RU2720859C1 - Method of horizontal methane coal well design selection - Google Patents

Abstract

FIELD: soil or rock drilling; mining.

SUBSTANCE: invention relates to extraction of methane from coal beds and can be used in designing methane-coal deposit development schemes. Method involves preparation of a one-dimensional geomechanical model based on research data in a draining vertical well or in nearby vertical structural and exploration wells, consisting of plotting, by well depth, a graph of formation pressure P variation by hydrodynamic analysis results, constructing a curve of geostatic stress σ_v from results of density logging, plotting curves of horizontal stresses σ_h and σ_H from data of acoustic logging. Obtaining in the range of the target coal bed values of stress tensor components characterizing its natural stress-strain state prior to construction of the horizontal well. Construction of two-dimensional geomechanics models for various versions of the horizontal well design in planes perpendicular to the horizontal section axis, using the stress tensor components as boundary conditions, determined according to the prepared one-dimensional geomechanical model. Estimation of coal bed permeability change at different versions of horizontal well design and selection of the version causing the greatest increase of coal bed permeability.

EFFECT: technical result consists in improvement of accuracy of determining the most efficient for existing mining and geological conditions of the structure of a horizontal methane coal-bearing well without the need for additional borehole investigations.

1 cl, 6 dwg

Description

The invention relates to the field of methane extraction from coal seams and can be used in the design of technological schemes for the development of methane deposits.

In the practice of developing coalbed methane deposits, as in the production of traditional hydrocarbons, vertical, directional and horizontal well profiles are used. At the same time, horizontal wells, due to the greatest contact with the productive coal seam and a significant drainage area, are characterized by the highest productivity. Nevertheless, the effectiveness of this type of methane-gas wells significantly depends on the geological conditions of the developed area and the type of construction used. These factors, taking into account the higher costs of horizontal drilling, confirm the need for a more thorough approach to the choice of horizontal methane-well designs taking into account existing mining and geological conditions.

In world experience, the following principle is used to determine candidate coal seams for horizontal drilling: low permeability coal seams with a thickness of more than 1 m are candidates for horizontal drilling (Rogers R., Ramurthy K., Rodvelt G., and etc. Coal Bed Methane: Principles and Practices. 2nd ed. Starkville, MS: Oktibbeha Publishing Co., 2007. 504 p.). But this criterion does not take into account the fact that the stress-strain state (SSS) existing in a rock mass can have a significant impact on the most important parameter of a coal seam, which makes it possible to extract desorbed gas - its permeability (Rogers R., Ramurthy K., Rodvelt G ., and etc. Coal Bed Methane: Principles and Practices. 2nd ed. Starkville, MS: Oktibbeha Publishing Co., 2007. 504 p .; Seidle J. Fundamentals of Coalbed Methane Reservoir Engineering. 1nd ed. Tulsa, USA: PennWell Corporation , 2011 .-- 401 p.). Thus, to ensure the scientific justification for choosing a particular type of horizontal methane-coal well, it is undoubtedly necessary to develop a technique that takes into account the stresses acting in the coal seam.

A known method of developing oil fields (patent RU 2556094 C1, IPC E21B 43/20, publ. 07/10/2015, bull. No. 19), including the conduct of geophysical exploration of wells by cross-dipole acoustic logging, the selection of oriented core, followed by determining the direction of maximum stresses of oil-saturated rocks, placement of injection wells along regional directions of maximum stresses, placement of production wells between injection wells with the formation of a development system. The disadvantages of the method are the need for expensive studies using cross-dipole acoustic logging and highly costly oriented core sampling, as well as the absence of the design of the used wells when implementing the method.

A known system and method for correcting the direction of the wellbore based on the stress field (patent RU 2496003 C2, IPC E21B 47/0224, E21B 44/00, publ. 10/20/2013, bull. No. 26), including measurements reflecting the geometry of the well with using the layout of the bottom of the drill string (BHA) rotated in the borehole, the geometry of which displays stimulated stresses in the formation, creating an image of the borehole based on measurements of its geometry, evaluating the azimuthal variation of the stimulated stress in the formation along the depth of the borehole, changing the drilling mode parameter for BHA using estimates of azimuthal variation over the well depth of stimulated stress in the formation. The disadvantages of this invention are the lack of preliminary determination of the most effective direction of the wellbore and the presence of a large number of risks associated with deciding on the path of the wellbore not at the design stage, but at the drilling stage.

