RU2515629C1 - Method for determination of brittle collector zones - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: method includes well surveys, laboratory tests of drill cores, identification of interdependencies of geomechanical formation characteristics and boring logs against the data set of the laboratory tests of drill cores and well surveys, identification against the data set of well surveys of geomechanical formation characteristics on the basis of the boring logs and identified interdependencies of geomechanical formation characteristics and boring logs. At that geomechanical formation characteristics are determined by means of multifactorial regression of the boring log variations which calculate clay content and porosity of the rock by the formula.

EFFECT: improving accuracy for determination and stimulation results for brittle collector zones, economic feasibility for surveys of new drilled wells.

1 ex, 1 dwg

Description

The invention relates to the oil industry, in particular to the study of the geomechanical properties of rocks for determining and stimulating zones of increased fragility, in particular, to methods for assessing geomechanical properties for determining zones with lower Poisson's ratio and higher values of Young's modulus and fragility index for determination and stimulation fragile zones of collectors.

A well-known technology for constructing a geomechanical model (Young's modulus, Poisson's ratio) of the borehole zone, in particular, for placing wells according to the results of reservoir structure studies (K. Edimann, JM Somerville, BGD Smart, SA Hamilton, BR Crawford. Predicting Rock Mechanical Properties from Wireline Porosities .SPE 47344), based on the inter-well distribution of logs, including geophysical surveys (GIS) throughout the reservoir interval, laboratory core studies (Young's modulus, Poisson's ratio), the identification of laboratory data cores and geophysical studies of wells, the interdependence of the geomechanical characteristics of the reservoir and porosity calculated on the data of logs and the calculation of the index of brittleness of the reservoir based on the above data:

$$BI=\frac{E}{\mathrm{twenty}}-\frac{\nu -0.5}{0.5},\text{(one)}$$

where BI is the fragility index, E is the Young's modulus, ν is the Poisson's ratio.

Determine the geomechanical characteristics of the formation (Young's modulus, Poisson's ratio, brittleness index) on the basis of well logs distributed in the interwell space and the revealed interdependence of geomechanical characteristics of the reservoir and logs. Well logs are distributed into the interwell space by analyzing the change in the shape of the log diagram throughout the interval of the formation along strike, given the geological variation of the formation along strike. Recommendations for determining and stimulating brittle zones of reservoirs are carried out according to the obtained geomechanical characteristics of the reservoir, by modeling on simulators and determining zones with maximum values of Young's modulus and fragility index and minimum values of Poisson's ratio.

The effectiveness of the prototype is confirmed by the consistency of the predicted and actual well logs in determining the Young's modulus.

The disadvantage of the prototype is that the dependence of the Poisson's ratio and formation porosity is extremely low (the correlation coefficient is close to zero), and thus, the prototype determines the fragility index with a high error or does not determine it at all. This is due to the fact that porosity characterizes well the geomechanical properties of rocks with a low clay content. If the clay content in the rock is high enough, then only one porosity cannot qualitatively characterize the geomechanical properties of the rocks. For this reason, on collectors with a high clay content, for example, the Bazhenov Formation, the prototype method does not work.

The problem to be solved is to increase the universality of the method for determining brittle zones, the possibility of studying reservoirs with any clay content by introducing gamma-ray and side logging into the GIS complex, increasing the accuracy of determining and the effectiveness of stimulation of brittle zones of collectors.

The technical result is with sufficient accuracy certain geomechanical characteristics of the collectors, which, in turn, increases the accuracy of determination and the effectiveness of stimulation of brittle zones (high values of the fragility index) of the collectors.

In addition, with respect to the prior art in general, an increase in profitability is achieved by reducing the cost of researching newly drilled wells by conducting a standard GIS complex.

The problem is solved in that the proposed method for determining the brittle zones of reservoirs, including geophysical studies of wells, laboratory studies of cores, identifying from the totality of data from laboratory studies of cores and geophysical studies of wells, the interdependence of geomechanical characteristics of the formation and logs, identifying the totality of data from geophysical studies of wells of geomechanical characteristics formation based on common logs and interdependence the dependence of the geomechanical characteristics of the formation and logging diagrams, characterized in that gamma-ray logging and lateral logging and geomechanical characteristics of the formation are additionally determined by multivariate regression of changes in the logging diagrams that take into account clay content and porosity of the rock, according to the formula

$$\{\begin{array}{c}\nu =A+B^{\ast }GK+C^{\ast }NK+D^{\ast }BK\\ E=A_2+B_2{}^{\ast }GK+C_2{}^{\ast }NK+D_2{}^{\ast }BK\left(\text{2}\right)\\ \text{BI}={\text{A}}_{\text{3}}+B_3{}^{\ast }GK+C_3{}^{\ast }NK+D_3{}^{{}_{\ast }}BK\end{array}$$

In formula (2) and below, GK is gamma-ray logging, NK is neutron logging, VK is lateral logging, ν is Poisson's ratio, E is Young's modulus, BI is fragility index, coefficients A, A ₂ , A ₃ , B, B ₂ , B ₃ , C, C ₂ , C ₃ , D, D ₂ , D ₃ - are determined as a result of constructing multivariate regression.

The proposed invention is implemented by the following sequence of operations.

1. Conduct geophysical research of wells (TIS) to obtain logs, and additionally relative to the prototype conduct gamma-ray logging and lateral logging.

