In recent years, high-frequency dielectric tools have become an increasingly accepted service for providing saturation, water salinity, and textural information. To obtain these petrophysical quantities from the measured dielectric data, a mixing law is used that relates the bulk formation complex permittivity (relative permittivity and conductivity) to the constituents of the system, including the water present in the pores. Several mixing laws have already been published in the literature. This paper is concerned with an integral part of all of them, which is the prediction of the water complex permittivity as a function of salinity, temperature, and pressure.The water complex permittivity is known to vary with salinity, temperature, pressure, and frequency of operation. We have established a dielectric water model as a function of these four quantities, and have recently validated this model with dielectric laboratory experiments on several water samples at high temperature and pressure. In this paper we fully disclose the model so that it can be used and adapted by the industry.We also demonstrate with a sensitivity analysis that using the correct water model is critical to the accuracy of the final petrophysical outputs.
We experimentally verified that material-averaging techniques can be used for numerically modeling the response of the crosswell electromagnetic (crosswell EM) system in a fractured medium. We have designed a scaled model of the crosswell EM system, and verified that the laboratory data are in good agreement with the simulated response for different cases of fractures present in a host medium. Simulations of realistic scenarios in a hydrocarbon-filled reservoir indicate that the presence of fracture clusters can significantly affect the crosswell EM system response. The magnitude of the effect primarily depends on the number of fractures and their dip relative to the transmitter and receiver wells. In particular, small clusters of fractures filled with saline connate water produce large artifacts on the response, provided that the relative dip to the wells is large enough. The presence of a fracture cluster creates a characteristic signature in the full crosswell EM tomographical data set; therefore, crosswell EM data provides information related to the main fractures that are present between the wells. The inclusion of the fracture information as an additional constraint to the inversion of the crosswell EM data is expected to provide an improved interpretation of the reservoir fluids in the interwell space.
Crosswell electromagnetic (X-well EM) resistivity is emerging as an intriguing technology for reservoir surveillance. It provides a cross-sectional resistivity image between two wells and has the potential to provide fluid distribution at an inter-well scale. It can be used for identifying bypassed hydrocarbons, monitoring macroscopic sweep efficiency, planning infill drilling, and improving effectiveness of reservoir simulation. It can be deployed for one-time or time-lapse surveys. A crosswell EM technology trial project is being conducted in an Upper Jurassic carbonate reservoir, at the Ghawar Field in Saudi Arabia, to monitor the movement of injected water flood front and map the fluid distribution. The project site is in Ghawar's southern region, Haradh Field, and consists of three wells in the oil-water contact zone where peripheral injection water may have produced an uneven flood-front distribution. Significant drilling and well deepening were required prior to the deployment of tools in the three-well triangle. In fact, one new well was drilled and two other wells were deepened by more than 200 m, so that good volumetric coverage could be obtained at the oil- water contact zone. Extensive logs, core and formation tests were also acquired to provide deterministic saturation profiles at the near wellbore region. Formation evaluation in the project area indicates that one of the wells was fully swept while a second well, some 400 m away, was not. In July 2007, crosswell EM surveys were acquired across the three Haradh wells. In spite of the large well separations, the acquired EM data had good quality, and good stations repeatability. Preliminary processing has revealed a structure consistent with the background structure but a clear image of the oil-water contact is yet to be made. Introduction The Haradh Field is in the southernmost part of the greater Ghawar Field - the largest single oil field in the world (Figure-1). Arab-D is a 100-m thick, highly prolific, upper Jurassic reservoir comprising a carbonate sequence of grainstones, packstones, and wackestones 1. The original sedimentary textures have been altered in many places by leaching, recrystallization, cementation, dolomitization, and fracturing, which have caused a variety of pore types 2 to coexist in Arab-D. Flood-front movement can be uneven in some parts of the reservoir. Reservoir porosity ranges from less than 10% at the base to over 30% at the top while permeability ranges from few millidarcies to more than one Darcy. The Arab-D reservoir in Ghawar has historically been operated at relatively low depletion rates. Flank water injection is being carried out to maintain pressure and to improve sweep efficiency in this reservoir. With current inter-well spacing, about 1 km, determining fluid distribution behind the flood front is a key challenge to maximizing recovery from this reservoir. Traditional reservoir fluid monitoring techniques, e.g. pulsed- neutron logs (PNL) and resistivity logs, have investigation depths ranging from few inches, for PNL logs, to about 3 m for the deepest resistivity logs 3. Therefore, they cannot be used effectively for flood-front monitoring at the inter-well scale.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe cross-well electromagnetic (EM) resistivity method is emerging as one of the technologies that can be deployed for very deep reservoir surveillance/monitoring measurements. It has the potential to provide fluid distribution mapping at interwell scale, and thus can be used for identification of bypassed hydrocarbon, monitoring of macroscopic sweep efficiency, planning infill drilling and improving effectiveness of reservoir simulation.A cross-well EM trial is being conducted in an Upper Jurassic carbonate reservoir in Saudi Arabia to monitor the flood-front and fluid distribution with a well separation at the upper limit of the operating envelope allowed by this technology. For best results, current technology limits the maximum distance between wells to circa 1000 m for open-hole to open-hole and 300 m for cased-hole to cased-hole. Extensive pre-job forward modeling was carried out to investigate the feasibility of obtaining useful results from the measurements with such large inter-well spacing. Several scenarios of flood-front movement, such as edge water due to super-K or fracture swarms, bottom water encroachment, coning and cusping, etc., were modeled.The modeling results were encouraging and many of the anticipated subsurface scenarios were successfully detected.
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