Journal of Geophysics and Engineering - Latest Papers

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Determining the segmentation point for calculating the fractal dimension from mercury injection capillary pressure curves in tight sandstone

Wed, 04/25/2018 - 7:00pm
Tight sandstone reservoirs usually exhibit complex pore structure and a high degree of microscopic pore structure heterogeneity. The fractal dimension of pores in tight sandstone calculated by a mercury injection capillary pressure (MICP) curve can be used for the quantitative analysis of pore structure characteristics. The MICP curve of tight sandstone in log–log coordinates can be divided into two straight lines using a segmentation point and the fractal dimension of the pores is calculated in each part. However, the selection of the segmentation point is currently one of the challenges in the studies on the pore fractal characteristics of rocks. The fractal dimensions are calculated based on the non-wetting phase saturation and the wetting phase saturation using the Swanson point, Capillary-Parachor point, and the point at r 30 as the segmentation point respectively. The results indicate that the fractal dimension of small pores calculated based on non-wetti...

Influence of data acquisition and signal preprocessing parameters on the resolution of dispersion image from active MASW survey

Wed, 04/25/2018 - 7:00pm
This article investigates the influence of the key preprocessing parameters on the quality of the dispersion image obtained from the active multichannel analysis of surface waves (MASW) survey. A dispersion image of the best resolution is a necessity to obtain a meaningful and reliable outcome from an active MASW survey. Based on the field experiments conducted at two different sites, having varying site characteristics, the effects of sampling frequency, sampling length, filtering and muting are highlighted on achieving a high-resolution dispersion image. It is observed that the sampling time is site dependent, and there exists an optimal combination of sampling time and sampling frequency to produce good quality dispersion images. Optimal sampling time should allow for the completion of phase propagation through the receiver array, while discarding noise adulteration arising due to excessive sampling. Frequency filtering of the signals is essential to remove adulterating noise...

The b -value analysis of bedded shale under cyclic loading tests

Tue, 04/17/2018 - 7:00pm
Research on the damage evolution of bedded shale which can be described by different scales of cracks from micro- to macro-damage is significant. The b -value representing the relative proportion of micro-cracking to macro-cracking could be a good index for estimating the damage process of granite, sandstone, and marble as most of the previous studies have reported. The variation of the b -value of shale specimens with different bedding plane orientations subjected to cyclic load was investigated here. To ensure the reliability of the determination of the b -values, both the least squares method and maximum likelihood estimate were applied. And the sample range of acoustic emission events in data processing was redefined so that the previously generated cracks as well as the current formed cracks in this time window could all be used to calculate the b -value which reflects the accumulation of damage in one loading cycle. Cyclic load tests on stress level...

An inverse Q filtering approach based on a varying stabilization factor

Tue, 04/17/2018 - 7:00pm
The stable approach of inverse Q filtering based on wavefield continuation theory can simultaneously perform amplitude compensation and phase correction. The instability of conventional full inverse Q filtering is overcome through adding a stabilization factor to the wavefield continuation, and a good application effect is achieved. However, the method is likely to lead to noise amplification while compensating for the effective signal of the seismic data, resulting in the decrease of the signal-to-noise ratio (SNR) of seismic data. This phenomenon is due to the constant stabilization factor being added to the inverse Q filtering method. This paper presents an inverse Q filtering method based on a varying stabilization factor, which is a function of the propagation distance of a seismic wave, frequency of seismic data, and quality factor Q . This method can avoid the amplification of noise energy which can be caused by using the constant stabili...

A consideration of sea effects on MT data obtained in an east Indonesian island: Lembata Island

Tue, 04/17/2018 - 7:00pm
Geothermal energy is renewable and emits only minute amounts of carbon dioxide gas. It can be the most effective energy source not only economically but also environmentally when considering power generation for remote and relatively small islands like those in Eastern Indonesia. For the development of geothermal energy in the islands, magnetotelluric (MT) surveys have been conducted, but failed to properly image the electrical conductivity structure of the subsurface due to the impact of the existence of the surrounding sea, known as the ‘sea effect’. MT data are severely distorted by the electrical conductivity of seawater and thus should be corrected before inversion or directly inverted to take into account the sea effects. However, both strategies can be effective only when the sea effects at a target area are properly simulated. This study investigated sea effects at Atadei geothermal field in Lembata Island to build a resistivity model that can accurately describe sea eff...

