Abstract

The determination of petrophysical properties of oil reservoirs is complicated by the limited volume of investigation of the heterogeneous object under study (about 10-6 of the total volume) and the indirect nature of the information obtained. For each property several ways of determination exist, giving values differing sometimes by 1–2 orders of magnitude. One of the principal causes of such discrepancy is the that the problems of property determination using conventional methods are ill-set.

The issues of solving ill-set problems are investigated in the special mathematical theory, which has a lot of applications in natural science generally and in petroleum science in particular. Many problems of reservoir characterization and simulation are ill-set as well; a priori information and multicriterial approach are used in there solution. These are very complex problems, for example, flow model history matching and seismic interpretation. However, the formal multicriterial approach is not well developed with respect to relatively simple tasks of geology, petrophysics and reservoir engineering, such as permeability determination from well logging data, and others. The purpose of this work is the formalization of comprehensive approach to the solution of such problems.

The problem of determining the well permeability profiles from well logging data using statistical relationships based upon core analysis data with a priori information incorporation is investigated in detail. The additional criteria options and ways of their application are considered. The setting and solution of a two-criterial problem of determining permeability from well logging data, care analysis data and well test data on a real West-Siberian filed is provided.

Introduction

It is well known that a comprehensive approach should be applied for determining most of the quantitative reservoir, fluid and well completion parameters. Each of these quantities can be determined in several ways, each of which, when used separately, rarely gives a reliable result. That is why geology, petrophysics and reservoir engineering specialists try to attain the value of each parameter by several independent methods. Usually, the issue of combining these sometimes widely different results to some final value arises. For example:

  1. Reservoir permeability can be determined with two direct methods - core sample laboratory analysis, which gives an absolute permeability on about 10 cm3 scale, and well testing, which gives effective permeability on the 1000 m3 scale. Also, permeability can be determined from well logging data using empirical relationships - this is an indirect method. After calculating average permeabilities for the field (reservoir) from all three methods, three values differing by orders of magnitude are produced.

  2. The original oil-in-place can be calculated with a volumetric method, the accuracy of which, in the first place, depends on the accuracy of petrophysical properties of the reservoir determination (porosity, initial water saturation). Also, the oil-in-place can be calculated using the material balance equation, which relies on the quality of the average reservoir pressures calculated for the field, as well as on how representative are the fluid samples recovered from the reservoir. The third method is displacement characteristics, which allow to evaluate the reserves on a developed field in waterflood; some calculated recovery efficiency value allows to get oil-in-place from reserves. Since all of these three methods are based on quite different processes and principles, the values produced may be significantly divergent, especially in the case of the most important initial stage of field development, or at the plateau stage.

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