Heterogeneous saturation cause analysis during petrophysical modeling of low permeability Achimov deposits

UDK: 550.8.056
DOI: 10.24887/0028-2448-2022-3-30-35
Key words: achimov deposits, low-permeability reservoirs, sub-reservoir, capillary barriers, heterogeneous oil saturation
Authors: I.N. Zhizhimontov (Tyumen Petroleum Research Center LLC, RF, Tyumen), I.R. Makhmutov (Tyumen Petroleum Research Center LLC, RF, Tyumen), A.A. Evdoshchuk (Tyumen Petroleum Research Center LLC, RF, Tyumen), E.V. Smirnova (Tyumen Petroleum Research Center LLC, RF, Tyumen)

The article considers the features of complex petrophysical modeling of low-permeability Achimov deposits. These deposits are characterized by a typical clinoform structure, extremely low reservoir properties, and heterogeneous oil saturation. Photographs of the core column in UV-light show intervals of different luminescence intensity: strong and weak luminescence, no luminescence, and in combination with a separate coding volume-component model - luminous carbonate interlayers. It is established that oil is contained mainly in luminous layers. Taking into account the frequent alternation of luminous and dark intervals in the reservoir and the uncertainty of their areal distribution, the authors did not considerethe reliability of the prediction of oil-water contact (OWC) as a single horizontal surface for the studied clinoform complex. More likely is the existence of independent oil and water saturated interlayaers above the OWC. In this case, the reservoir rocks identified according to well logging data and characterized by the absence of luminescence (sub-reservoirs), due to low porosity and permeability properties, are not capable of receiving oil at a given capillary pressure and contain mainly bound water and a small amount of free water. It is difficult to identify sub-reservoir intervals only according to well logging data, because reservoir properties and electrical resistivities of luminous and conditionally dark intervals do not differ. The article presents an approach to assessing effective thicknesses and identifying sub-reservoirs, based on the theory of capillary barriers. According to this approach, it is assumed that at the stage of reservoir formation, oil could not overcome the inlet displacement pressure and the rock remained water saturated. For oil-saturated intervals the capillary pressure must exceed the displacement pressure. The inlet displacement pressure is the pressure at which the wetting phase saturation is below 100%. The relationship between the displacement pressure in the water-oil system and permeability was obtained according to the capillary studies data. In petrophysical modeling such an approach means substantiating the dependence of the reservoir cut-offs on the height above the OWC.


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