A nonlinear seepage model of gas and water transport in multi-scale shale gas reservoirs based on dynamic permeability

Tue, 04/17/2018 - 7:00pm
Flow in extremely low permeability shale gas reservoirs undergoes a transition from a Darcy regime to other flow regimes including slip flow, transition flow and free molecular flow, due to the significant effect of molecular collisions with pore walls on gas transport. These various flow regimes in shales and their effect on actual gas production are not clearly understood, and multi-fractured horizontal wells are crucial for the economic production of unconventional resources. Therefore, a numerical model which is valid for the entire Knudsen range (continuum flow, slip flow, transition flow and free molecular flow) in shale gas reservoirs has been developed, with the effect of gas–water flow and the simulation of hydraulic fracturing cracks taken into consideration as well. Furthermore, the effect of different flow regimes on both the dynamic permeability of shales and then the actual gas production rate are analyzed thoroughly, and we study the influencing factors such as ga...

3D paleotectonic stress field simulations and fracture prediction for marine-continental transitional facies forming a tight-sandstone reservoir in a highly deformed area

Tue, 04/17/2018 - 7:00pm
Marine-continental transitional facies coal measures that form tight sandstone reservoirs in heavily deformed areas are a new field for natural gas exploration. The degree of fracture development is the core factor in the enrichment of the potential natural gas resources. In this paper, using the Shanxi Formation in southern Qinshui Basin of eastern China as an example, the characteristics of fracture development in tight sandstone reservoirs were studied, and the paleotectonic stress field was reconstructed. On this basis, the fracture development of the area was predicted. The results show that fractures of different scales were developed in the tight sandstone of the Shanxi Formation and that over 90% of these fractures are tectonic. The types of fracture filling in the tight sandstone of the Shanxi Formation are mainly unfilled and half-filled. Fine geological modeling and paleostress simulations were conducted on the II sand group of the Shanxi Formation using the 3D finite...

Application of extended elastic impedance (EEI) inversion to reservoir from non-reservoir discrimination of Ghar reservoir in one Iranian oil field within Persian Gulf

Tue, 04/17/2018 - 7:00pm
The aim of this study is to discriminate reservoir from non-reservoir facies of the Ghar formation in one Iranian oil field within the Persian Gulf, using extended elastic impedance (EEI) inversion. The Ghar reservoir, as a member of the Asmari Formation, consists of reservoir unites with interbedded shales. It is often difficult to separate these units. The lithological facies discrimination of the Ghar reservoir was applied, using the simultaneous application of EEI attributes. The EEI inversion method used best ##IMG## [] {$\chi $} angles from cross-correlation based on a model-based inversion algorithm to generate the EEI attributes. In the facies discrimination, the lithological facies volume was provided based on EEI Lambda-Rho versus EEI Mu-Rho cross-plot mapping together with porosity, water saturation, shale volume, sand volume and dolomite volume, which attributes indicated good defini...

Impact of salinity and connate water on low salinity water injection in secondary and tertiary stages for enhanced oil recovery in carbonate oil reservoirs

Tue, 04/17/2018 - 7:00pm
Connate water saturation and its composition, as well as the salinity of injected water, have significant impact on the performance of low salinity water injection (LSWI). Due to the lack of experimental data and difficulties in performing experiments on carbonate rocks, the mechanisms and impact of pertinent parameters involved in the low salinity water flooding in carbonate oil reservoirs is not well understood. In this study, several core flood experiments were conducted on tight carbonate rocks under highly controlled conditions, and using seawater at various dilution ratios as the injected and connate water. The cores were initially established with different connate waters, in terms of saturation and salinity, and the injection scenarios were carried out under secondary and tertiary stages at various salinities. Oil recovery as well as composition and pH of effluent brine were measured to investigate the impact of connate water and salinity on LSWI. The results depicted th...

Integrating gravity and magnetic field data to delineate structurally controlled gold mineralization in the Sefwi Belt of Ghana

Tue, 04/17/2018 - 7:00pm
Gravity and magnetic surveys were used to delineate potential gold mineralization zones in the Sefwi belt of Ghana. The study area is an intrusive dominated area that hosts pockets of small scale mining operations locally referred to as Galamsey. These Galamsey operations are not guided by a scientific approach to back the trend of gold mineralization which is conventionally mined. The study aimed at mapping lithological units, structural setting and relating Galamsey sites to delineate potential zones of gold mineralization. A Scintrex CG5 gravimeter and GEM’s Overhauser magnetometer were used for gravity and magnetic data acquisition respectively. The magnetic data were corrected and enhancing filters such as reduction to the pole (RTP), analytical signal and first vertical derivative were applied using Oasis montaj 7.1. Gravity data were also reduced to the geoid using the Oasis montaj software to produce a complete Bouguer anomaly map. The regional/residual separation techni...