2. Conduct laboratory tests of cores (Young's modulus, Poisson's ratio).

3. By the totality of the data of laboratory research of cores and geophysical studies of wells, the interdependence of the geomechanical properties of the formation and the shape of the log diagrams is revealed:

$$\{\begin{array}{c}\nu =A+B^{\ast }GK+C^{\ast }NK+D^{\ast }BK\\ E=A_2+B_2{}^{\ast }GK+C_2{}^{\ast }NK+D_2{}^{\ast }BK\\ \text{BI}={\text{A}}_{\text{3}}+B_3{}^{\ast }GK+C_3{}^{\ast }NK+D_3{}^{{}_{\ast }}BK\end{array}$$

Presentation of the curves of the logs in the form (2) allows you to take into account the clay content and porosity of the rock and to carry out a more accurate analysis of the logs in the wells. Moreover, local heterogeneity of the formation in depth is taken into account under conditions of any clay content in the rock, which, in turn, leads to a significant improvement in the accuracy of determination and the effectiveness of stimulation of brittle reservoir zones. The obtained amount of information on the geomechanical characteristics of the logs in the form of decomposition coefficients of the standard logs GK, NK, VK allows a fairly objective characterization of the reservoir.

4. The logging diagram for each investigated well is distributed into the interwell space along the strike of the formation by reproducing the decomposition coefficients of the logging diagram on the basis of standard logs GK, NK, VK in the field of lack of data.

5. Accordingly, the geomechanical characteristics of the formation (Young's modulus, Poisson's ratio, brittleness index) are determined on the basis of well logs distributed in the interwell space and the interdependence of the geomechanical characteristics of the formation and well logs (according to Clause 3 of the list of operations).

6. Based on the obtained geomechanical characteristics of the formation (Young's modulus, Poisson's ratio, fragility index), modeling on simulators and determining zones with maximum Young's modulus and fragility index and minimum Poisson's ratio justify the determination of brittle zones and reservoir stimulation.

The advantage of the present invention over the prototype is that clay content and rock porosity, local heterogeneity of the formation in depth, and geological variation of the formation along strike are effectively taken into account, which as a result allows more accurate determination of the geomechanical characteristics of the reservoir by means of multivariate regression of the change in the shape of logs in wells.

This increases the accuracy of determination and the effectiveness of stimulation of the fragile zones of the collectors.

An example of a specific implementation of the method.

For one of the sections of the Salym field, it is clearly shown that the proposed method significantly increases the accuracy of determination and the effectiveness of stimulation of the fragile zones of the Bazhenov formation reservoirs.

When implementing the proposed method, the interdependence of the geomechanical characteristics of the formation and well logs was substantiated by the totality of the data of laboratory core research and geophysical well surveys and, through multifactor regression of the change in the shape of the log diagram in the interwell space, the geomechanical structure of the formation was restored.

As a result, fragile zones of the Bazhenov formation reservoirs were substantiated and stimulated with high efficiency.

The application of the prototype method leads to large errors in determining the fragile zones of the Bazhenov formation collectors and reduces the stimulation efficiency of the Bazhenov formation collectors. This is explained by the impossibility of the prototype method to explicitly establish the relationship of the Poisson coefficient and porosity determined from the logging. At the same time, the application of the proposed method in the selected area significantly increased the accuracy of determining the brittle zones of the Bazhenov formation reservoirs, which is confirmed by an increase in the coincidence of model (obtained by the present method) and actual geomechanical properties (the correlation coefficient increased from 0.2 to 0.7) and a decrease relative error of up to 8%, which is 41% less compared to the prototype.

The figure shows a comparison of the logging diagrams of the Poisson coefficient obtained in different ways, as well as the actual logging diagram in the drilled well: the left column is the prototype, in the center is the proposed method, and on the right is the actual logging diagram. It can be seen that the proposed method with higher accuracy predicts the behavior of the logs and, accordingly, the structure of the reservoir, including the geomechanical characteristics of the reservoir.

In practice, in order to more accurately determine the geomechanical properties of the rock, an expensive method is often used based on density and cross-dipole broadband acoustic logs, the claimed method allows to reduce the cost of studying the geomechanical properties of formations, since it does not require the use of density and cross-dipole broadband acoustic logs and allows do with a standard GIS complex. The claimed method is more effective than the prototype, as it allows:

- more accurately determine the geomechanical characteristics of the formation and the relative position of the parts making up the formation, and it is more efficient to stimulate the reservoir, for example, the Bazhenov Formation;

- to make a complete and consistent reinterpretation of many old wells with the involvement of the most common standard well logging system and core data to obtain more reliable data.

Claims (1)

A method for determining brittle zones of reservoirs, including geophysical studies of wells, laboratory studies of cores, identification of the interdependence of geomechanical characteristics of the formation and well logs from a combination of data from laboratory studies of cores and geophysical studies of wells, revealing from a set of data from geophysical studies of wells geomechanical characteristics of the reservoir based on common logs and the revealed interdependence of the geomechanical characteristics of the reservoir and logging charts, characterized in that gamma-ray logging and lateral logging are additionally carried out, and the geomechanical characteristics of the formation are determined by multivariate regression of the logging patterns that take into account clay content and rock porosity, according to the formula
$$\{\begin{array}{c}\nu =A+B*GK+C*NK+D*BK\\ E=A_2+B_2*GK+C_2*NK+D_2*BK,\left(\text{one}\right)\\ \text{BI}={\text{A}}_{\text{3}}+B_3*GK+C_3*NK+D_3*BK\end{array}$$

where GK is gamma-ray logging, NK is neutron logging, VK is lateral logging, ν is Poisson's ratio, E is Young's modulus, BI is fragility index, coefficients A, A ₂ , A ₃ , B, B ₂ , B ₃ , C , C ₂ , C ₃ , D, D ₂ , D ₃ - are determined by the construction of multivariate regression.

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