Different Depths Of Investigation Research Articles

Assessment of True Formation Resistivity and Water Saturation in Deeply Invaded Tight-Gas Sandstones Based on the Combined Numerical Simulation of Mud-Filtrate Invasion and Resistivity Logs

The process of mud-filtrate invasion involves immiscible fluid displacement and salt mixing between mud filtrate and formation fluids in porous and permeable rocks. Consequently, the post-invasion spatial distribution of fluids and electrolyte concentration around the borehole affects resistivity measurements with different depths of investigation (DOI). In the presence of deep mud-filtrate invasion, the assessment of water saturation in the uninvaded zone based on the deep resistivity log can be inaccurate. Deep and electrically conductive filtrate invasion coupled with shoulder-bed effects can artificially increase water saturation (Sw) estimations by 20 saturation units (s.u.) in the Barik reservoir, resulting in pessimistic estimates of hydrocarbon pore volume if no corrections are applied. The Barik sandstone reservoir, which is characterized by low porosity ( up to 14%), low-to-medium permeability (up to 40 md), and high residual gas saturation (40 to 50%), exhibits low storage capacity to admit the critical filtrate volume necessary for building an impermeable mudcake. Combined with multiple days of overbalanced exposure to saline water-based mud (WBM), mud-filtrate invasion results in deep and smooth radial transition zones where the uninvaded formation is far beyond the depth of investigation of laterolog tools. Deep resistivity values are, therefore, lower than the true formation resistivity. Additionally, numerical simulations of resistivity logs show that the resistivity reduction by conductive invasion is further aggravated by shoulder-bed effects when individual reservoir thickness falls below 2.5 m. This paper describes the implementation of a compositional fluid-flow simulator to numerically model WBM-filtrate invasion and mudcake buildup in vertical boreholes. The algorithm allows the simulation of physical dispersion and fluid displacement around the borehole in a multilayer model. Time-dependent radial profiles of Sw and salinity are combined with core-calibrated porosity and electrical properties to compute electrical resistivity via Archie’s formulation. Subsequently, numerically simulated logs are generated using vendor-specific forward model processing and compared against field measurements. This workflow was extensively tested in various reservoir intervals with a wide range of petrophysical rock types and drilling conditions. Results show that the deep laterolog exhibits low sensitivity to conductive filtrate invasion when reservoirs’ porosities are lower than 8%. Above that threshold value, invasion length is a nontrivial process involving multiple variables. Even though exposure time to openhole conditions is a key factor leading to deep invasion, certain reservoir characteristics can lead to deeper invasion at short exposure times and significantly increase uncorrected Sw estimates.

Intelligent Classification of Carbonate Reservoir Quality Using Multisource Geophysical Logging and Seismic Data

Reservoir quality evaluation is an integral part of oil–gas exploration and development in fracture-vug carbonate formations. Varieties of geophysical data with different advantages are used in reservoir evaluation. These multisource data include conventional logs (CLs), borehole imaging logs (BILs), borehole acoustic reflection imaging logs (BARILs), and even seismic data. Obviously, combining these data can get richer formation information, which is more reliable than single-source data. In this work, the fusion dataset was constructed by calculating the principal components of CLs, BILs, and BARILs. The fusion dataset describes the fracture-vug information in different depths of investigation. Then, the agglomerative hierarchical clustering was used to analyze the fusion data and evaluate the reservoir quality in the borehole. Finally, taking the borehole-side seismic data and borehole evaluation results as training data, an intelligent model was trained based on the random forest algorithm. Also, the intelligent model can be used for reservoir quality classification in a 3-D space. In the case study, the evaluation results of reservoir quality are consistent with the validation data, and the accuracy of the final intelligent model is 85.15%.

