Forward modeling and inversion of induction logs from shaly sand reservoirs using petrophysical conductivity models

Algorithms and software have been designed for petrophysical 2D forward simulation and inversion of induction logs from oil wells drilled in shaly sandstone using conductivity models for different clay types. As comparative analysis of relative amplitudes and phases of tool responses has demonstrated, it is possible to reliably and accurately estimate fluid saturation and porosity from induction logs. The applied 2D numerical inversion of both theoretical and well logs yielded quantitative estimates of petrophysical parameters of reservoirs, with clay volume fraction assumed according to the chosen conductivity model. Correction for the presence of clay improves inversion quality if clay content as well as type are taken into account.     

Series Expansion-Based Genetic Inversion of Wireline Logging Data

An evolutionary approach is applied to solve the nonlinear well logging inverse problem. In the framework of the proposed interval inversion method, nuclear, sonic, and laterolog resistivity data measured at an arbitrary depth interval are jointly inverted, where the depth variation of porosity, water saturation, and shale volume is expanded into series using Legendre polynomials as basis functions. In the interval inversion procedure, the series expansion coefficients are estimated by using an adaptive float-encoded genetic algorithm. Since the solution of the inverse problem using traditional linear optimization tools highly depends on the selection of the initial model, a heuristic search is necessary to reduce the initial model dependence of the interval inversion procedure. The genetic inversion strategy used in interval inversion seeks the global extreme of the objective function and provides an estimate of the vertical distribution of petrophysical parameters, even starting the inversion procedure from extremely high distances from the optimum. For a faster computational process, after a couple of thousand generations, the genetic algorithm is replaced by some linear optimization steps. The added advantage of using the Marquardt algorithm is the possibility to characterize the accuracy of the series expansion coefficients and derived petrophysical properties. A Hungarian oil field example demonstrates the feasibility and stability of the improved interval inversion method. As a significance, the genetic inversion method does not require prior knowledge or strong restrictions on the values of petrophysical properties and gives highly reliable estimation results practically independent of the initial model and core information.

     

Exploratory Factor Analysis of Wireline Logs Using a Float-Encoded Genetic Algorithm

In the paper, a novel inversion approach is used for the solution of the problem of factor analysis. The float-encoded genetic algorithm as a global optimization method is implemented to extract factor variables using open-hole logging data. The suggested statistical workflow is used to give a reliable estimate for not only the factors but also the related petrophysical properties in hydrocarbon formations. In the first step, the factor loadings and scores are estimated by Jöreskog’s fast approximate method, which are gradually improved by the genetic algorithm. The forward problem is solved to calculate wireline logs directly from the factor scores. In each generation, the observed and calculated well logs are compared to update the factor population. During the genetic algorithm run, the average fitness of factor populations is maximized to give the best fit between the observed and theoretical data. By using the empirical relation between the first factor and formation shaliness, the shale volume is estimated along the borehole. Permeability as a derived quantity also correlates with the first factor, which allows its determination from an independent source. The estimation results agree well with those of independent deterministic modeling and core measurements. Case studies from Hungary and the USA demonstrate the feasibility of the global optimization based factor analysis, which provides a useful tool for improved reservoir characterization.     

Conductivity of layered reservoirs in induction data processing: continuous-function approximation

We suggest to approximate the depth-dependent conductivity of layered formations by continuous functions and use this approach to design an algorithm for 2D inversion of induction logging data. The logs represent formations consisting of alternating thin clay and sand layers of varying thickness, in which the sand layers can involve a developed invaded zone of a complex structure. Comparison of synthetic logs in models with piecewise constant and continuous approximations of conductivity shows that the continuous-function approximation is accurate within 5%, and the accuracy improves as the average conductivity of the layered formation is taken into account.     

