Imparting Directional Dependence on Log-Derived Permeability
- J.H. Schön (Joanneum Research) | D.T. Georgi (Baker Atlas) | O. Fanini (Baker Atlas)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 2003
- Document Type
- Journal Paper
- 48 - 86
- 2003. Society of Petroleum Engineers
- 4.3.4 Scale, 5.6.4 Drillstem/Well Testing, 5.1 Reservoir Characterisation, 1.6.9 Coring, Fishing, 5.5.11 Formation Testing (e.g., Wireline, LWD), 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.5.2 Core Analysis, 2.4.3 Sand/Solids Control, 5.5.3 Scaling Methods, 5.5 Reservoir Simulation, 5.2.1 Phase Behavior and PVT Measurements, 5.3.1 Flow in Porous Media, 5.1.1 Exploration, Development, Structural Geology, 5.6.1 Open hole/cased hole log analysis, 5.7.2 Recovery Factors, 1.2.3 Rock properties, 4.1.5 Processing Equipment
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Cost-effective, efficient production of hydrocarbons depends on accurate reserves estimates, reservoir architecture, and hydrocarbon distribution. In addition, a complete reservoir description, including both horizontal and vertical permeability, is critical to efficient development strategy. Hydrocarbon recovery efficiency depends on many factors, but one key parameter is the ratio of vertical to horizontal permeability.
There are numerous ways to estimate permeability. It is now possible to estimate permeability from continuous formation evaluation measurements. However, it remains difficult to determine directional permeability. Nuclear magnetic resonance (NMR) logs are used routinely to estimate permeability, but NMR-derived permeability is based on scalar properties (e.g., f, T2 distributions from NMR, and bulk volume irreducible water) and is inherently a scalar property itself.
Few formation evaluation measurements provide directional information. Dip and image logs provide bed thickness and layer dip information, while the multicomponent induction instrument (3DEX*) and crossed-dipole shear-wave acoustic tools provide direct measurements of macroscopic formation anisotropy.
In this paper, we explore theoretically and with real data the computation of permeability anisotropy. We use laminated sand models as well as macroscopic models based on the resistivity anisotropy measurements to estimate kV :kH ratios. In addition to multicomponent induction data, we explore stand-alone and joint interpretation of NMR property variations to predict the macroscopic reservoir permeability anisotropy.
Thinly bedded laminated reservoirs exhibit "macroscopic anisotropy" of physical properties, including electrical conductivity and permeability. We define macroscopic to be the scale of the logging- tool measurement. Macroscopic anisotropy is manifested in thinly layered sedimentary formations where geologic processes deposit sediments in layers much thinner than logging tools can resolve.1-4
There are two types of layering, which we discuss in two separate sections:
Finely layered anisotropic sands (bimodal sands) - the fine layering can give rise to transverse anisotropic sands composed of layers of different grain sizes, porosity, and/or sorting (e.g., aeolian sands and turbidite sequences). When oil-saturated, the coarse fractions exhibit a relatively high resistivity as a result of low Swirr and high permeability, while the fine sands are characterized by lower resistivity values (high Swirr ) and lower permeabilities.
Laminated shaly sands - the sediments consist of thinly bedded sand/shale sequences. Sand layers are characterized by high resistivity and high permeability; shale layers are characterized by low resistivity and extremely low permeability (on the order of nanodarcies).
Generally, the higher contrast, in both resistivity and permeability, is associated with laminated shaly sands. This is especially the case for the permeability contrast.
The goals of detailed reservoir analysis for layered anisotropic sands are the same as they are for thicker, homogeneous reservoirs: volumetric analyses and flow-property determination. The volumetric reservoir description includes matrix volume fractions, porosity, permeability, and fluid properties and volumes. The flow properties are characterized by permeability, but permeability is inherently a tensorial property that varies with direction. Although permeability anisotropy can originate at the pore scale, we will focus on the macroscopic permeability associated with laminated systems. In this paper, we will deal only with transversely isotropic (TI) media, which can be described by the horizontal and vertical permeability and resistivity of the constituent thin beds.
New, multicomponent induction-logging hardware makes possible the direct measurement of resistivity and resistivity anisotropy. In two component-layered sediments, the resistivity tensor data allow a decomposition of the macroscopic conductivities into the contributions from the two components of the layered sediment and, therefore, are key to the determination of the volumetric properties (e.g., porosity and saturation). Further, the data can be used to infer permeability anisotropy.
Permeability and Permeability Anisotropy - Background
Permeability is a critical reservoir property that controls hydrocarbon production. Permeability controls the production rate, and the reservoir permeability controls the recovery efficiency. Thus, a goal of reservoir description has been to obtain permeability from log measurements since the first continuous wireline log data were collected. Generally, log-derived permeabilities have relied on the transformation of scalar properties to permeability. Most frequently, they are derived from core-based porosity/permeability correlations. Recently, log porosity data have been supplemented with NMR log-determined rock properties (e.g., mineralogy, independent porosity, and bound water saturation). This has improved the accuracy of log-derived permeabilities, particularly in clastic reservoirs.5
Log-derived permeabilities often are referred to as permeability indices with little regard to whether the computed permeabilities represent absolute or effective permeability, even though relative permeability effects are well understood by reservoir engineers and petrophysicists. Further, little regard is paid to whether the computed permeabilities represent horizontal or vertical permeability.
It is not too surprising that no directionality is attributed to log-derived permeabilities because they are generally based on scalar properties. Scalar properties are by definition independent of direction; thus, it is unreasonable to imply directionality to NMR log-derived permeabilities. Newly introduced multicomponent induction resistivity data (3DEX)6 carry directional information; in this paper, we examine a means for imparting directional properties to log-derived permeabilities.
Core-Derived Permeability Anisotropy.
The permeability of cores is routinely determined with small-diameter core plugs (typically a few inches in length and about an inch in diameter). These small plugs are usually cut either parallel to bedding or perpendicular to bedding. This makes it possible to estimate vertical and horizontal permeability (assuming that the bedding is horizontal).
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