Multi-Layered High Permeability Conduits Connecting Onshore and Offshore Coastal Aquifers

Groundwater resources in coastal regions are facing enormous pressure caused by population growth and climate change. Few studies have investigated whether offshore freshened groundwater systems are connected with terrestrial aquifers recharged by meteoric water, or paleo-groundwater systems that are no longer associated with terrestrial aquifers. Distinguishing between the two has important implications for potential extraction to alleviate water stress for many coastal communities, yet very little is known about these connections, mainly because it is difficult to acquire continuous subsurface information across the coastal transition zone. This study presents a first attempt to bridge this gap by combining three complementary near-surface electromagnetic methods to image groundwater pathways within braided alluvial gravels along the Canterbury coast, South Island, New Zealand. We show that collocated electromagnetic induction, ground penetrating radar, and transient electromagnetic measurements, which are sensitive to electrical contrasts between fresh (low conductivity) and saline (high conductivity) groundwater, adequately characterize hydrogeologic variations beneath a mixed sand gravel beach in close proximity to the Ashburton River mouth. The combined measurements - providing information at three different depths of investigation and resolution - show several conductive zones that are correlated with spatial variations in subsurface hydrogeology. We interpret the conductive zones as high permeability conduits corresponding to lenses of well-sorted gravels and secondary channel fill deposits within the braided river deposit architecture. The geophysical surveys provide the basis for a discharge model that fits our observations, namely that there is evidence of a multilayered system focusing groundwater flow through stacked high permeability gravel layers analogous to a subterranean river network. Coincident geophysical surveys in a region further offshore indicate the presence of a large, newly discovered freshened groundwater system, suggesting that the offshore system in the Canterbury Bight is connected with the terrestrial aquifer system.

Response simulation and theoretical calibration of a dual-induction resistivity LWD tool

In this paper, responses of a new dual-induction resistivity logging-while-drilling (LWD) tool in 3D inhomogeneous formation models are simulated by the vector finite element method (VFEM), the influences of the borehole, invaded zone, surrounding strata, and tool eccentricity are analyzed, and calibration loop parameters and calibration coefficients of the LWD tool are discussed. The results show that the tool has a greater depth of investigation than that of the existing electromagnetic propagation LWD tools and is more sensitive to azimuthal conductivity. Both deep and medium induction responses have linear relationships with the formation conductivity, considering optimal calibration loop parameters and calibration coefficients. Due to the different depths of investigation and resolution, deep induction and medium induction are affected differently by the formation model parameters, thereby having different correction factors. The simulation results can provide theoretical references for the research and interpretation of the dual-induction resistivity LWD tools.

Residual Oil Saturation Determination for EOR Projects in Means Field, a Mature West Texas Carbonate Field

Summary The technical success of an enhanced oil recovery (EOR) project depends on two main factors: first, the reservoir remaining oil saturation (ROS) after primary and secondary operations, and second, the recovery efficiency of the EOR process in mobilizing the ROS. These two interrelated parameters must be estimated before embarking on a time-consuming and costly process for designing and implementing an EOR process. The oil saturation can vary areally and vertically within the reservoir, and the distribution of the ROS will determine the success of the EOR injectants in mobilizing the remaining oil. There are many methods for determining the oil saturation (Chang et al. 1988; Pathak et al. 1989), and these include core analysis, well-log analysis, log/inject/log (LIL) procedures (Richardson et al. 1973; Reedy 1984), and single-well chemical tracer tests (SWCTT) (Deans and Carlisle 1986). These methods have different depths of investigation and different accuracies, and they all provide valuable information about the distribution of ROS. No single method achieves the best estimate of ROS, and a combination of all these methods is essential in developing a holistic picture of oil saturation and in assessing whether the oil in place (OIP) is large enough to justify the application of an EOR process. As Teletzke et al. (2010) have shown, EOR implementation is a complex process, and a staged, disciplined approach to identifying the key uncertainties and acquiring data for alleviating the uncertainties is essential. The largest uncertainty in some cases is the ROS in the reservoir. This paper presents the results from a fieldwide data acquisition program conducted in a west Texas carbonate reservoir to estimate ROS as part of an EOR project assessment. The Means field in west Texas has been producing for more than the past 75 years, and the producing mechanisms have included primary recovery, secondary waterflooding, and the application of a CO2 EOR process. The Means field is an excellent example of how the productive life and oil recovery can be increased by the application of new technology. The Means story is one of judicious application of appropriate EOR technology to the sustained development of a mature asset. The Means field is currently being evaluated for further expansion of the EOR process, and it was imperative to evaluate the oil saturation in the lower, previously undeveloped zones. This paper briefly outlines the production history, reservoir description, and reservoir management of the Means field, but this paper concentrates on the residual oil zone (ROZ) that underlies the main producing zone (MPZ) and describes a recent data acquisition program to evaluate the oil saturation in the ROZ. We discuss three major methods for evaluating the ROS: core analysis, LIL tests, and SWCTT tests.

