Improved Petrophysical Methods and Techniques for Shaly Sands Evaluation

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractClean sands have null or minimal shale and clay, and can adequately be evaluated using petrophysical techniques like neutron-density cross-plot and Archie's equation. Routine core analysis can be enough for validating log-analysis results in clean sands. On the other hand, shaly sands have considerable shale and clay; consequently the above petrophysical evaluation techniques are not adequate. Porosity logs, e.g. neutron, density and sonic, and saturation logs, i.e. deepresistivity, are affected by the shale and clay present in shaly sands. Additionally, shale and clay affect the electrical properties, the capillary pressure and the transmissibility in shaly sands. The effects of shale and clay, on logs and loganalysis, need to be modeled and incorporated in the petrophysical evaluation in order to obtain a more accurate assessment of shaly sands.Soft and hard data was acquired in some shaly sands, in Saudi Arabia, to evaluate their storage and flow capacity. Density, neutron, sonic, resistivity and gamma-ray logs were routinely acquired in wells drilled in these formations. Logs like Elemental-Capture-Spectroscopy (ECS) andNuclear-Magnetic-Resonance (NMR) were acquired in key wells. In some key wells, the core was cut and core analysis was conducted to obtain formation porosity, electrical properties and clay attributes, e.g. clay-type and clay-abundance. The above data was exploited in improving shaly sands petrophysical evaluation, which is discussed and demonstrated in this paper.

Accuracy Analysis of Water Saturation Models in Clean and Shaly Layers

“…are logging suites of resistivity , neutron , GR and density for wells A & B.GR was used to calculate shale volume ( V = 0.33 (2 2*IGR -1), when Vsh is <.10, section is considered clean sand and when Vsh is < 0.75, it is taken as shale section Neutron and density logs readings have been corrected from shale effect to give corrected porosity values for using either in Archie's formula or in shaly water saturation model The studied field examples produce from clean sand sections and shaly sand sections In clean sand, water saturation is computed by Archie's formula S w = (a R w /φ m R t ) 1/ n ……………………………… (8) An accurate computation of water saturation in shaly sand is subjected to many uncertain parameters It is necessary to integrate information from several types of logs using different interpretation models and local experience, in order to accurately determine the desired formation properties GR log is used to determine shale volume that has been used to correct density and neutron porosity values from shale effect to give corrected effective porosity used in water saturation computation In the analysis program; clean sand is assumed for Vsh < 0.10 and use Archie' formula to find Sw , shaly sand section for 0.10 < Vsh < 0.75 and use shaly sand water saturation model to find Sw and Shale section for Vsh > 0.75 and neglect it Water saturations have been computed for both clean sections and shaly sand sections using Archie parameters calculated by conventional , CAPE , 3-D techniques and common values This has been done by repeating the computer program four times, each tune with different Archie parameters a, in and n values. Considering the fact that water saturation has been computed in high shale volume sections as well as in low shale volume sections, we used three shale models and final water saturation value is taken as the average value of the three shaly sand water saturation models In effect clays are dispersed within sand grains, so, we selected three dispersed clay models In fact that there is no general shaly sand model can be applied in all cases For example Doll model often gives optimistic results, but in very shaly pay zone can be unrealistic and Poupon et al & Hossin models often become pessimistic in shaly pay sand located in transition zones and their application may condemn pay zones of commercial swnificance, [9][10][11][12][13][14][15][16][17][18] Figure 5 shows water saturation profiles for well A using Archie's parameters a, in and n calculated by four techniques Water saturation was computed in clean sand section with Archie's formula while in shaly sand section it was computed using shaly sand water saturation models Water saturation has been measured in the laboratory on 10 core samples from the studied section in the well, the average water saturation of all core samples was 30.5 %. Fig.…”

Development of Water Saturation Error Analysis Charts for Different Shaly Sand Models for Uncertainty Quantification of Volumetric In-Place Estimate (Russian)

“…Таким образом, мы сделали допущение, что все выбранные модели для настоящего исследования (модели Simandox 5 , Indonesian 6. 7, 8, 9, 10 , Waxman & Smits 11,12,13,14,15,16,17,18 ,Dual Water 19,20,21,22,23,24 , и Effective Medium 25, 26, 27 ) практически применимы к исследуемому пласту, не отдавая предпочтения ни одной из моделей. Таким образом, мы сталкиваемся с неопределенностью в оцененных петрофизических и электрических параметрах по каждой выбранной модели, т.е.…”

Uncertainty analysis of Archie’s parameters determination techniques in carbonate reservoirs

“…Petroleum literature presents an extensive review of the results determining Archie's parameters and also water saturation computation processes. Accuracy water saturation values rely on the uncertainty of Archie's parameters used either in Archie saturation equation in clean formation or in a shaly sand water saturation model in shaly formation (Archie 1942;Dewhite 1950;Atkins and Smits 1961;Simandoux 1963;Waxman and Smits 1968;Clavier et al 1984;Sen 1997;Kennedy et al 2001;Bori 1987;Al-Ruwaiili and Alwaheed 2004;Dernaika et al 2007). This paper presents the results of the application of three techniques to determine Archie's parameters: (1) three-dimensional regression (3D) technique, which is based on the analytical expression of three dimension plot of R t /R w versus S w and /, (2) core Archie's parameters estimate (CAPE) and (3) conventional technique.…”