RU2487239C1 - Method for determination of oil-filled formations - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: bore core is selected and examined, induction logging and induced gamma-ray logging or neutron-neutron logging is made and log curves are analysed for the roof of production tier. At that formations with apparent resistivity are identified with values less than 6-8 Ohm/m during induction logging and against values at curves of induced gamma-ray logging or neutron-neutron logging making less 85% and less than values of lower formations. Among these formations it is necessary to select strata without loamy lintels and strata of carbonate oil-filled formations and values of apparent resistivity not less than 15 Ohm/m against data of induction logging. Then sedimentary types for the selected formations is defined and if oil-saturated sandstone is present then conclusion is made about terrigenous origin of these formations. Then values are specified for porosity coefficient, permeability and oil-saturation coefficients and when lower limits for this region are exceeded the indentified formations will be referred to productive formations.

EFFECT: increase of operational efficiency during installation of the bottom-hole complex, improvement of level of detail and authenticity of GIS data for identification of geological rating for rock masses.

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Description

The invention relates to the oil industry and may find application in determining oil-saturated formations in the context of the well.

There is a method of determining the component composition and oil and gas saturation of terrigenous rocks of reservoirs and determining calculation parameters based on processing data from various complexes of geophysical studies of well sections (GIS), including identifying reservoir intervals in a well section, determining their effective thicknesses, clay content and the nature of the distribution of clay material in the rock (dispersed, structural, layered), full and effective porosity, absolute and phase permeability, oil and gas assessment the saturation and composition of the fluids extracted from the hydrocarbon-containing rock, determining the coefficient of oil and gas displacement from productive formations ("Guidelines for determining the estimated parameters of oil and gas deposits based on the materials of geophysical well surveys using the results of core analyzes, tests and productive formations" / Ed. B.Yu. Vendelshtein, V.F. Kozyara, G.G. Yatsenko, Kalinin, NPO Soyuzpromgeofizika, 1990, 261 pp.).

The known method is implemented in accordance with the "Instructions for the use of field geophysical research materials using the results of core studies and well tests to determine and justify the estimated parameters of oil and gas deposits", Moscow, VNIGNI, 1987, 20 pp.) And allows to determine the geological characteristics only previously selected reservoir strata in the borehole section using, when processing logging readings, specific theoretically based empirical data determined for reservoir rocks petrophysical models and stochastic petrophysical relationships established on the basis of the study of core samples taken from wells in the intervals of bedding and analysis of core-core, core-GIS and GIS-GIS relationships.

The known method has limitations in determining the geological characteristics of rocks in the section of the well, is used only to determine the parameters of reservoir rocks and does not fully determine the structural-mineralogical and fluid model of the rock, and also does not implement an assessment of the geological characteristics of rocks of non-reservoirs in the section of the well.

Closest to the invention in technical essence is a method for determining the geological properties of terrigenous rock in the near-wellbore space according to well logging data, including performing geophysical surveys in the well and processing the information obtained by isolating reservoir intervals and assessing their geological properties (BCAfanasyev, G.A.Shnurman, V.Yu. Terentyev, Methodology for assessing porosity and component composition of sandy-siltstone-clayey rocks from field geophysical data, Oilfield geophysics, Issue 5. Ufa, BashNIPIneft, 1975, pp. 88-94 - prototype).

The known method allows to determine the porosity, component composition and oil saturation of sand-silt-clay reservoirs. The method is based on the use of petrophysical models when processing GIS data, taking into account the influence on the readings of various GIS methods of an elevated content of silt material and clays in the rock skeleton with different distributions (dispersed, structural, layered) in the rock mass. In the process of processing well log data, the porosity coefficient, the content of sand, silt and clay fractions in the rock skeleton, as well as the coefficient of total water saturation of the rock are determined.

The disadvantage of this method is that it is focused on the study of the properties of only previously selected reservoir strata in the well section and uses simplified theoretical and stochastic petrophysical models when processing logging data that are applicable for an approximate assessment of the geological properties of reservoir rocks.

The objective of the invention is to increase the detail and reliability of determining according to GIS geological characteristics of the rocks that make up the terrigenous sequence - the definition of oil-saturated layers.

