Petrophysical Evaluation Of Low-Contrast Low-Resistivity Thin-Bedded Shale And Sand Beds (Page 1 of 6) By Adrian Manescu, Baker Hughes; Sze-Fong Kho and Kwong-Tak Ha, Sarawak Shell Berhad., 1 Mar 2011

Geologically, thin beds are classified as medium–to-thick (greater than 10 cm), thin (3-10 cm) and very thin (less than 3 cm), with the lamina less than 1 cm thick (Campbell, 1967). In contrast, petrophysical thin beds are more loosely defined as beds that are below the resolution of the measuring tool's capability of reading a true value for the bed in question; for example, sandstone resistivity. For a gammaray device a thin bed might be about 1 m, for a high-resolution density/neutron it might be 30 cm, and for a borehole image log it could be approximately 3 cm. Very few conventional wireline logs can resolve below 10 cm without special inversion-processing methods.

In thin beds, if the bed is thick enough, the representative petrophysical measurement value is often taken at the measurement peak. The measured peaks, however, comprise some proportional average of the over and underlying beds. In the case of alternating thin-bedded shale and sandstone, shale beds are usually conductors. Due to the averaging effect, the apparent resistivity of hydrocarbonbearing sandstone is reduced and can result in underestimating the true hydrocarbon saturation or even missed pay.

Petrophysical evaluations of thin-bedded, laminated reservoirs, such as turbidite reservoirs, are incorrect if traditional effectiveporosity and effective-saturation models are employed. These models view thinbedded formations by bulk responses of the measurement instruments, and then proceed to compute bulk-volume shale and effective porosity/saturations without accounting for the petrophysical properties of the individual laminae. It is well known, however, that shale laminae have no effect on the intrinsic reservoir properties of the interbedded sand laminae.

Until the development of resistivity anisotropy measurements, image logs and Nuclear Magnetic Resonance (NMR) were the most important logs for evaluation of thin-bedded shale sand sections, such as the turbidite reservoir in our case study. While the vertical resolution for image logs is about 3 cm, NMR vertical resolution is poorer, and both logs do not provide measurements in the deep, uninvaded, virgin reservoir formation. This results in difficult quantification of sand laminae saturation in low–contrast, low–resistivity reservoirs.

Resistivity anisotropy measurements, such as those from the 3D Explorer (3DEX Elite™) Induction Logging Service, are deep-reading measurements. The tool acquires at each logging depth the full nine components of the magnetic field, at multiple frequencies in the range of 20–220 Khz. These components enable measurement of formation dip and azimuth, and formation horizontal and vertical resistivities. Using these measurements enables the transfer from a conventional scalar parallel–resistivity model to a tensor petrophysical model, focusing on the intrinsic reservoir petrophysical properties of the interbedded laminae: the sand and shale. Laminar sand porosity is derived from the Thomas–Stieber (Thomas et al., 1975) model, and the solution of the hydrocarbon saturation moves from an error-prone, bulk-volume effective saturation model to simple clean sand (Archie) or clean sand with dispersed shale (Waxman-Smits) total porosity models.

For the case study well, a logging program was devised to mitigate this turbidite, thin bed problem and to acquire data that enabled accurate saturation and net pay information. The key elements of the logging program were multicomponent induction logging and NMR.


Multi-Component Induction Logging

Traditional array–induction resistivity measurements are made using transmitter and receiver sensors with their axes parallel to the borehole and the tool's vertical axis. As the sensors are aligned along the borehole axis, these induction measurements are limited to a single dimension: horizontal in vertical holes. Over thin–bedded and shale intervals, horizontal resistivity is heavily biased toward low–resistivity shale and is less sensitive to the hydrocarbon-bearing sandstone resistivity (see the horizontal resistivity equation in Figure 1). For an accurate evaluation of low-resistivity pay in thinly bedded or laminated reservoirs, the addition of a vertical resistivity measurement provides much better sensitivity to the presence of hydrocarbons.

This vertical resistivity measurement is provided by an instrument with advanced induction logging technology. The instrument performs resistivity measurements in three dimensions, and the data interpretation yields horizontal and vertical resistivities that lead to reliable identification and accurate petrophysical evaluation of low-resistivity pay. The tool has three mutually orthogonal transmitter–receiver arrays that acquire a full 9–component magnetic tensor at multiple frequencies. Horizontal transmitter-receiver arrays on the instrument are sensitive to current flow perpendicular to formation bedding; hence the measured vertical resistivity is dominated by the highly resistive elements, which is usually the hydrocarbon-charged sandstone (see the serial resistivity equation in Figure 1).