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In contrast to monopole logging tools, dipole acoustic devices can excite a low-frequency flexural wave in the borehole at shear velocity. Low-frequency ( 1 kHz) dipole sources allow for shear-velocity determination that is much closer to seismic shear waves and permits acquisition of direct-shear velocities in slow and fast formations. However, increased noise (i.e., a lower signal-to-noise ratio) is one limitation of low-frequency operation. Noise has been reduced through improved acquisition electronics, the use of semi-rigid tool designs, and by choosing the operational mode of the dipole source. A semi-rigid tool body not only reduces the influence of the tool body on the measurement but also permits operation in deviated wells.
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.
Martinov, M E (TNK-Nyagan) | Kozlov, A V (TNK-Nyagan) | Platunov, A A (TNK-Nyagan) | Filimonov, A Y (Schlumberger) | Ezersky, D M (Schlumberger) | Egorov, S S (Schlumberger) | Charupa, M V (Schlumberger) | Tsiklakov, A M (Schlumberger) | Mikhalteva, I V (Schlumberger) | Weinheber, P (Schlumberger)
Abstract In a recent Em-Egovskoe well an extended logging suite was performed with the aim to evaluate the petrophysical properties of Jurassic and Paleozoic formations as well as to improve the structural geological model of this part of the oilfield and to do detailed characterization of the dynamic model by desired properties of formations and fluids. Apart from a standard "triple combo" logging suite the following advanced technologies were applied in the well: neutron-gamma spectroscopy, nuclear magnetic resonance, formation micro imager, formation testers in different modes of reservoir and fluid properties evaluation. Noteworthy, the zone of interest was considered to contain only oil-saturated reservoirs – no gas cap was expected. Indeed on the initial triple combo log data there were no routine gas attributes were observed. Gas-saturated reservoirs were only observed based on integrated analysis of standard and advanced log data, particularly, nuclear magnetic resonance and cross-dipole sonic measurements. Gas-saturated intervals were fully proven by formation tester using downhole fluid analysis (DFA) As a result, one of the Jurassic layers was acknowledged as gas/gas-condensate saturated down to the bottom and the rest of Jurassic intervals were found to be oil saturated. The Abalak formation was also encountered in this well and evaluated. Thin carbonate streaks were identified with the micro-imager and were tested with the dual-packer module of the wireline formation tester. The result was the first ever Abalak oil sample in this field. Furthermore, based on pressure transient analysis of the build-up from the pressure test it was suggested that these tight streaks are laterally discontinuous. Finally, we created a stress profile in the Jurassic and Paleozoic layers. Based on formation micro-imager and acoustic scanning measurements the maximum horizontal stress directions and magnitudes were estimated. Then dual-packer formation tester micro-stress measurements were made to acquire direct measurements of fracture closure pressure. These measurements were used to calibrate our stress model.
Previous studies of resistivity anisotropy have neglected crossbedding effects. This article analyzes induction response in crossbedded reservoirs using a new computer modeling code. The code computes the response of an induction logging tool as it orthogonally traverses many beds, each of which possesses different crossbedding characteristics. The crossbedding in each medium is described by a uniaxial conductivity tensor whose principal axes have strike and dip angles oriented arbitrarily with respect to the bedding planes. The code is numerically efficient; response for a tool logging through several beds can be generated in less than 15 minutes on a modem workstation. Results show that, for anisotropy coefficients less than 5, computed responses for both two-coil and multicoil devices vary in a continuous manner as the sondes cross a single bed boundary separating two infinitely thick beds. Furthermore, after correction for skin effect, the limiting log values far from the bed boundary are entirely predictable from a previously published formula. However, in vertical wells, when the crossbedding dip angle is 75" or greater and the anisotropy coefficient greater than or equal to 5, anomalously large readings appear in the vicinity of the bed boundaries. These large readings are similar to the polarization horns that occur in dipping beds at high-contrast isotropic interfaces. In the case of a thin bed (e.g., < 5 ft) located between two massive shoulder beds, the large anomalies from the bed boundaries merge into a single anomaly at the center of the bed. This behavior was not expected and can be quantified only by modeling. Modeled results are also used to analyze the Schlumberger AIT Array Induction Imager instrument response in a crossbedded reservoir in the Nugget formation where we expect different values of R, and Rh.