TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA comprehensive petrophysical study in a gas field has demonstrated the effective application of saturation-height functions for calculating water saturation. The results show that linking depositional and diagenetic rock fabric to hydraulic units, and then linking the hydraulic units to zones with similar core capillary pressure relationships, improved the accuracy of the models. In the study field, saturationheight functions provided accurate water saturation, and they can potentially overcome uncertainties associated with log interpretation, using Archie or shaly sand models. The saturation-height models were developed from core capillary pressure (Pc) data to calculate water saturation versus depth, which is independent of logs. Consequently, the core-based saturation height functions can be useful in the calibration of log-based petrophysical models.Capillary pressure curves from special core analysis (SCAL) studies were distributed into corresponding hydraulic units (HUs), based on the calculated flow zone indicators. Saturation-height function was then developed for each HU and used to calculate water saturation in the study field. The most accurate saturation model that evolved is a function of only porosity and height above free water level. It is a modification of Cuddy's 1 saturation function, which relates the bulk volume water (BVW) to height above free water level. Cuddy referred to the function as FOIL, but did not define the acronym. Like FOIL, the Modified FOIL function, that has been developed, does not require permeability in its application, and performed better than the Leverett J-function in this field.Modified FOIL model that has evolved out of rigorous core-log integration will provide more accurate petrophysical interpretation in shaly sands and thin bed units.

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.

No 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.

Cyclic Borehole Effects in Deviated Wells. Abstract Highly deviated wellbores sometimes suffer from a cyclic variation in borehole size. Even though the caliper oscillations may be relatively small, a salty mud can combine with the periodic hole size variation to produce wireline data that has been severely compromised. Interestingly, it may be the deepest reading tool (resistivity) which suffers the largest degradation. A straight-forward solution, calibrated to specific wellbore conditions, has been developed which facilitates a much more representative formation evaluation. Introduction Upon occasion, the bottom-hole drilling assembly in a deviated well can yield a cyclic, or corkscrew, hole which has small (+/− 1/4") oscillations about the mean wellbore radius. Figure 1 is the CBIL image of the borehole in one such instance, and it's seen that the borehole radius has a distinct, periodic character, with an approximate 4' wavelength. In this image, the wireline tool is rotating while being pulled out of the hole, giving a corkscrew appearance. The actual hole size, however, is oscillatory in that the hole dia meter systematically changes, along the wellbore (coaxial) direction. Typically, the problem arises when changing from sliding to rotating drilling modes. While sliding, the bent housing steerable mud motor produces an in-gauge hole, but when one switches to rotating (in order to drill ahead Lister), the bent assembly will often interact with the stabilizer, to produce alternating hole sizes. The period of the oscillation is determined by the bit-to-stabilizer length. While the volumetric variation is extremely small in comparison to the total volume seen by the deep resistivity tool (or even the porosity logs), the periodic nature of the caliper oscillation call yield a log anomaly (noise) which nearly overwhelms the basic formation response. By working in the frequency domain, however, it is possible to characterize the "noise" via a Fourier Transform of the caliper log, which is the physical manifestation of the problem. And to then design a filter which may be applied to the remainder of the logs. In effect, one removes from the resistivity and porosity data, those spatial frequencies which correspond to the "noise" found in the caliper log, leaving the basic formation response. We have typically encountered the problem in highly deviated wells. Since the formations of interest are usually thick and nearly horizontal, true formation variations in the historical sense (along the wellbore) are gradual. As a result, the high cut filter designed by examination of the caliper Fourier spectra, does not seriously compromise the resulting evaluation. The approach developed here is general, and may be applied in other deviated wells as necessary. The Fourier Transform concept can further be used to quantify bed boundary resolution, at various logging speeds, thereby allowing one to optimize (particularly uphole passes, for correlation purposes) wireline operations (minimizing rig time). The Fourier Transform The Fourier Transform amplitudes are key to proper filter design, so that one must be familiar with basic Fourier principles. P. 627