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One role of the petrophysicist is to characterize the fluids encountered in the reservoir. Detection of a change in fluid type in the rocks while drilling is usually straightforward with the use of gas and chromatographic measurements. Gas shows and oil shows while drilling are time-honored indicators of zones that need further investigation through logs, testers, and cores. In the rare case of gas-bearing, high-permeability rock drilled with high overbalance, gas will be flushed from the rock ahead of the bit, will not be circulated to the surface in the mud, and will not produce a gas show. Because hydrocarbons are not always part of a water-based-mud formulation, sophisticated analytical chemical techniques can be used on the oil and gas samples circulated to the surface and captured to determine the properties of hydrocarbons in a given zone penetrated by the drill bit.
Nuclear logging has been used in some form since the late 1920s to provide information on lithology and rock characteristics. Continued technological advances have provided improved methods for analyzing the measurements of natural and induced nuclear readings. Even with better tool designs, the long-standing problem remains that logging tools do not directly measure the formation properties that engineers, geologists, and petrophysicists need to describe a reservoir. The goal of log analysis is to map out the downhole values of reservoir characteristics chiefly as porosity, fluid saturations, and permeability. Unfortunately, nuclear-logging tools only measure gamma ray or neutron count rates at cleverly positioned detectors. Nuclear-log interpretation rests on smarter processing of these tool readings. Understanding what the tools really measure is the key to better log analysis. Consider some of the limitations of the current technology. Grouping nuclear logs according to their underlying nuclear physics highlights the blurry relationship between what they measure and what we expect from them. First, because a nuclear tool averages over a shallow bulk volume, the borehole often represents a major part of the tool's response. Second, even if all borehole effects can be removed, the fact remains that nuclear tools do not respond directly to reservoir properties. Sometimes, the reservoir parameter of interest does not even dominate the underlying physics of the tool. Historically, such problems have been addressed with calibrations at a few points accessible in the laboratory; these are then generalized into correction charts. Two books serve as excellent general introductions to the convoluted physics of logging tools. Nuclear logs work because gamma rays and neutrons are penetrating radiation. They can even penetrate casing, giving them a near monopoly in cased-hole formation evaluation.
Nuclear log interpretation is simply the practice of solving tool-response mixing-law equations with the judicious application of some assumptions and constraints. All interpretation is an approximate model. As more factors are taken into account, the interpretation usually improves, but the model becomes more complicated. For the neutron-porosity log, the simplest interpretation model is to naively accept the raw log reading. If the reservoir is shaly, or if the fluid density is not the same as water, a hydrogen-index linear-mixing law will generally do.
The term "petrophysics" was coined by G.E. Archie and J.H.M.A. Thomeer in a quiet bistro in The Hague. By their definition, petrophysics is the study of the physical and chemical properties of rocks and their contained fluids. It emphasizes those properties relating to the pore system and its fluid distribution and flow characteristics. These properties and their relationships are used to identify and evaluate hydrocarbon reservoirs, hydrocarbon sources, seals, and aquifers. The petrophysicist provides answer products needed and used by team members, as well as physical and chemical insights needed by other teammates. The reservoir and fluid characteristics to be determined are thickness (bed boundaries), lithology (rock type), porosity, fluid saturations and pressures, fluid identification and characterization, permeability (absolute), and fractional flow (oil, gas, water). It is easy to define these characteristics and to appreciate their part in the assessment of reserves. The difficult part comes in determining their actual value at a level of certainty needed to make economic decisions leading to development and production. The seven characteristics listed are interdependent (i.e., to properly determine porosity from a wireline log, one must know the lithology, fluid saturations, and fluid types). The science of petrophysics is then used to unscramble the hidden world of rock and fluid properties in reservoirs from just below the Earth's surface to ones more than four miles deep. The petrophysicist then takes on many characteristics of the fictional sleuth Sherlock Holmes to extrapolate, from the most meager of clues, the true picture of the subsurface reservoir using dogged determination to wrest all possible information from the available data, all the while enjoying the thrill of the hunt. How does the petrophysicist solve this difficult problem? Archie's general method is to subdivide the problem into smaller segments and iterate using all data until all data agree. One starting point is to determine rock types (petrofacies) wherein we identify pore type, pore size distribution, pore throat type, and pore throat distribution.
The near-wellbore environment is usually altered by the drilling process in several ways, one of which is mud filtrate invasion as a result of overbalance and/or imbibition. The size of the invaded zone depends on many parameters. The exact shape of the invaded zone is unknown but is assumed to be cylindrical. The radial extent of this invaded zone can be determined with multiple-spaced resistivity tools if the invasion process has altered resistivity, unless the depth of invasion is beyond the zone of investigation of the resistivity tool. If one is comfortable that the shallow-reading resistivity device responds solely from the invaded zone, then one can use the Archie relationship to compute water saturation, as discussed previously.
Nuclear magnetic resonance (NMR) log data can be analyzed independently or in combination with conventional-log and core data. As an independent logging service, NMR can provide porosity, permeability index, and complete information on fluid type and saturation of the flushed zone. Some data-interpretation methods operate in the echo-decay time domain, while others operate in the T2-relaxation domain. Estimation of residual-oil saturation is one of the oldest applications of NMR logging. Unlike resistivity-log analysis, NMR analysis does not rely on formation-water salinity to obtain water saturation.