The problem is solved by the method of determining oil-saturated formations, according to which well logs are analyzed in the roofing of the productive layer, formations with apparent resistivities of induction log (IR) of not more than 6-8 Ohm · m and readings on neutron gamma-ray logging curves (NGC) or neutron-neutron logging (NOC), comprising 85% or less of the values of the lower located formations, among the identified formations, formations are selected without clay bridges with formations of carbonated oil-saturated rocks and values of apparent resistivities by IR of at least 15 Ohm · m, clarify the lithological composition of the identified strata, if there is oil-saturated sandstone in the rocks, make a conclusion about the terrigenous origin of these strata, specify the values of the coefficients of porosity, permeability and oil saturation, when exceeding the lower limits of the porosity coefficients, The permeability and oil saturation of petrophysical coefficients for a given region are identified as productive reservoirs.

SUMMARY OF THE INVENTION

When interpreting geophysical material, problems arise in explaining the relatively low readings of induction logging (6-8 Ohm · m) in the roof of the Tournaisian layer with reservoir rocks 1-4 m thick, below which are the differences in carbonate oil-saturated rocks without a clay bridging between reservoirs with specific resistances exceeding 15-20 Ohm · m.

The problem is that carbonate rocks (aquiferous limestones) with an apparent specific electrical resistivity of IR equal to 6-8 Ohm · m are classified as aquifers, and they are located above the oil-bearing strata. Moreover, between them there is no insulating clay layer or layer, i.e. there is no clay bridge between the layers. It turns out that below the aquiferous limestones without an insulating clay bridge, oil-bearing limestones are immediately located. This contradicts the oil reservoir model, according to which the aquifers should be lower than the oil ones.

Aquiferous limestones or sandstones cannot be located above oil-bearing limestones directly without a bridge due to the higher density of formation water compared to oil. Less dense oil due to gravitational distribution is located above the produced water, as it floats.

The explanation of this paradox lies in the following: lithotype of rocks with a resistivity of 6-8 Ohm · m, occurring on oil-bearing limestones, refers to oil-bearing terrigenous rocks (sandstones).

The characterization of the roofing (anomalous) part of the Tournaisian layer is noted by lower readings of the curves of OGCs or NOCs in comparison with the underlying oil-bearing limestones (by 15% or more). Such strata are widely developed on the Fedotovskaya area of the Novo-Elkhovskoye field in the zones of erosive incisions of the Visean age in the deposits of the Tournaisian stage.

The correct choice of the lithotype of the reservoir during interpretation will allow us to correctly determine its petrophysical parameters (coefficients of porosity, permeability, clay, oil saturation) and thereby allow us to increase oil reserves and involve them in development. So, with the same neutron gamma-ray measurements of NGK-60, the porosity of quartz sandstone exceeds the porosity of limestone by 4-5%. This means that when determining the oil saturation coefficient according to the Archie formula, a higher value of the oil saturation coefficient will correspond to a higher value of the porosity coefficient with the same specific electrical resistivity of the formation.

With a porosity coefficient of 9% and an electrical resistivity of 15 Ohm · m according to the Archie formula, the oil saturation coefficient of limestones of the Tournaisian stage of the Novo-Elkhovskoye field is 55%, this is the lower limit for industrially productive formations.

The oil saturation coefficient (Kn) of the reservoir is determined by the well-known Archie formula:

Kn = 1-s / (Kn ^{m / n} × Rn ^{1 / n} )

where C is the coefficient of proportionality equal to:

C = (a * b * RB ) ^{1 / n}

a, b, m, n - coefficients determined by core;

Kn ^{m / n} is the formation porosity coefficient in the degree ^{m / n} .

Rn ^{1 / n} - electrical resistivity of the studied formation in the degree of ^{1 / n} .

The laboratory determines the electrical characteristics of core samples - the porosity parameter Pn and the saturation parameter Rn:

Рп = Rвп / RВ = а / Kn ^m

Rn = Rp / Rvp = b / Cove ⁿ

where - Rp - electrical resistivity of the studied formation in Ohm · m,

Rvp - electrical resistivity of the formation with its full water saturation in Ohm · m.

RВ - electrical resistivity of formation water in Ohm · m.

