Predicting Reservoir Properties of Carbonate Rocks on the Basis of their Sedimentation Heterogeneity and Secondary Transformations
Authors: Andrey Dushin , Gulnara Gaymaletdinova , Aleksandr Melnikov 
Published: October 2017

The object of the work is to study the factors influencing the formation of the void space of a carbonate reservoir in order to predict reservoir properties, as well as to implement the typification of rocks according to the degree of their transformation and the creation of appropriate recommendations for prediction of reservoir properties.

The article considers sedimentation processes and secondary transformations as well as the influence of these processes on the formation of a void space and the distribution of reservoir properties of carbonate rocks of the Upper Silurian and Lower Devonian deposits named after R. Trebs and A. Titov, which calls for a complex consideration of all factors affecting porosity and permeability, as well as reservoir connectivity and the application of a targeted approach if the influence of the afore-mentioned factors increases.

Methods for prediction of reservoir properties are based on identifying the range of factors influencing filtration-volumetric characteristics of rocks, their ranking in terms of their impact, as well as the establishment of remote and direct methods for their detection. This, during the first stage a sedimentary model of the Deposit was created and depositional factors controlling void space were identified.

Further, in the course of a detailed layer-by-layer description of the core and petrographic observations a number of secondary changes were identified, including dolomitization, cementation, compaction, fissuring and clay alteration.

Also, the influence of sedimentation processes and secondary changes on reservoir properties of rocks was studied and recommendations for their prediction were given.

Depending on the degree of influence of secondary transformations on the void space and properties of the reservoir, all the studied rocks were divided into three groups:

Carbonate rocks that retained their primary properties. In this case, the filtration-volumetric characteristics control the sedimentation conditions to a greater degree. The collector is unchanged or practically unchanged by secondary processes. In this case, the prediction of the properties can be based on the data obtained through direct sedimentation modelling, determining the relationship between the facial heterogeneity and filtration-volumetric characteristics of the reservoir. Carbonate rocks that partially lost their primary properties. The empty space of such reservoirs is modified due to the influence of secondary processes (mineralization, leaching, dolomitization and others). In such rocks, the in the reservoir changes can be detected by analysing the core scales. Petrophysical studies help in monitoring and predicting of heterogeneity by means of petrophysical classification and the heterogeneity data can afterwards be reconstructed basing on the geophysical data coming from the whole range of wells. Carbonate rocks that completely lost their primary properties. In such rocks, there is a significant change in the reservoir. The connection between the distribution of reservoir properties and the facial heterogeneity is very weak or practically absent. Prediction of properties should include studies performed on a larger scale (seismic) and must be normalized in accordance with the production well performance indicators (well production rate, the falling rate of production).

Thanks to the obtained data, approaches were revised in the modelling and distribution of filtration-volumetric characteristics and their connectivity.

The use of such complex methods which are based on heterogeneity as much as they possibly can, made it possible to predict the performance of operating wells, and was also confirmed by new drilling. Also, the results obtained are used in planning the location of new prospecting and production wells.