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Contents
E1.0 SHALY SHALY FORMAT FORMATION IONS.......................................... S................................................................. .............................................. .............................................. .........................1 ..1
E1.1 E1.1 INTRODU INTRODUCTIO CTION N .............................................. ..................................................................... .............................................. .............................................. .........................1 ..1 E1.2 POROSITY IN SHALY FORMATIONS.....................................................................................3 E1.3 EVALUATION OF SHALE VOLUME ( V )........................................ )............................................................... ........................................4 .................4 SH
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Introduction to Openhole Logging
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E1.0
Shaly Formations
E1.1 INTRODUCTION Shales are one of the most important common constituents of rocks in log analysis. Aside from their effects on porosity and permeability, this importance stems from their electrical properties, which have a great influence on the determination of fluid saturations.
Archie's water saturation equation relating formation resistivity to water saturation, assumes that formation water is the only electrically conductive material in the formation. The presence of another conductive material (e.g., shale) requires changes to either Archie's equation or the model relating resistivity to water saturation. As well, the presence of clay in the formation complicates the concept of porosity. The water associated with the clays can represent a significant amount of porosity. However, this porosity is not available as a potential reservoir for hydrocarbons. To this point, we have dealt with tool responses from our porosity devices that yield total porosity φ T . At this time we have to introduce a new term, effective porosity, φ e , which is that portion of the formation porosity available to contain and produce fluids.
The presence of shale in formations generally affects the response of the logging devices. In our discussions we usually speak of shaly sands; however, the presence of shale in carbonates can often be treated in a similar manner. As briefly mentioned before, we categorize the distribution of shaly material in formations in three possible ways (see Figure E1): 1) Laminar Shale: occurs when shale exists in the form of laminae or thin layers between thin layers of sand. The shale streaks do not actually influence the effective porosity of the sand layers in the formation; however, as the bulk volume of shale increases, the overall formation porosity decreases. The presence of the shale may have considerable influence on the logging tool responses. 2) Structural Shale: is defined as the type of shale that exists as grains or nodules in the formation matrix. It is considered to have properties similar to laminar shale.
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3) Dispersed Shale: occurs where the shaly material is dispersed through the sand to occupy part of the intergranular space. Dispersed shale reduces the pore space available for fluid accumulation and also reduces formation permeability. The evaluation of shaly sands requires that we assume some distribution model. With the advent of computers we can analyze formations on the basis of sedimentation principles. Here we determine the silt and wet clay content of the shale; the former is a maximum near the main sand body (high-energy deposition) and the wet clay becomes predominant as distance
from the main sand body increases (lowenergy deposition). When shales consist of wet clay and silt, the bulk volume fractions may be expressed as: V sh = V silt + V clay
Another commonly used expression is the silt index ( I silt ) where I silt = V sil / Vsh t
also V clay = V sh ( I – I silt ).
Figure E1: Forms of Shale Classified by Manner of Distribution in the Formation Pictoral Representations Above, Volumetric Representations Below
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E1.2
POROSITY IN SHALY FORMATIONS When a sand contains shale we cannot obtain an accurate value of effective porosity from one porosity log. The responses of the density and neutron logs to shale content in sands is considered to be the same as in nearby bedded shales, no matter what model of shale distribution is considered. On the other hand, sonic logs have quite a different response between laminated-structural and dispersed shales.
b) Neutron (CNL/SNP) Logs - Neutron tools respond to the amount of hydrogen in the formation. Because shales contain bound water, the porosity recorded by neutron devices in shaly sands is always higher than the effective porosity.
- In liquid-filled shaly sands, the neutron relationships may be written as φ N = φ e + V sh (φ Nsh )
a) Density Logs - When shale and sand matrix densities are close to each other, the density log is least affected by shale and reads close to the effective porosity. - When the shale matrix density is less than 2650 kg/m3 the density log in shaly sands will record porosities higher than the effective porosity. - When shale matrix density is greater than 2650 kg/m3, the density log in the shaly sands will record porosities lower then the effective porosity. - The relationship for liquid-filled shaly sands can be written as
ρb = ρ f φ e + ρma (1 - φe – V sh ) + ρsh V sh or ρb = (1 – φe )ρma + φ e ρ f + V sh (ρsh – ρma )
c) Sonic Logs - Sonic traveltime in shales rises because of the fluid content of the shales; hence, sonic porosities in shaly formations are always higher than the effective porosity. To further enable sonic porosity determination, we must also know what shale model is present, and also whether a compaction correction is necessary. - In compacted formations with shales present, a general sonic relationship may be written as
∆t log = (φ e – V sh )∆t m a + (V lam + V str )∆t sh + (φ e – V dis)∆t f - In uncompacted zones, sonic porosities derived from this relationship must also be corrected downward for the lack of compaction.
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E1.3
EVALUATION OF SHALE VOLUME ( V sh)
Basic methods of shale (clay) volume calculation use the following indicators: - Gamma ray - NGS tool - Spontaneous potential - φ N versus φ D crossplot φ N versus φ S crossplot a) Gamma Ray If the radioactivity of the shale content is constant and if no other mineral in the formation is radioactive, the gamma ray reading may be expressed as a function of clay content. The formula can be written as GR zone – GRclean
†V sh = GRshale – GRclean
b) NGS Natural Gamma Ray Spectrometry Tool By using only thorium and potassium components of the gamma ray signal, the radioactive uranium element not associated with shales will be eliminated. The same method is then applied to the NGS as that for a regular gamma ray.
CGRzone - CGRclean †Vsh = CGRshale - CGRclean These formulae will not hold true for zones that contain radioactive matrix materials or radioactive waters (e.g., granite wash sands). Similarly, this method will not hold true where nonradioactive shales occur. Some typical values for formations are - Clean Sandstone: GR = 15–30 API - Clean Carbonates - Dolomite: GR = 10–20 API - Limestone: GR = 8–15 A.P.I. - Shallow Cretaceous Shale: GR = 100– 140 API †Strictly speaking, all GR values should be corrected for borehole effect and formation density. However, this approximation is usually satisfactory.
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c) Chart Calculation The linear equations in (a) and (b) of this section are good first estimates of shale volume. Chart Vsh -1 (Figure E2) allows us to correct for the non-linear relationship between V sh and the GR deflection denoted as “x”. Line (1) is generally used, yielding good interpretation results.
d) Spontaneous Potential (SP) In waterbearing sands of low to moderate resistivity, the ratio of SSP (static SP) to PSP (pseudostatic SP) is indicative of clay content, where
α = PSP / SSP and V sh = 1 - α If hydrocarbons are present, α will be decreased because of the further reduction of PSP by the hydrocarbons. Also, when using this method to calculate V sh , suitable bed thickness must be present to obtain PSP and SSP .
Figure E2: Chart V sh - 1: Shale Model Correction
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