1Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran
2Ahwaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iran
Determination of shale volume distribution is one of the most important factors that has to be considered in formation evaluation, since existence of shale reduces effective porosity and permeability of the reservoir. In this paper, shale volume and distribution (dispersed, laminar and structural) and formation effective porosity are estimated from well log data and cross-plots. Results show that distribution of shale is mainly dispersed with few of laminar ones, and the quality of reservoir (effective porosity) decreases with depth resulting in low productivity of gas wells drilled in lower zones. Good agreement of estimated shale volumes and effective porosities from neutron-density cross-plot with the values determined from gamma ray log (CGR) and core analysis demonstrates the accuracy and applicability of these plots in determination of petrophysical parameters from conventional log data.
Shale is a clay-rich heterogeneous rock which contains variable content of clay minerals (mostly illite, kaolinite, chlorite and montmorillonite) and organic matter [1, 2]. Presence of shale in the formation has sever effects on petrophysical properties and reduces effective and total porosity and permeability of the reservoir [3, 4]. Moreover, existence of shale causes uncertainties in formation evaluation and proper estimation of oil and gas reserves .
Shale distribution influences the evaluation of all principal reservoir characteristics e.g. effective porosity, water saturation, and permeability [6, 7]. Dispersed shale is composed of clay particles, fragments or crystals to be found on grain surface that occupy void spaces between matrix particles and reduce the effective porosity (φe) and permeability significantly . Structural shale exists in the form of fragments or crystals which are an integral part of the rock framework and is considered as a portion of rock matrix . Laminar shale exists as layer of shale which does not exceed 0.5 in. (1.27 cm) thickness within clean formations. The effect of two last shale types on porosity and permeability is assumed to be negligible [2, 10]. In this paper, shale volume and distribution type, and effective porosity of the formation are determined from well log data depicted on triangle density-neutron porosity cross-plots which are introduced as a quick and accurate method in determination of rock petrophysical parameters .
A triangle neutron-density porosity (φN Vs. φD) cross-plot is used to determine shale type and volume, and effective porosity (Fig. 1). Three distinct points (F, M, Sh) are shown in this cross-plot; Point F represents fluid or water point where φD= φN=100%. Point M represents matrix point; if density and neutron tools are calibrated in terms of the existing matrix, then φN=φD=0. Point SH represents shale point; the coordinate of point SH [φNSh , φDSh] must be determined for shaliest portion of well and this coordinate varies from well to well and has to be estimated for each case.
Data points representing φN and φD values in clean formations (i.e Vsh=0) fall on M-F line and their position on the line indicates effective porosity values. Line M-Sh represents φe=0 line and value of each point on this line indicates shale volume of the formation with zero effective porosity. Because of porosity values do not exceed 50%, line M-F is plotted till 50% porosity to make full use of cross-plot ([12, 13]. The following equations are used to construct this triangle cross-plot :
The laminar, dispersed and structural shale points areas fall on or around LS-Sh, DIS and STR lines, respectively. For each point within triangle VSh is estimated on M-Sh line parallel to clean formation line, and also, φe is determined on clean formation line parallel to M-Sh line. For example, point A in Fig. 1 represents a shaly formation that has values of φe=9% and VSh= 23% with dispersed shale content.
Figure 1. Triangle neutron-density porosity cross-plot
If formation contains hydrocarbons, neutron and density porosities have to be corrected before points are plotted since calculated shale volumes will be too low in gas-bearing intervals [14, 15]. The procedure of hydrocarbon correction is as follows .
where P is salinity of the mud (PPM×10), φN is neutron porosity, φNcorr and φdcorr are corrected neutron and density porosities, ρh is hydrocarbon density and Shr is residual hydrocarbon saturation. Hydrocarbon density ρh can be estimated by:
Residual gas saturation is used since neutron and density tools investigate the flushed zone. The portion of hydrocarbon in the invaded zone is given by Archie’s equation :
where φ is porosity, m is cementation factor, a is formula constant, Rmf is resistivity of mud filtrate and Rxo is resistivity of flushed zone and residual gas saturation is Shr= 1–Sxo.
In this study, Vsh estimated from cross-plot method is validated with Vsh calculated from gamma ray log (CGR). The following equations are used to determine shale volume:
where IGR is the gamma ray index, GRlog is the gamma ray response in the zone of interest, GRmin is the gamma ray response in cleanest formation, GRmax is the gamma ray response in shale layer. The shale volume (Vsh) can be calculated from the gamma ray index :
The studied formation is a lower Triassic sequence composed of eolithic calcite with few of anhydrite and secondary dolomite. Five depth intervals are selected so that different shale volumes exist in each part in order to extend the working criteria to a wide range of shaliness. The coordinate of shale point is determined from well logs to be φNsh=42% and φDsh=8%. Cross-plots for studied intervals are shown in Figures 2-7.
Raw data of neutron and density logs (PHI-N, PHI-D) and estimated shale volumes and effective porosities (PHIE) from cross-plot (VSH-CP) and gamma ray log (VSH-GR) for mentioned intervals are also presented in Tables 1-5.
It is observed that shale volumes estimated from cross plots are in good agreement with gamma ray log which proves the accuracy of this approach for shale characterization in formation evaluation process (Figures 8 and 9 are presented as sample comparison plots). Integration of neutron-density cross-plot analysis for above depth intervals shows that shale distribution in the studied field is mainly dispersed with few of laminar shale and the main reason of low productivity of wells is pore throat plugging by dispersed clay minerals. Based on effective porosity values estimated, it is also observed that the reservoir quality decreases with increasing depth and the majority of gas production is from upper zones.
Figure 8. Vsh from cross-plot Vs. Vsh from gamma ray log for 2718.5-2719.5m depth interval. Figure 9. Vsh from cross-plot method vs Vsh from gamma ray log for 2743-2744 m depth interval
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World Multidisciplinary Earth Sciences Symposium (WMESS 2016)
IOP Conf. Series: Earth and Environmental Science 44 (2016) 042002