The criteria for the allocation of layers in oil-bearing in the roof of the Tournaisian tier are the following:

1. Induction logging. Indications 6-8 Ohm · m. Oil-bearing carbonate reservoirs (limestones) lying without a clay or compacted lintel have specific resistivities of 15 or more Ohm · m according to this method.

2. Neutron methods (neutron gamma-ray logging or neutron-neutron logging. Indications of the roofing part are less than 15 percent or more compared to the underlying carbonate stratum.

3. Nuclear magnetic logging. The presence of a free fluid index in this formation, as well as in the underlying oil-saturated carbonate rocks. That is, the reservoir is a reservoir and during testing can give out the fluid contained in the pores.

4. Confirmation by core sampling of the terrigenous origin of lithology and the nature of saturation (ie oil saturation) of the formation, which lies without a bridge in the roofing of the Tournaisian stage in two or three wells in a specific section of the developed oil field. During core analysis, the following are determined: lithology, coefficients of porosity, permeability, clay content, oil and water saturation of selected rocks.

5. The coefficients of porosity, permeability, clay, oil saturation are calculated according to petrophysical algorithms, based on the terrigenous origin of the roof of the formation. When confirming the values of the calculated petrophysical parameters (i.e., at their close convergence) with the core analysis data and exceeding them in terms of the boundary values of the coefficients for this region, it is concluded that productive sandstones are located in the roofing of the Tournaisian layer.

Concrete example

In well No. 1 of the Fedotovskaya area of the Novo-Elkhovskoye field, according to the hydrodynamic studies of the wells, the roof of the Tournaisian layer was determined at a depth of 1009.7 m.

Data for well 1 are shown in table 1

According to the data of geophysical surveys of wells (GIS), a carbonate reservoir with a porosity of 16% and oil saturation of 52% is located in the roofing part of the Tournaisian layer in the interval 1009.7-1011.6 m (according to table No. 1, such a formation is not productive in terms of the coefficient saturation) and specific resistance of 6 Ohm · m. Below, in the range of 1011.6-1027 m, there is a carbonate oil-saturated reservoir with a porosity of 10.8%, oil saturation of 71.3%, and a specific resistance of 19 Ohm · m. According to the core data, the interval 1009.7-1011.6 m is represented by oil-bearing sandstones. Calculate the values of the petrophysical parameters of the sandstone formation in the interval 1009.7-1011.6 m according to the standard for the interpretation of GIS. The obtained values: porosity coefficient - 20%, oil saturation coefficient - 62%. Those. a collector in the roofing part of the Tournaisian layer in the range of 1009.7-1011.6 m, with the correct determination of lithology, passes from the category of non-productive ones according to the oil saturation coefficient (oil saturation coefficient - 52%) to the productive category (oil saturation coefficient - 62%).

When calculating the balance sheet and recoverable oil reserves for this reservoir, the following data were obtained.

The reservoir is carbonate (limestone). Initial balance reserves of 80028 tons, initial recoverable reserves of 18406 tons, oil recovery coefficient of 23%.

The reservoir is terrigenous (sandstone). Initial balance reserves of 133,380 tons; initial recoverable reserves of 66690 tons. Oil recovery factor of 50%.

Those. after applying the proposed method with the correct determination of the lithology of the reservoir, the recoverable oil reserves increased by 48,284 tons, and the oil recovery coefficient increased by 27%.

The application of the proposed method will identify previously not detected oil-saturated formations.

Claims (1)

A method for determining oil-saturated formations, according to which core samples are taken and examined in the well, induction logging and neutron gamma-ray logging or neutron-neutron logging are carried out, well logging curves in the roofing of the productive layer are analyzed, formations with apparent resistivities of induction logging of no more than 6 8 Ohm · m and readings on the neutron gamma-ray or neutron-neutron-log curves, representing 85% or less of the values of the lower layers, among the identified layers in formations without clay bridges with formations of carbonate oil-saturated rocks and with apparent resistivity values of induction logging of at least 15 Ohm · m are selected, the lithological composition of the identified formations is clarified, if oil-saturated sandstone is present in the rocks, the conclusion is made on the terrigenous origin of these formations, and the values the coefficients of porosity, permeability and oil saturation, when exceeding the lower limits of the coefficients of porosity, permeability and oil saturation of petrophysical coefficients Ienti for this region, identified strata are classified as productive.