Estimation of Variable Fluid-Mixture Density With 4D-NMR Logging

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 109051, "Estimation of Variable Fluid-Mixture Density With 4D-NMR Logging," by Chanh Cao Minh, SPE, Schlumberger; Emmanuel Caroli, Total; and Padmanabhan Sundararaman, SPE, Chevron, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11-14 November. The paper has not been peer reviewed. No technique is available to estimate the apparent fluid density measured by the density log. In wells drilled with oil-based mud (OBM), the near-wellbore-fluid composition and distribution can be very complex. Current practice for computing density porosity is to use an ad hoc constant fluid density. This method could lead to erroneous estimation of porosity. Radial fluid profiling with advanced magnetic-resonance tools provides radial information that can be inverted jointly with conventional 3D nuclear-magnetic-resonance (NMR) data to determine fluid mixture and radial distribution. Introduction Computing total porosity corrected for hydrocarbon effect from density/neutron logs gives an estimate of the hydrocarbon density when the matrix density is known. It assumes that the fluids are distributed uniformly within the investigated zones. Noting that near-wellbore fluids can vary radially and accounting for the different depths of investigation (DOIs) of the density and neutron logs enable computing total porosity in gas-bearing formations. Density and NMR log data can be combined to estimate total porosity in gas-bearing formations in the so-called DMR technique. However, the various sensors assess the same fluids. In spite of their shallow DOI, each sensor has a distinct DOI and fluid-averaging scheme. For example, NMR has a shallow DOI on the order of 1 in. to a few inches and a narrow sensitivity zone in the formation. Classical nuclear tools have deeper DOIs, ranging from a few inches for the density tool to 1 ft for the neutron tool, along with much broader investigation zones than NMR tools. Even though NMR and density logs might agree in many circumstances, they may not assess the same fluids. 3D NMR is a standalone technique to evaluate near-wellbore fluids at a fixed DOI. In some cases, complex fluid mixtures including water, oil, OBM filtrate, and gas are observed. Slow logging speed and unfavorable vertical resolution prevent routine use of the technique to estimate total porosity. 4D-NMR logging adds a radial dimension and can deliver continuous fluid mapping at multiple DOIs called shells, typically at 1.5, 2.7, and 4 in., as shown in Fig. 1. The simultaneous processing of data from all shells is intended to optimize the fluid results. Therefore, the variable fluid-mixture density can be estimated with 3D NMR at multiple DOIs and then used in the density-log-porosity computation, assuming that the matrix density is known.

Forward modeling of fracture-induced sonic anisotropy using a combination of borehole image and sonic logs

We develop a methodology to model and interpret borehole dipole sonic anisotropy related to the effect of geologic fractures, using a forward-modeling approach. We use a classical excess-compliance fracture model that relies on the orientation of the individual fractures, the elastic properties of the host rock, and the normal and tangential fracture-compliance parameters. Orientations of individual fractures are extracted from borehole-image log analysis. The model is validated using borehole-resistivity image and sonic logs in a gas-sand reservoir over a [Formula: see text] (50 m) vertical interval of a well. Significant amounts of sonic anisotropy are observed at three zones, with a fast-shear azimuth (FSA) exhibiting 60° of variation and slowness difference between 2% and 16%. Numerous quasivertical fractures with varying dip azimuths are identified on the image log at the locations of strong sonic anisotropy. The maximum horizontal-stress direction, given by breakouts and drilling-induced fractures, is shown not to be aligned with the strike of natural fractures. We show that using just two adjustable fracture-compliance parameters, one fornatural fractures and one for drilling-induced fractures, is an excel-lent first-order approximation to explain the fracture-induced anisotropy response over a depth interval of [Formula: see text]. Given the presence of gas and the absence of clay filling within the fractures, we assumed equal normal and tangential compliances. The two inverted normal compliances are [Formula: see text] and [Formula: see text]. Predicted FSA matches measured FSA over [Formula: see text] (40 m) of the [Formula: see text] (50 m) studied interval. Predicted slowness anisotropy matches the overall variation and measured values of anisotropy for two of the three strong anisotropy zones. Analysis of the symmetries of the modeled anisotropic response shows that the medium is mostly a horizontal transverse isotropic medium, with small azimuthal variation of the symmetry axis. Analysis of each independent fracture type shows that the anisotropy is mainly driven by open or partially healed fractures, but also consistent with stress-related, drilling-induced fractures. Therefore, the measured sonic anisotropy is caused by the combination of stress and fracture effects where the predominance of one mechanism over the other is depth-dependent. This method provides a consistent approach to data interpretation by integrating borehole image and sonic logs that probe the formation at different depths of investigation around the borehole.

