Summary Textural and wettability variations are two main vexing problems in carbonate reservoirs. Adding heavy oil presents an incremental challenge to the reservoir description. Ineffectual analysis often leads to poor completion and high water production. Integrating multifrequency-dielectric and nuclear-magnetic-resonance (NMR) logs leads to improved carbonate-reservoir evaluation. Dielectric and NMR logs have similar depths of investigation, both probing the flushed zone. It is difficult to distinguish bound water from heavy oil in rocks from NMR tools alone. The addition of accurate water-filled-porosity (ϕW) measurements from the dielectric log allows us to differentiate the fluids. Using ϕW as a reference and accounting for restricted molecular diffusion in small pores, NMR analysis on the basis of diffusion relaxation provides more-accurate oil viscosity and fluid volumes. The computation of viscosity from NMR assumes that the oil relaxation is dominated by bulk relaxation; oil-wetting of pore surfaces causes viscosity to be overestimated. Conversely, the wetting state can be inferred if the viscosity is known. Dielectric dispersion with frequency provides a measure of the water-phase tortuosity MN_De, which in turn is affected by texture and wettability. We compared the in-situ NMR signal of oil to the surface NMR measurement of a bulk-oil sample. We also identified a correlation between MN_De and the Archie cementation exponent, mainly driven from NMR. This leads to interesting conclusions about texture and wettability, which are found to be coherent with the existing reservoir data. Together with the viscosity profile, the comparison between the pore-fluid volumes in the flushed zone and the bulk volume of water from deep resistivity is used to identify the zones of movable oil and to estimate residual oil saturation. The method was applied to data sets from three wells in a shallow carbonate reservoir. The estimated viscosity was validated by pressure/volume/temperature (PVT) analysis. In two wells, the prediction of movable oil and water zones was confirmed by downhole test results. In the third well, the free-water zone was confirmed by production logging.
The Paleocene[Eocene age 1st Eocene Reservoir is the shallowest producing interval of Wafra Field in the Partitioned Neutral Zone (PNZ), Saudi Arabia and Kuwait. Characterization of this heavy oil reservoir is challenging due to observed variations in oil viscosity, heterogeneity related to complex mineralogy, a possible dual porosity system, and the presence of fractures at varying scales. This case study of the 1st Eocene reservoir characterization in the steam flood pilot area will improve our understanding of the range and distribution of formation properties which is critical for management of the current pilot project. This study presents several aspects of an integrated approach to characterize the 1st Eocene reservoir. The approach includes the quantification and distribution of the evaporite minerals and porosity, analysis of a possible dual porosity system, and evaluation of permeability using a new porosity partitioning technique. Data used in this study includes conventional open-hole well logs, borehole images, nuclear magnetic resonance (NMR) logs, Elemental Capture Spectroscopy (ECS) logs, formation pressure measurements, and routine core analysis. The porosity partitioning model allows the core porosity-permeability relationship to be reproduced from logs by incorporating dual porosity information. The integration of NMR logs and image logs used in the porosity partitioning model provides a well log based permeability that has a good correlation with core plug data. Introduction The 1st Eocene reservoir of wafra filed is located in the Divided Zone (DZ), Saudi Arabia and Kuwait (Fig. 1). Currently, a large scale Steamflood pilot (LSP) consisting of sixteen, 2.5 acre inverted 5-spot patterns is being installed. This will be the first multi-pattern steam flood of a carbonate reservoir in the Middle East. Fig. 2 shows the locations of the LSP Steamflood project. Oil was first discovered in the 1st Eocene in 1954 and full scale development began in 1956. The 1st Eocene has produced only 4% of OOIP and primary recovery is not expected to exceed 10%. The oil API in this heavy oil reservoir varies from 14–20ºAPI. The 1st Eocene is a depletion drive reservoir with partial solution gas drive and limited aquifer support. The estimated gross OOIP within the currently delineated field limit is more than 10 billion reservoir barrels. A generalized Stratigraphic column for PNZ is given in Fig. 3. The reservoir mainly consists of dolomitized packstones and grainstones deposited under arid to semi-arid conditions on a shallow, very gently dipping, low to moderate energy inner shelf or ramp setting in a gently dipping, restricted ramp environment. The presence of minor interbedded evaporites suggests restriction was occasionally sufficient for the development of hyper-saline lagoons and sabkhas. The shallowing-upward cycles are capped by mud-dominated rocks, hardgrounds and exposure surfaces. The average porosity is 35% and the average permeability is 250 md over the gross interval. Well log and core plug porosity values over 50% are common and measured permeability values range up to 5000 md. The 1st Eocene reservoir has an average depth of about 1000 feet and a gross thickness of about 750 feet.1 A structure map is given in Fig. 2
TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractOptimizing production from existing boreholes requires the detection and evaluation of bypassed hydrocarbon and the ability to track fluid movement in the reservoir. Logging through casing is an important acquisition technique for this application. There are a number of measurements which provide reservoir saturation monitoring through casing, however each technology has limitations. The factors limiting the application of different measurement types relate to formation properties, completion type and production characteristics of the well. The selection of the most suitable technology requires consideration of the effect of the completion, well dynamics and formation characteristics on different measurements in addition to an understanding of the measurement physics.This case study is from Wara sandstone reservoir Wafra Field located in partitioned neutral zone (PNZ) between Kuwait and Saudi Arabia. The wells are producing with high water cut and reservoir surveillance requires an efficient method for saturation monitoring. The saturation monitoring in Wara is challenging due to relatively low formation water salinity, varying amount of clay content, and the depleted state of the reservoir. Application of slim, cased-hole formation resistivity measurements is successfully used to evaluate the saturation in challenging conditions. A lesson learned from this study is that proper job planning is critical for cased-hole logging. The planning phase is divided into two steps.The first part simulates the responses of the various tools using available reservoir information to identify the most appropriate saturation monitoring technique. The second part analyzes the logging conditions and implements appropriate well preparation measures to ensure representative data acquisition. The criteria developed for the selection of cased-hole resistivity measurements and the appropriate steps taken to ensure reliable data acquisition are presented in this paper.Proper job planning provided good quality time lapse formation resistivity logs in several cased-hole wells without the need to kill the wells. The study also contains a comparison between resistivity and sigma log evaluation of fluid contacts and quantification of water saturation in this shaly sandstone reservoir. It was found that due to low contrast and shallow depth of investigation, the sigma measurement does not provide a robust saturation analysis in the Wara formation in some cases. The cased-hole resistivity is used to evaluate time lapse saturation, which helped to identify by-passed oil zones. In addition, the zones which are most prone to high water production were identified and a work over plan is proposed and implemented. The time lapse cased-hole resistivity logs and the proper job planning has helped in successful saturation monitoring in this shaly sandstone reservoir.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.