Monitoring of wastewater percolation in unsaturated sandy soil using geoelectrical measurements at Gabal el Asfar farm,northeast Cairo,Egypt

The use of wastewater for irrigation in sandy soil increases the pollution risk of the soil and may infiltrate to the shallow groundwater aquifer. In such environment, some important parameters need to be obtained for monitoring the wastewater in the unsaturated zone over the aquifer. These parameters include clay content, heterogeneities of the upper soils, depth to the aquifer and the variations of groundwater quality. In the present work, the efficiency of DC resistivity method in forms of 1-D and 2-D measurements was studied for wastewater monitoring in the Gabal el Asfar farm, northeast of Cairo, Egypt. Forty-one Schlumberger soundings (VES) were performed then followed by three pole-dipole 2-D profiles along some considered regions within the area. The resistivity measurements were integrated with the boreholes, hydrogeological and hydrochemical (surface and groundwater samples) information to draw a clear picture for the subsurface conditions. The obtained results were presented as cross sections and 3-D visualization to trace the clay intercalations within the unsaturated zone. In addition, a vulnerability map was created using the obtained results from 1-D Schlumberger survey and confirmed with the 2-D resistivity profiling. The obtained results have shown that the 2-D resistivity imaging technique is a powerful tool for mapping the small-scale variability within the unsaturated zone and the wastewater infiltration. However, limitations of resistivity techniques were observed in the area with limited resistivity contrast such as thin clay layers with brackish water background. Under that condition, the measured pattern of resistivity distributions depends on the applied electrode array, electrode spacing and using the available geological information during the inversion process.     

Reservoir characterization of basal sand zone of lower Goru Formation by petrophysical studies of geophysical logs

The lower Indus basin is one of the largest hydrocarbon producing sedimentary basins in Pakistan. It is characterized by the presence of many hydrocarbon-bearing fields including clastic and carbonates proven reservoirs from the Cretaceous to the Eocene age. This study has been carried out in the Sanghar oil field to evaluate the hydrocarbon prospects of basal sand zone of lower Goru Formation of Cretaceous by using complete suite of geophysical logs of different wells. The analytical formation evaluation by using petrophysical studies and neutron-density crossplots unveils that litho-facies mainly comprising of sandstone. The hydrocarbons potentialities of the formation zone have been characterized through various isoparameteric maps such as gross reservoir and net pay thickness, net-to-gross ratio, total and effective porosity, shaliness, and water and hydrocarbons saturation. The evaluated petrophysical studies show that the reservoir has net pay zone of thickness range 5 to 10 m, net-togross ratio range of 0.17 to 0.75, effective porosity range of 07 to 12 %, shaliness range of 27 to 40 % and hydrocarbon saturation range of 12 to 31 %. However, in the net pay zone hydrocarbon saturation reaches up to 95%. The isoparametric charts of petrophysically derived parameters reveal the aerial distribution of hydrocarbons accumulation in basal sand unit of the lower Goru Formation which may be helpful for further exploration.     

潮白河顺义段再生水补给区包气带理化参数相关性分析

土壤阳离子交换容量(CEC)、总有机碳(TOC)、粘土矿物总量和粘粒含量是影响再生水入渗过程中污染物在包气带及地下水含水层介质中迁移转化的四项重要指标,是反映包气带对污染物吸附性及生物作用的主要参数。本文选取北京市潮白河顺义段再生水补给区包气带作为对象,对CEC、TOC、粘土矿物总量及粘粒含量4项指标进行了两两相关性分析及函数拟合。结果表明,上述四项指标的大小均与埋深无关,只与介质本身特性有关,且两两之间存在着显著的相关性,相关系数均大于0.8。     

巴东组非饱和红土强度与变形特性试验研究

宜昌-巴东段高速公路修建中形成了大量的路堑边坡,上覆为巴东组红层残坡积红土。降雨条件下,土体强度急剧下降,导致大量路堑边坡失稳。目前针对巴东组残坡积红土强度与变形特性开展非饱和试验的研究较少,为探索基质吸力对红土强度的影响,对红土进行了土水特征曲线、三轴强度试验,采用GDS三轴系统对红土进行了饱和及非饱和剪切试验,非饱和三轴试验采用了常含水量试验方法。结果表明:基质吸力对巴东红土强度影响很大。土体强度随吸力增大而呈非线性增长; 而基质吸力随着含水量增大而减小,即巴东组非饱和红土强度特性随土体含水量增大而变差。最后拟合出巴东非饱和红土的经验抗剪强度公式。研究成果对该区域工程实践具有一定的参考意义。     