A 3D parametric inversion algorithm for triaxial induction data

We develop a parametric inversion algorithm to determine simultaneously the horizontal and vertical resistivities of both the formation and invasion zones, invasion radius, bed boundary upper location and thickness, and relative dip angle from electromagnetic triaxial induction logging data. This is a full 3D inverse scattering problem in transversally isotropic media. To acquire sufficient sensitivity to invert for all of these parameters, we collect the data using a multicomponent, multispacing induction array. For each transmitter-receiver spacing this multicomponent tool has sets of three orthogonal transmitter and receiver coils. At each logging point single-frequency data are collected at multiple spacings to obtain information at different depths of investigation. This inversion problem is solved iteratively with a constrained regularized Gauss-Newton minimization scheme. As documented in the literature, the main computational bottleneck when solving this full 3D inverse problem is the CPU time associated with constructing the Jacobian matrix. In this study, to achieve the inversion results within a reasonable computational time, we implement a dual grid approach wherein the Jacobian matrix is computed using a very coarse optimal grid. Furthermore, to regularize the inversion process we use the so-called multiplicative regularization technique. This technique automatically determines the regularization parameter. Synthetic data tests indicate the developed inversion algorithm is robust in extracting formation and invasion anisotropic resistivities, invasion radii, bed boundary locations, relative dip, and azimuth angle from multispacing, multicomponent induction logging data.

Assessing the performance of urban BMPs in Scotland

Best Management Practices (BMPs) in urban drainage have been promoted in Scotland for a number of years in response to the need to combat pollution arising from diffuse sources in urban areas. This has led to the construction of around one hundred BMP installations using both source and end of pipe control. These are principally in the East of Scotland, and the number is growing rapidly as education and knowledge increases. A programme of investigations into the factors which influence the performance of the systems commenced in 1997. A range of types of BMP are under study including both source control and end of pipe systems. The sites have been divided into groups for which different depths of investigation are being undertaken. The programme involves a number of parallel investigations requiring field studies, data reviews and enquiries to confirm applicability and performance in situ. Many of the factors which influence the selection, installation and operation of a particular system are social, legal and administrative in addition to those which relate to its design and construction. The different strands of the study have been developed to ensure that all influences are identified and evaluated.

Assessing the performance of urban BMPs in Scotland

Best Management Practices (BMPs) in urban drainage have been promoted in Scotland for a number of years in response to the need to combat pollution arising from diffuse sources m urban areas. This has led to the construction of around one hundred BMP installations using both source and end of pipe control. These are principally in the East of Scotland, and the number is growing rapidly as education and knowledge increases.A programme of investigations into the factors which influence the performance of the systems commenced in 1997. A range of types of BMP are under study including both source control and end of pipe systems. The sites have been divided into groups for which different depths of investigation are being undertaken. The programme involves a number of parallel investigations requiring field studies, data reviews and enquiries to confirm applicability and performance in situ. Many of the factors which influence the selection, installation and operation of a particular system are social, legal and administrative in addition to those which relate to its design and construction. The different strands of the study have been developed to ensure that all influences are identified and evaluated.

