Determination of Residual Water and Oil Saturation from NMR Data in Reservoirs with Mixed Wettability
Authors: Chashkov A. V. , Kolesov V. A. 
Published: October 2012

The Verkhnechonskoye oil and gas condensate field was discovered in 1978 by drilling the parametric well #122, which yielded: commercial inflow of gas (62 thou m3/day) from sandstones of the Lower Motskaya Suite and inflow of gas (28 thou m3/day) from the Osinsky horizon. The field exploration and appraisal stage was finished in 1993.

Several pay horizons are delineated in the field: sandy-argillaceous deposits of the Lower Motskaya Sub-Suite: VC1 and VC2 formations; carbonate deposits of the Middle Motskaya Sub-Suite: PR formation (Preobrazhensky Horizon); carbonates of the Top Motskaya Sub-Suite: the Ustkutskiy Horizon, formations UK1 and UK2; Carbonated of the Usolskaya Suite: the Osinsky Horizon (OS).

Main oil reserves are associated with VC1 and VC2 formations, that is why the problems related to reservoirs delineation, evaluation of their saturation nature, as well as quantitative assessment of rock porosity & permeability properties, first of all occur in the process of the Verkhnechonsky sediments study. Thereby the development of methodological approaches, related to an increase of geological efficiency of various methods of the Verkhnechonsky deposit sections study, is the high-priority issue.

The formation of deposits and accumulation of sediments of the Verkhnechonsky Horizon took place in submontane and possibly coast-deltoid conditions; diagenetic and catagenetic transfigurations, metasomatism processes led to great heterogeneity of both mineral composition of solid part and complex structure of rocks pore space. All these factors significantly complicated the petrophysical model of rocks and reservoirs of the Verkhnechonsky Horizon; led to reduction of efficiency of "traditional" approaches to quantitative assessments of rock porosity & permeability properties for logging and to increase of uncertainty in the process of geological models development.

Lithological formations VC1 and VC2 are presented by fragmentary material with the size of grain from pelitic to gravelite fractions, predominantly of quartz or quartz-feldspar composition. Sandstones are anisometric with different extent of grading. Argillaceous cement, occasionally carbonaceous and/or sulphated, bituminous; by type from film to porous-film. The characteristic feature of the Verkhnechonsky Horizon reservoirs is almost all-round but very uneven salinization; in some areas the content of halite in pore space grows up to a complete loss of filtration properties. Besides, separate differences of rocks contain significant (up to several percent of rock volume) volumes of accessory minerals (monazite, ferriferrous minerals, etc.) that have anomalous physical properties (radioactivity, density, etc.) [1].

The distinctive feature of the Verkhnechonsky deposits is their hydrophobic property. According to the laboratory examination of extracted core samples by the method of Amott and Tulbovich 80% of samples are completely or partially hydrophobic. Moreover, the results of laboratory core examination show that the wettability is influenced by the content of halite, which occurs in significant volumes (up to 30%) in pay sandstone of the Verkhnechonsky Horizon. This hydrophobic property was also identified by the results of relative permeability test.

Estimations of impacts of wettability on distribution of formation fluids and oil recovery factor are still challenging. Such estimations are needed for evaluation of fields in terms of petrophysics and also for field development planning. But currently no such model was developed that would help to do it based on the well logging data.

In this paper we try to develop the method of determination of rock wettability in the Verkhnechonsky Horizon by using well logging data. For this purpose we successively analyse the results of special laboratory-based core analyses, methods of high-frequency electromagnetic well logging with simultaneous recording of phases difference and fading of electromagnetic waves, as well as nuclear magnetic logging data.