弹性参数直接反演技术在储层流体识别中的应用

叠前地震反演是目前应用于流体识别的主要技术,但在具体研究区储层地质地震特点的应用过程中,存在岩石物理基础研究薄弱、多解性等问题。为提高流体地震识别精度,以KD地区为例,开展了陆相碎屑岩典型储层的岩石物理基础、叠前地震反演技术研究及应用。利用河道砂岩实验室岩石物理测试数据,分析了岩性、物性及孔隙流体对岩石物理参数的影响;基于弹性阻抗方程,通过弹性参数直接反演获取对储层流体敏感的参数,对KD地区河道砂岩储层进行了流体识别,提高了流体地震识别精度,取得了较好的地质效果。     

Numerical modeling and inversion of electromagnetic logs in the wells drilled with biopolymer and oil-based mud

Algorithms and software for numerical modeling and inversion of electromagnetic logs in the wells drilled with biopolymer and oil-based mud are developed. The algorithms are based on linearized solutions of the forward and inverse problems of electromagnetic logging and permit fast modeling of induction logs and efficient recovery of electric conductivity around the well. Mathematical modeling is based on numerical-analytical solution of the 2D forward problem taking into account high conductivity contrast between the well and the formation. Linear inversion is based on SVD-decomposition of information matrix. The results of numerical modeling and inversion of synthetic and field logs at the intervals of fluid-saturated terrigenous and carbonate formations drilled using biopolymer and oil-based mud are given.     

Evaluation of probabilistic flow in two unsaturated soils

A variably saturated flow model is coupled to a first-order reliability algorithm to simulate unsaturated flow in two soils. The unsaturated soil properties are considered as uncertain variables with means, standard deviations, and marginal probability distributions. Thus, each simulation constitutes an unsaturated probability flow event. Sensitivities of the uncertain variables are estimated for each event. The unsaturated hydraulic properties of a fine-textured soil and a coarse-textured soil are used. The properties are based on the van Genuchten model. The flow domain has a recharge surface, a seepage boundary along the bottom, and a no-flow boundary along the sides. The uncertain variables are saturated water content, residual water content, van Genuchten model parameters alpha (α) and n, and saturated hydraulic conductivity. The objective is to evaluate the significance of each uncertain variable to the probabilistic flow. Under wet conditions, saturated water content and residual water content are the most significant uncertain variables in the sand. For dry conditions in the sand, however, the van Genuchten model parameters α and n are the most significant. Model parameter n and saturated hydraulic conductivity are the most significant for the wet clay loam. Saturated water content is most significant for the dry clay loam. Electronic Publication     

Mineralogical,petrophysical, and acoustic features of serpentinites,indicators of the paleodynamic conditions of their genesis (by the example of the Main Uralian Fault zone)

The composition, structure, and properties of different genetic types of serpentinites developed after ultrabasic rocks are studied. The rocks were sampled from massifs located along the Main Uralian Fault, in a Paleozoic subduction zone. We consider specific features of serpentinites developed after chromite-bearing ultrabasic rocks, in metamorphic zones of carbonate rocks, and within the zone of weathering of ultrabasic rocks. A comparative study of the microstructure and chemical and phase compositions of serpentinites is carried out by IR spectroscopy, scanning electron microscopy, and thermal, X-ray diffraction, and microprobe analyses. The petrophysical properties and acoustic emission of the rocks are examined. Several genetic and structural groups of serpentinites developed after different protoliths (mantle and crustal) in different geologic conditions are recognized.© 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.     

