**Reservoir Properties of Low-Permeable Carbonate Rocks: Experimental Features**

#### **Aliya Mukhametdinova 1,\*, Andrey Kazak 1, Tagir Karamov 1, Natalia Bogdanovich 1, Maksim Serkin 2, Sergey Melekhin 2 and Alexey Cheremisin 1**

1 Center for Hydrocarbon Recovery, Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, 121205 Moscow, Russia; A.Kazak@skoltech.ru (A.K.); Tagir.Karamov@skoltech.ru (T.K.); N.Bogdanovich@skoltech.ru(N.B.);A.Cheremisin@skoltech.ru(A.C.);


Received: 12 March 2020; Accepted: 28 April 2020; Published: 3 May 2020

**Abstract:** This paper presents an integrated petrophysical characterization of a representative set of complex carbonate reservoir rock samples with a porosity of less than 3% and permeability of less than 1 mD. Laboratory methods used in this study included both bulk measurements and multiscale void space characterization. Bulk techniques included gas volumetric nuclear magnetic resonance (NMR), liquid saturation (LS), porosity, pressure-pulse decay (PDP), and pseudo-steady-state permeability (PSS). Imaging consisted of thin-section petrography, computed X-ray macro- and microtomography, and scanning electron microscopy (SEM). Mercury injection capillary pressure (MICP) porosimetry was a proxy technique between bulk measurements and imaging. The target set of rock samples included whole cores, core plugs, mini cores, rock chips, and crushed rock. The research yielded several findings for the target rock samples. NMR was the most appropriate technique for total porosity determination. MICP porosity matched both NMR and imaging results and highlighted the di fferent e ffects of solvent extraction on throat size distribution. PDP core-plug gas permeability measurements were consistent but overestimated in comparison to PSS results, with the di fference reaching two orders of magnitude. SEM proved to be the only feasible method for void-scale imaging with a spatial resolution up to 5 nm. The results confirmed the presence of natural voids of two major types. The first type was organic matter (OM)-hosted pores, with dimensions of less than 500 nm. The second type was sporadic voids in the mineral matrix (biogenic clasts), rarely larger than 250 nm. Comparisons between whole-core and core-plug reservoir properties showed substantial di fferences in both porosity (by a factor of 2) and permeability (up to 4 orders of magnitude) caused by spatial heterogeneity and scaling.

**Keywords:** reservoir properties; void space structure; porosity; permeability; complex rocks; NMR; MICP; CT; SEM