Analysis of the zoning excavation monitoring data of a narrow and deep foundation pit in a soft soil area

Tue, 04/17/2018 - 7:00pm
In order to study the influence of subarea excavation on the deformation and stress behavior of a subway foundation pit, the deep horizontal wall displacement, supporting axial force and surface deformation were measured and compared with other similar pit cases according to the case of a soft soil deep foundation pit for a metro station in Hangzhou. The results of the theoretical and experimental studies show that zoned excavation can reduce the aspect ratio of a single foundation pit and then the spatial effect of small excavation can be mobilized, which can effectively control lateral wall deformation and wall deformation. Higher stiffness can strengthen the integrity of the foundation pit and provide more lateral force to prevent excessive axial force fluctuation in the deep foundation pit, and its restraining effect is greater than the first excavation foundation pit. The difference in the distribution of buildings on both sides of the long and narrow pit will lead to a dif...

Influence of prefabricated fissure angle on sandstone damage and infrared radiation temperature characteristics

Fri, 04/13/2018 - 7:00pm
To study the influence of sandstone fissure angle on its characteristics, we performed sandstone uniaxial compression experiments with different prefabricated fissure angles. The infrared radiation temperature (IRT) response characteristics of the prefabricated fissure sandstone and the influences of the fissure angles on mechanical properties, failure modes and infrared radiation indicated the following: (1) the peak stress and failure rate increased as the prefabricated fissure angle increased; the stress peak σ f had a quadratic functional relationship with the prefabricated fissure angle α , and the elastic modulus E was linearly related to the angle of the prefabricated fissure. (2) When0° ≤ α < 45°, the prefabricated fissure tip formed wing cracks that were nearly perpendicular to the prefabricated fissure. When45° ≤ α < 75°, the prefabricated fissure tip formed wing cracks that were nearly parallel to the prefa...

Iterative deblending using the POCS algorithm in the approximate flattened domain

Thu, 04/12/2018 - 7:00pm
We proposed an improved method to eliminate the interference generated by simultaneous-source acquisition, which can help shorten the acquisition period and improve the quality of seismic data. An iterative mathematical framework is devised, which uses the projection onto convex sets algorithm to estimate the blending noise subtracted from the pseudo-deblended data to separate the blended data in an iterative way. Differently to the conventional method using the coherent-promoting operator only based on the curvelet transform, we combine the curvelet transform and the approximate flattened operator (AFO) to improve the deblended result, which can flatten seismic events approximately to preserve the details of useful signals. This is the first time that the AFO and the curvelet transform are combined to enhance the effect of the coherent-promoting operator and improve the performance of deblending. To display the advantages of the improved method, we use both simulated synthetic ...

Use of electromagnetic radiation from fractures for mining-induced stress field assessment

Thu, 04/12/2018 - 7:00pm
This paper presents an electromagnetic radiation-based method to qualitatively assess mine-induced stress field at the longwall face. We first analyze the correlation between electromagnetic (EM) signals and stress, and then measure the signals and drilling bits in both the strike and dip directions at the 11702 working face of Nuodong Coal Mine, China, which indirectly verifies this method. Results show that there is a positive correlation between EM intensity and stress. In the same coal seam, the frequency of EM signals is inversely proportional to the extended size of the cracks inside the coal rock, and the dominant frequency (DF) of the EM signal is negatively related to external stress. Test results in Nuodong Coal Mine show that the EM intensity and DF can well reflect a stress relief zone, concentration zone and original zone. In addition, through field test and theoretical calculation, we can ascertain that the crack propagation length is from several centimeters to gr...