Discrimination of pore fluid and lithology of a well in X Field,Niger Delta,Nigeria

Quantitative rock physics analyses were used to determine the lithology and pore fluid of a reservoir in the Niger Delta. Inaccurate prediction of lithology and pore fluid results in the inaccurate determination of other petrophysical properties and parameters such as porosity, permeability, and net pay. This research is to predict lithology and pore fluid using rock physics analysis. However, reservoir zones were also predicted. Density, compressional wave velocity, and shear wave velocity logs were used as input to calculate elastic parameters such as velocity ratio, Poisson’s ratio, and bulk modulus. The calculated velocity ratio log was used to differentiate between sand and shale. Poisson’s ratio and velocity ratio using Goodway interpretation template were carried out and used to delineate pore fluid content, gas sand, oil sand, and sandstone formation from crossplot analysis.     

降雨入渗非饱和黏土边坡稳定性的极限平衡条分法研究

考虑非饱和黏土边坡的基质吸力和渗流力,根据水位线位于滑面上、下的临界平衡状态,分别建立了滑体条块极限平衡状态下力和力矩平衡式,获得了降雨条件下非饱和黏土边坡稳定性的极限平衡条分法计算式。通过试验、参量变换以及作用力的位置关系可以确定相关参量,并采用数值计算求解临界平衡状态下滑体条块的相互作用力系数和非饱和边坡安全系数。案例结果表明：考虑渗流力时的非饱和黏土边坡安全系数比不考虑渗流力的安全系数降低约13.8%,且考虑渗流力作用的条间力作用系数变化率明显大于不考虑渗流力的结果;当降雨强度超过一定值时,坡面径流很快形成,边坡出现不稳定的时间基本相同。     

Petrophysical zoning of the Natalka ore field

The petrophysical properties of 1168 rock and ore samples collected across the sections of the Natalka ore field have been determined. 235 of them were prepared as thin and polished sections and studied by geological-mineralogical methods. The general two-level petrophysical zoning of the ore field was established and substantiated. Its relations with ore areas, the interzonal space, altered wall rocks, and the mineral composition were determined. Both levels of the revealed petrophysical zoning are traced in the ore field over 4 km along the strike and 800 m down the dip. The subvertical conductivity zone (confined to the Omchak ore cluster) has spatial-genetic relations with the petrophysical changes, which reflect the established mineralogical composition and structure of the metasomatites and ores.     

The Sensitive Properties of Hydrate Reservoirs Based on Seismic Stereoscopic Detection Technology

Higher-precision determinations of hydrate reservoirs, hydrate saturation levels and storage estimations are important for guaranteeing the ability to continuously research, develop and utilize natural gas hydrate resources in China. With seismic stereoscopic detection technology, which fully combines the advantages of different seismic detection models, hydrate formation layers can be observed with multiangle, wide-azimuth, wide-band data with a high precision. This technique provides more reliable data for analyzing the distribution characteristics of gas hydrate reservoirs, establishing velocity models, and studying the hydrate-sensitive properties of petrophysical parameters;these data are of great significance for the exploration and development of natural gas hydrate resources. Based on a velocity model obtained from the analysis of horizontal streamer velocity data in the hydrate-bearing area of the Shenhu Sea, this paper uses three VCs(longitudinal spacing of 25 m) and four OBSs(transverse spacing of 200 m) to jointly detect seismic datasets consisting of wave points based on an inversion of traveltime imaging sections. Accordingly, by comparing the differences between the seismic phases in the original data and the forward-modeled seismic phases, multiangle coverage constraint corrections are applied to the initial velocity model, and the initial model is further optimized, thereby improving the imaging quality of the streamer data. Petrophysical elastic parameters are the physical parameters that are most directly and closely related to rock formations and reservoir physical properties. Based on the optimized velocity model, the rock elastic hydrate-sensitive parameters of the hydrate reservoirs in the study area are inverted, and the sensitivities of the petrophysical parameters to natural gas hydrates are investigated. According to an analysis of the inversion results obtained from these sensitive parameters, λρ, Vp and λμ are simultaneously controlled by the bulk modulus and shear modulus, while Vs and μρ are controlled only by the shear modulus, and the latter two parameters are less sensitive to hydrate-bearing layers. The bulk modulus is speculated to be more sensitive than the shear modulus to hydrates. In other words, estimating the specific gravity of the shear modulus among the combined parameters can affect the results from the combined elastic parameters regarding hydrate reservoirs.     

