Maclean O. Amabeoku

Evaluation and Application of Digital Rock Physics (DRP) for Special Core Analysis in Carbonate Formations

A pilot study to evaluate the quality and validity of special core analysis (SCA) data from Digital Rock Physics (DRP) has provided results that are comparable to laboratory measurements. The DRP technique applied in this study employs the Lattice Boltzmann Method (LBM) for computing relative permeability (Kr(Sw)) and capillary pressure (Pc(Sw)) curves from high resolution digital pore structures obtained from micro-CT image data. The DRP processes, results, and comparisons with laboratory measurements on carbonate rock samples from different Saudi Arabian carbonate reservoirs are presented. DRP conventional core analysis (DRP-CCA) computations include porosity, permeability, formation factor, and dynamic elastic properties. DRP special core analysis (DRP-SCA) computations include Kr(Sw) and Pc(Sw). The translation of DRP-CCA and DRP-SCA determinations from imaged 4 mm subsamples to the 38 mm core plug-scale was achieved by upscaling the data for the various flow units and porosity structures in each plug. The number of flow units within each plug varied between one and four. The process of assembling plug-scale DRP-CCA and DRP-SCA properties is discussed. DRP-SCA results and laboratory measurements from similar rock types in the same wells are comparable and show inherent process and inter-lab uncertainties. The dynamic range of the computed relative permeability curves is superior to the laboratory measurements. The comparisons further showed the benefit of the DRP images and computations in capturing the detailed pore structure and fabric of the rock, especially in the capillary pressure responses. The DRP-SCA computations accentuate spontaneous imbibition and the transition to forced imbibition, a region that traditional laboratory methods may not adequately capture. Computations for different wetting conditions provide relative permeability data that cover all possible rock-fluid wettability states. Similar attempts in traditional laboratory experiments would be long, tedious and expensive. This work shows that DRP can provide satisfactory and complementary data for reservoir studies. The images are readily available and can be used for sensitivity studies. The workflow allows users to conduct their own validation tests, just as we have done, to determine the applicability of the method.

Successful Application of the Key Well Concept to Enhance Formation Evaluation

An integrated system of acquiring data from key wells to calibrate petrophysical models has proven effective in formation evaluation programs. In the key wells, extensive logging, coring and fluid sampling programs provide the data used to develop better predictive models for water saturation, lithology, porosity, and permeability. These enhanced models have reduced uncertainties and resulted in more accurate hydrocarbon-in-place calculations.

Core analysis with emphasis on carbonate rocks — quality assurance and control for accuracy and representativeness

Accurate and representative core data should be used as the ground truth in formation evaluation, reservoir surveillance, and other relevant geoscience applications because the measurements are more direct than estimations from other petrophysical measurements such as logs. To ensure accuracy and representativeness, the rock and fluid samples should represent the reservoir; test conditions, test equipment, and test procedures need to be fit for purpose. In addition, the core analysis program should satisfy clearly defined objectives, which, unfortunately, is not generally applied in the industry. In this paper, we discussed general core analysis data quality assurance and control, with emphasis on carbonate core analysis when applicable. We started with recommended workflows for coring and coring fluid selection, conducting routine and fit-for-purpose special core analysis tests, and core analysis fluid selection and preparation. We then provided some tips and potential pitfalls useful to know in core analysis data quality assurance and control, including core sample screening and selection, criteria used in core sample cleaning and drying, and potential issues related to test conditions and procedures. We also illustrated applications of quality-controlled core data in formation evaluation and reservoir surveillance.