Modeling electrical properties in complex carbonate reservoirs using well logs with geological and petrophysical evaluation
Date
2012
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Abstract
Electrical properties including the cementation or porosity exponent (m) and the water saturation exponent (n) are the most uncertain parameters in water saturation (Sw) calculations, and by extension in determination of hydrocarbons in place and reserves evaluation in carbonate rocks. Analogy constant values are used during the exploration phase and average values based on laboratory measurements of a small number of samples are used during the development phase. The use of these constant and average values can result in significant errors in the calculation of water saturation. As a result, this dissertation develops new approaches for estimating values of m and n from well logs at each depth point in complex carbonate reservoirs of the Middle East Manifa and Lower Ratawi reservoirs. The geological and petrophysical classification of the studied samples are important for understanding the pore and rock types present in the two reservoirs. Petrographic work reveals that Manifa reservoir has a greater variety of petrophysical properties and lower rock quality than Lower Ratawi reservoir. The non-connected (e.g. vuggy, moldic and fenestral) porosities are associated with lower quality rocks. Grain size has no relationship with rock quality in these carbonate rocks where cementation and dissolution are usually more influential. Comparison of petrophysical rock typing techniques shows that Winland/Aguilera rp35 technique provides robust results for classifying rock types at Manifa and Lower Ratawi. One of the advantages of Winland/Aguilera rp35 is that it does not require extensive quality control on results from well logs. However, quality control should be a regular and careful process with all datasets. An improved triple porosity model is developed to calculate m in vuggy and fractured carbonate reservoirs. The variation in m depends on the relative contribution of influential pore types, interparticle (r/Jb), fracture (rp2) and non-connected porosities (r/Jnc). The contribution of each category changes continuously with depth and as a result a method to calculate each proportion from conventional porosity logs is developed in this study. Manifa reservoir has higher contribution of non-connected porosity, r/Jnc, as compared with fracture porosity, r/)2, which leads to an increase in the estimated average m value to 2.18. Lower Ratawi reservoir has a slightly higher contribution of r/Jnc making the average m value equal to 2.04. The validity of the estimated m values from the improved triple porosity model developed in this thesis is compared with previous models and also against laboratory measurements of m. The comparison with cores indicates that the improved triple porosity model is a solid tool for estimating m values. A new method is also presented for estimating the water saturation exponent (n) for water-wet carbonate rocks with the use of the NMR log. The slope of the relationship between the resistivity index and water saturation is related to the average surface to volume ratio (S/V) of pores and pore throats. For the rocks considered in this research higher S/V ratio lead to larger n values and vice versa. A general correlation is shown to exist between n from laboratory measurements and the geometric mean of the T2 distribution. A regression model is established from this relationship for application in carbonate reservoirs. Estimated n values from the regression model show a better agreement with laboratory measurements as compared with previous models for Manifa and Lower Ratawi. The same approach is anticipated to have application in other carbonate reservoirs around the world. As the estimated electrical properties (m and n) using logs compare well with laboratory measurements, it can be concluded that the methodology developed in this thesis provides higher confidence in Sw calculations and reserves estimation. The Sw calculation using assumed constant values of m and n show large discrepancies when compared with Sw results using variable exponents with depth developed in this dissertation. The study provides proper understanding of m and n exponents coupled with sound theoretical models. The need for the developed models arises particularly in the case of exploration wells where there is generally a lack of experimental data on a timely basis.
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Bibliography: p. 196-205
Many pages are in colour.
Many pages are in colour.
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Citation
Al-Ghamdi, A. A. (2012). Modeling electrical properties in complex carbonate reservoirs using well logs with geological and petrophysical evaluation (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/5112