Case Histories and Applications of a New Slimhole MWD Multiple-Depth-of-Investigation Resistivitiy Sensor

Summary The use of measurement while drilling (MWD) resistivity tools in boreholes 8½ in. and greater in diameter has evolved over the past 10 years from a little-used, qualitative service to a routinely used quantitative service in directional, horizontal, and extended-reach wells. Today's applications include reconnaissance logging, geo-steering, pore pressure evaluation, wireline replacement, and enhanced formation evaluation. Over the last several years, there has been an increasing trend worldwide toward drilling smaller boreholes to optimize the costs of tubulars and gain greater drilling efficiencies. Until recently, there have been no MWD resistivity tools available for logging 6-in. diameter boreholes. A new multiple-depth-of-investigation MWD resistivity sensor housed in a nominal 4$34-in. diameter drill collar has been developed for use in slimhole drilling operations. This new MWD resistivity sensor provides eight different depths of investigation in any mud type and allows for derivation of the flushed zone resistivity, true resistivity, and the diameter of invasion across a wide range of formation resistivities. The tool is designed for boreholes larger than 5⅞ in. in diameter and can be used in medium-radius drilling applications. Extensive operations in the North Sea, Gulf of Mexico, south Texas, Middle East, Alaska, and Australia have demonstrated a variety of applications. Slimhole MWD resistivity has been shown to have use in geosteering horizontal wells to maintain and optimize wellbore placement. Additionally, it allows determination of true formation resistivity and diameter of invasion for formations with invasion effects and provides for application of advanced modeling techniques to enhance formation evaluation and log interpretation.

Well logging—A 25‐year perspective

Developments in the field of well logging over the last 25 years are reviewed. Surface and borehole instrumentation have evolved significantly, taking advantage of modern digital and analog integrated circuits. Most open‐hole petroleum well logs are now recorded digitally. Digital logs are also frequently acquired in cased‐hole petroleum, mineral, and geotechnical applications. Nuclear well‐log measurements have become accepted and reliable. New measurements include borehole compensated density and neutron‐porosity, sidewall epithermal neutron‐porosity, and most recently litho‐density. The neutron decay log, developed early in the 25‐year period, has undergone a number of major improvements since its introduction. Probes which make spectral measurements of natural gamma‐ray emission, and gamma‐ray emission from neutron interactions with matter have also been developed. Resistivity measurements are now made with probes which combine three or more sensors each with different depths of investigation so that information about the borehole invasion profile can be acquired. Acoustic logging methods have expressed major developments and improvements. The compensated sonic measurement was introduced early in the period along with the cement bond logging method. Interest in measurement of shear‐wave velocity has produced new direct shear‐wave measurements as well as improved acoustic probes for full‐waveform acoustic logging. Other interesting or promising methods which have been developed or improved during the period include the borehole televiewer, the borehole gravimeter, and the nuclear magnetic resonance log. The digital computer provides powerful capabilities for well‐log analysis both at the well site and in the office. Analysis of complex sand‐shale and carbonate formations using two or more logs in a simultaneous solution of a litho‐porosity model is now routine. Powerful signal processing techniques are being applied to “deconvolve” well logs, to enhance or synthesize images of the wellbore, and to estimate or extract information from full‐waveform acoustic logs. While new or improved measurements have been introduced and log analysts now have access to powerful computers and graphic work stations, understanding of the petrophysical significance of the measurements lags behind the basic hardware measurement and interpretation technology.

The Use of Well Logs To Monitor a Surfactant Flood Pilot Test

A logging program has been developed and used successfully to monitor the performance of a surfactant flood pilot test in Loudon field, Fayette County, IL. This program consisted of induction and carbon/oxygen (C/O) log measurements that were made repeatedly in five observation wells, producing time-lapse records of oil saturation changes. To enable use of induction logs, the observation wells were completed with fiberglass casing. Special efforts were made to maximize the accuracy of the oil saturations calculated from logs. These efforts included use of pressure core measurements of residual oil saturation for calibrating log responses, laboratory measurements of the electrical properties of cores, and special wellsite calibration and quality control procedures. The log responses in each observation well provided essential data for evaluating pilot performance. The logs measured pre- and postflood oil saturations as well as the arrival time, oil saturation, and duration of the bank of mobilized oil. Vertical stratification effects also were observed. The combination of induction and C/O logs proved an effective means of monitoring a surfactant flood. Saturation measurements from the C/O log complemented the induction log results very well, providing better vertical resolution and a response independent of salinity and formation electrical properties.