method can be extensively applied to the shale crack detection.

The complex structure and surface property of porous media have significant impact on its accumulation and adsorption capacity. Based on the fractal theory, this paper presents a fractal pore structure model for shales. The effect of different pore structures on fractal dimension is discussed, and the influence of fractal dimension and pore size distribution on porosity is also analyzed. It is shown that the fractal dimension D decreases with the increase of structure parameter q/m for a certain pore diameter ratio, and porosity has positive relationship with fractal dimension. This paper also presents a multilayer fractal adsorption model which takes into account the roughness of adsorption surface by using fractal theory. With the introduction of pseudo-saturated vapor pressure in the supercritical temperature condition, the proposed adsorption model can be applied into a wider range of temperature. Based on the low-pressure nitrogen adsorption and methane isothermal adsorption experiments, the effect of fractal dimension on the adsorption behavior of shales is discussed. Fractal dimension has significant impact on the surface adsorption property and adsorption layer number n. The monolayer saturated adsorption volume V_m increases with the increase of D, while parameter C has the opposite variation trend. Finally, the optimal combination of fractal parameters for describing pore structure of shale samples is selected.

The apparent liquid permeability (ALP) of shale is challenging to be characterized due to complex wettability and nanopore size distribution. The nanopores in organic matter of shale are usually hydrophobic and the nanopores in inorganic matter are hydrophilic. The flow behaviors in these two different nanopores are quite different, and accurately predicting the ALP of shale is difficult. This paper proposes a fractal model for predicting the ALP of shale with dual wettability. The nonflowing boundary layer effect of water in inorganic pores and the slip effect in organic pores are considered, and the equations for describing the flow rate in single organic pore and inorganic pore are derived, respectively. With the assumption of the fractal distributions of organic pores and inorganic pores in shale, the analytical expression for predicting the ALP of shale is derived, and the key parameters influencing shale ALP are analyzed with sensitivity study. The research results show that the nonflowing boundary layer can reduce the ALP of inorganic pores, but slip effect will increase the ALP of organic pores. ALP of inorganic pores is affected by the thickness of nonflowing boundary layer, which is determined by the displacement pressure gradient, fluid viscosity, and pore size distribution. ALP of inorganic pores is more affected by contact angle and pore size distribution.

Formation damage evaluation is a key and basic link in optimizing working fluids. It is widely accepted that formation damage is the reduction of core plugs permeability caused by working fluid invasion. However, the measurement of permeability faces a huge challenge for shale formation, such as overspending, time-consuming and the scarcity of unbroken core plug samples. A new method of fractal analysis derived from Scanning Electron Microscopy (SEM) image of shale pore structure was used to quantify the shale formation damage. This method needs to select optimal magnification and segmentation threshold value of SEM image to obtain exact Fractal Dimension (FD) of pore structure. In this paper, we take the black shale outcrops from Sichuan Basin for an example. The results shows that the optimal magnification for observation of the pore structure using SEM imaging in this area is 1000×, and the optimal threshold value for binary image is 29 (RGB). Microscopic pore structure of the shale follows the fractal law, and the FDs increase with increasing measurement scales. It is evident that the evaluation results of shale formation damage when exposed to 2 wt.% NaOH solution and 2 wt.% brine solution using microstructure fractal are exceptionally in good agreement with permeability reduction results. The microstructure fractal obtained from SEM images provides a new method for evaluation of shale formation damage. And it can be applied to optimize the screening working fluids used in shale formation in real time under the condition of high temperature and high pressure.

The multiphase flow behavior in shale porous media is known to be affected by multiscale pore size, dual surface wettability, and nanoscale transport mechanisms. However, it has not been fully understood so far. In this study, fractal model of gas–water relative permeabilities (RP) in dual-wettability shale porous media for both injected water spontaneous imbibition and the flow back process are proposed using fractal geometry. The shale pore structure is described as tortuous with different pore sizes and morphologies including slit pore, equilateral triangle, circular pore and square pore. The proportion of each pore morphology can be obtained from SEM/FIB-SEM pore structure characterization results. Injected water spontaneous imbibition after hydraulic fracturing is modeled as the capillary force dominated process and injected water flow back is modeled as a non-wetting gas phase drainage process in inorganic matter. The organic pores are deemed to be not accessible by injected water. The boundary slip of water and free gas flow in the inorganic matrix are considered while both free gas flow and adsorbed gas flow are modeled in organic matter. The proposed gas–water RP fractal model is verified via comparisons with the available experimental data and is discussed in detail. Study results reveal that gas phase RP increases with increasing pore fractal dimensions and tortuosity fractal dimensions, whereas it decreases with increasing Total Organic Carbon (TOC) volumes. Water phase RP decreases with increasing of pore fractal dimensions and tortuosity fractal dimensions, whereas it increases with increasing TOC volumes.