Abstract Selection of the chemical composition of the mud, as well as an appropriate strategy for the future development of the well, requires knowledge of properties of the invaded zone and reservoir quality. The main objective of this study was to determine the evolution stages and properties of the invaded zone using induction logging while drilling (LWD propagation resistivity) and repeated measurements. For this purpose, numerical modeling of the invasion zone evolution, modeling of the induction logging signals, evaluation of mud/mudcake properties (measured in the lab) and analysis of drilling data were performed. As input data to simulate invasion zone evolution, the following parameters were used: formation fluid and drilling mud filtrate viscosities; Corey-Brooks constants in phase-permeability functions; formation brine and mud-filtrate resistivity; Waxman-Smits equation constants; mudcake filtration properties and drilling data. All these parameters are measured at the surface level both while drilling and while performing petrophysical measurements. The invasion simulation enables water saturation and salinity distribution to be obtained in the near-wellbore area at different moments in time. These characteristics are used to calculate resistivity distribution using the Waxman-Smits equation. The obtained distributions are utilized to calculate synthetic induction log signals. Joint analysis of the synthetic and measured signals make it possible to correct the formation evaluation. After the formation properties are corrected, the invasion simulation and induction log modeling is iterated again until the formation properties are found providing the best match between synthetic and measured signals. In other words, the induction time-lapse measurements were inverted to obtain the porosity, permeability and water saturation of the formation at given mudcake parameters. The obtained formation properties were compared against the reservoir properties determined for the neighboring wells with full logging suite and showed good correlation. The main advantage of the method is the use of additional available measurements such as drilling data and mudcake tests that are usually available on the rig site, but are not applied for formation evaluation. Extra core filtration tests with mudcake growth are used for fine model adjustment. In addition, the method can be recommended to estimate reservoir properties in sidetrack wells that are usually drilled with a limited set of LWD tools (gamma ray and induction logging). The main condition for successful application of the method is a presence of repeated time-lapse LWD induction logs.
Abstract In studying and development of unconventional reservoirs, it is important to know vertical component of resistivity. This information can be obtained by HRLA or Rt-Scanner type devices, but due to low oil prices, using of them in Russia is becoming economically inexpedient. This leads to the necessity to develop a quantitative interpretation of the data of traditional Russian electrologging methods, which can provide information of the structure and properties of deposits with minimal cost. In vertical wells, in high-resistivity deposits similar to the Bazhenov formation, gradient sondes are sensitive to anisotropy, which was shown by the results of modeling and interpretation of signals on a one-dimensional cylindrical-layered model (Sukhorukova et al. 2013). A set of gradient sondes with different lengths is included in the equipment of the lateral logging sounding (BKZor also Russian lateral logging) method and is often used in Russia in the vertical wells survey. However, the application of the method is difficult if the thickness of the beds is small. In this paper, I propose a technique for processing BKZ data measured in complicated geological medium with small thicknesses of interlayers. To increase the accuracy of determining the parameters of an anisotropic medium, a two-dimensional geoelectric model and finite-element algorithms for solving direct problems are used. The geological object of study is a major source of unconventional oil in Russia – the Bazhenov formation, but the proposed technique is applicable to other objects. Based on the results of data processing for 18 wells from the Fedorovskoe and Russkinskoye fields, the features of resistivity anisotropy coefficient distribution by depth at intervals of the Bazhenov formation have been revealed.
Abstract The presented study demonstrates the effect of different technological activities of drilling through a reservoir interval on induction logging while drilling and near wellbore resistivity. The drilling operations include drilling with different penetration rates, shutdowns for drill-pipe addition, back reaming and so on. To estimate this effect, numerical modeling has been applied for simulation of mud filtrate invasion followed by electromagnetic modeling of induction tool signals. When modeling the invaded zone, the developed algorithm takes into account the drilling mode and mudcake formation. The computed near wellbore distributions of invasion zone parameters such as water saturation and salinity are then used to determine the resistivity distribution in the near wellbore area. The obtained geoelectric model of the well and the invaded zone makes it possible to compute the induction signals measured while drilling. It has been shown that neglecting features of the drilling operation may lead to improper interpretation of logging measurements.
Podberezhny, M.. (Salym Petroleum Development) | Polushkin, S.. (Salym Petroleum Development) | Marketz, F.. (Salym Petroleum Development) | Makarov, A.. (Baker Hughes Incorporated) | Sviridov, M.. (Baker Hughes Incorporated) | Mosin, A.. (Baker Hughes Incorporated)
Abstract The Salym Petroleum Company uses a perforated liner to complete horizontal wells. Selection of the chemical composition of the mud, as well as an appropriate strategy for the future development of the well, requires knowledge of properties of the invaded zone. The main objective of the study was to determine the evolution stages and properties of an invaded zone using induction logging while drilling (LWD) and repeated LWD measurements. For this purpose, numerical modeling of the invasion zone evolution and modeling of the induction logging signals were performed. The modeling showed a good correlation between LWD measurements taken at different times after reservoir drilling. We confirmed that invasion zone evolution modeling enables determination of the true properties of the uninvaded formation and further optimizes well development. Specifically, evolution of the invaded zone can be studied by using sets of induction LWD measurements acquired at different times.