Wavelet threshold de-noising of rock acoustic emission signals subjected to dynamic loads

Thu, 04/12/2018 - 7:00pm
The discrete wavelet transform theory was used to process non-stationary rock acoustic emission signals recorded in split Hopkinson pressure bar tests. Wavelet basis db10 was selected as the optimum wavelet for processing such signals by calculating the reconstruction root mean square error (RMSE) of each wavelet basis candidate, and the unbiased estimation procedure SURE was used to determine the threshold value for each decomposition level. Denoising by wavelet thresholding and conventional low-pass filter were compared by evaluating the signal to noise ratio and RMSE. Results show that discrete wavelet transform has a favorable denoising effect and better signal reconstruction capability, and also indicate that the wavelet technique is especially suitable for denoising non-stationary signals like the transient signals of rock acoustic emission under impact loading.

Characteristics and generation mechanism of ULF magnetic signals during coal deformation under uniaxial compression

Thu, 04/12/2018 - 7:00pm
Low-frequency magnetic signals with the advantages of fewer interfering factors and longer propagation ranges are more useful for improving the accuracy of monitoring and early warning technology for coal or rock dynamic disasters. In this paper, the characteristics of low-frequency magnetic signals generated during uniaxial compression failure of coal are investigated in the time and frequency domain. The generation mechanism of low-frequency magnetic signals is proposed based on the correlation between low-frequency magnetic signals and micro-seismic signals. The experimental results show that (1) abnormal ultra-low-frequency (ULF) magnetic signals can be clearly observed in the process of coal failure. The signals occur as oscillating pulses, with amplitudes between 13 and 156 nT. (2) Both the maximum amplitude and energy of the signals are both positively correlated with the strength of samples and loading speed. (3) Signals in the direction perpendicular to the crack surfac...

Evaluation of the pore structure of tight sandstone reservoirs based on multifractal analysis: a case study from the Kepingtage Formation in the Shuntuoguole uplift, Tarim Basin, NW China

Thu, 04/12/2018 - 7:00pm
Both the microscopic pores and the pore structure of a reservoir affect its properties, including permeability and the distribution of oil and gas. Due to the stronger pore structure heterogeneity in tight sandstone reservoirs, it is difficult to characterize the pore structure of tight sandstone with traditional Euclidean geometry. In this study, based on the comprehensive utilization of reservoir data, such as petrophysical property data, mercury injection capillary pressure data and thin sections images, image processing and multifractal analyses are used to classify and describe the pore structure of tight sandstone in the Kepingtage Formation in the Shuntuoguole low uplift, Tarim Basin. The relationships among pore structure parameters, physical properties, and multifractal feature parameters are discussed in detail. Finally, a pore structure classification method using multifractal parameters based on K-means clustering analysis is proposed and applied to tight sandstone. ...

Drilling rate prediction from petrophysical logs and mud logging data using an optimized multilayer perceptron neural network

Thu, 04/12/2018 - 7:00pm
Rate of penetration (ROP) enhancement serves as a key factor in reducing drilling time and hence drilling costs. ROP enhancement requires identification of the parameters affecting this rate. However, the large number of effective parameters, which are further immersed in noise, makes it difficult to present a highly accurate and comprehensive model. In the present research, in order to predict the drilling ROP in one of the vertical wells drilled into the Karanj Oilfield, a hybrid model composed of a multilayer perceptron (MLP) neural network together with either a particle swarm optimization (PSO) algorithm or a cuckoo optimization algorithm (COA) was used. For this purpose, first petrophysical logs and drilling data were denoised using the Savitzky–Golay filter. Then, the ‘plus- l -take- r ’ method was used to select superior features. Feature selection results indicated that an increase in the number of input parameters tends to reduce the error associated with the...

Joint velocity updating for anisotropic PP and PS prestack time migration based on hyperbolic correction of nonhyperbolic moveout

Thu, 04/12/2018 - 7:00pm
The analysis of anisotropic velocity has been well researched to support multicomponent seismic migration of anisotropic media. Because of anisotropy and asymmetric ray paths, PS-wave reflection moveout is more nonhyperbolic than PP-wave reflection moveout, which makes conventional root-mean-square velocity analysis no longer applicable. Hence, additional work is needed to reform the existing seismic data processing software or to develop new velocity analysis modules. For this reason, we propose a method of joint anisotropic velocity updating for PP and PS prestack time migration based on the hyperbolic correction of nonhyperbolic moveout. Our velocity analysis method can be performed using conventional velocity analysis. According to the PP-wave moveout equation in VTI media, we derive an accurate double-square-root moveout equation for PS-waves, which is then used to generate PP and PS common-imaging-point gathers. The travel time and offset of each sample are maintained when...