3-D P-wave velocity structure of the crust beneath the Loess Plateau and surrounding regions of China

The crustal P-wave velocity structure beneath the Loess Plateau and surrounding regions, which is a transition zone between the Tibetan Plateau and the Huabei and Huanan Blocks of China, was tomographically imaged for the first time down to the depth of 80 km. The seismic sources comprised both local and regional earthquakes and the reconstruction was accomplished using a newly developed simultaneous inversion procedure, which solves for both the earthquake hypocentres and the 3-D velocity field. Special features of the procedure include a modified shortest path algorithm for the bent ray tracing, an analytic Jacobian matrix for solution updating, and a damped, minimum norm, constrained non-linear solver based on a CG approach. The velocity structure obtained is more complex than previously thought. The lateral velocity variations are consistent with the U-shaped seismic ring structure but the vertical variations along the Fenwei seismic belt are consistent with the mechanism of the Mountain-Basin generalized system formation. The velocity images of Loess Plateau, the transition zone may help to build a geodynamic mechanism for inland China, to explain the formation of ground fissures so prevalent in the Loess Plateau.     

GEOPHYSICAL LOGGINGS AT TAZAWA LAKE DISTRICT,AKITA PERFECTURE

Test drilling was performed at Yoroibata of Tazawako-machi, Sempokugun, Akita prefecture, where uranium-bearing deposits were previously discovered by the car-borne radioactivity survey party of the Geological Survey of Japan. In the Ohrnata formation at the bottom of drill hole No. 3, a slight radioactivity anomaly was recognized at a depth of about 10 m, and a fractured zone was recognized from the electric logs. — Author.     

广西红粘土击实样强度特性与胀缩性能

通过对广西贵港红粘土重型击实样的室内试验研究,探讨了其力学特性、胀缩性能、孔径分布特征与含水量之间的关系。结果表明：干密度指标总体上能反映红粘土击实样的强度规律,但非饱和击实样强度峰值对应的含水量因基质吸力作用而偏小,饱和后土体由于吸水膨胀与基质吸力的消失,使得强度峰值对应含水量较饱和前明显增大,红粘土在最优含水量下压实,虽可获得很高的压实度,但饱和后的强度并非最大;红粘土击实样的胀缩性能主要由含水量决定,同时,受到干密度的影响;孔隙主要以孔径在0.01～0.05μm范围内的小孔隙为主,为进一步掌握红粘土的工程力学特性提供了帮助。     

Petrophysical properties assessment using wireline logs data at well#3 of Srikail gas field,Bangladesh

This study focused on the quantitative analysis of the petrophysical parameters in characterizing the reservoir properties of the Srikail gas field using multi-scale wireline logs. Petrophysical parameters (shale volume, porosity, water saturation and hydrocarbon saturation) were estimated from the combination of gamma ray log, resistivity log, density log and neutron log for three hydrocarbon (gas)-bearing zones at well#3. At the first time, log records at 0.1 m and 0.2 m intervals were read for this study. Result showed the average shale volume is 21.07%, 53.67% and 51.71% for zone-1, zone-2 and zone-3, respectively. For these zones, the estimated average porosity was 35.89%, 29.83% and 28.76%, respectively. The average water saturation of 31.54%, 16.83% and 23.39% and average hydrocarbon saturation of 68.46%, 83.17% and 76.61% were calculated for zone-1, zone-2 and zone-3, respectively. Thus zone-2 is regarded the most productive zone of well#3. It was found that the values of some parameters (porosity, hydrocarbon saturation and permeability) are higher than the existing results. Therefore, this study confirmed that the log reading at minute/close interval provides better quantitive values of the reservoir’s petrophysical properties. It is expected that this result will contribute to the national gas field development program in future.©2022 China Geology Editorial Office.