Known methods for determining the effect of stresses caused by the construction of degassing wells on the permeability of a coal seam (Xie J., Gao M., Yu B., Zhang, and etc. Coal permeability model on the effect of gas extraction within effective influence zone // Geomechanics and Geophysics for Geo-Energy and Geo-Resources - 2015, No. 1-2; Zhang H., Cheng Y., Liu Q., and etc. A novel in-seam borehole hydraulic flushing gas extraction technology in the heading face: Enhanced permeability mechanism, gas flow characteristics, and application // Journal of Natural Gas Science and Engineering - 2017, No. 46), including two-dimensional geomechanical modeling of a coal seam exposed by a degassing well, followed by an assessment of its permeability along the monitoring lines under consideration. The disadvantage of these methods is the low accuracy of determining the most effective design of a degassing well due to the lack of information about the actual stress values in the coal seam before the construction of the well.

Closest to the technical nature of the claimed invention is a system and technology for the optimal choice of the method of completion and construction of the well (patent US 7181380 B2, IPC G06G 7/48, publ. 02.20.2007), including obtaining the results of reservoir, geomechanical modeling and modeling of materials information on reducing pore pressure, stress values, their orientation and rock strength to determine the optimal location, trajectory and completion method. The disadvantage of the system and technology data is the lack of accounting for changes in the permeability of the formation after well construction.

The technical result of the claimed invention is to improve the accuracy of determining the most effective for the existing geological conditions of the design of a horizontal methane-gas well without the need for additional downhole research. The specified technical result is achieved by the fact that in the method for selecting the design of a horizontal methane-coal well based on a standard complex of seismic, geophysical and hydrodynamic studies of methane-coal deposits, the natural VAT of the target coal seam is determined by the method of one-dimensional geomechanical modeling, two-dimensional geomechanical modeling of the coal seam is performed for various horizontal well design options and assessing changes in the natural permeability of the coal seam for each of the considered options. The results of the assessment of changes in permeability allow us to determine the design of the well, which allows the most efficient drainage of the target coal seam.

The invention is illustrated by the following figures.

FIG. 1. An example of a one-dimensional geomechanical model constructed according to a vertical exploratory well.

FIG. 2. An example of a design scheme for a U-shaped horizontal methane-well (1 - horizontal well, 2 - vertical well, 4 - sectional planes under consideration).

FIG. 3. An example of the design scheme for a system of U-shaped horizontal methane wells (1 - horizontal well, 2 - vertical well, 4 - sectional planes under consideration).

FIG. 4. An example of a design scheme for a multi-sided horizontal methane-well (1 - horizontal well, 2 - vertical well, 3 - sidetracks, 4 - sectional planes under consideration).

FIG. 5. An example of the result of two-dimensional geomechanical modeling of a coal seam with a horizontal multilateral well.

FIG. 6. An example of the result of assessing changes in the natural permeability of a coal seam in the case of the construction of a horizontal multilateral well.

Let us consider in more detail the procedure for implementing the method on the example of one of the coal seams of the Naryksko-Ostashkinskoye methane coal field of Kuzbass, suitable for horizontal drilling.

Based on the results of a standard set of studies (seismic, geophysical and hydrodynamic) in a vertical exploratory well, which can subsequently serve as a drainage well or in the nearest vertical structural and exploratory wells, a one-dimensional geomechanical model is being prepared. Preparation includes plotting the depth of the reservoir pressure P (based on the results of hydrodynamic studies), plotting a geostatic (vertical) stress curve σ _v (based on density logging), plotting the horizontal stress curves σ _h and σ _H using the well-known formula (Rogers R ., Ramurthy K., Rodvelt G., and etc. Coal Bed Methane: Principles and Practices. 2nd ed. Starkville, MS: Oktibbeha Publishing Co., 2007. 504 p.):

,

,

where σ _h is the minimum horizontal stress, Pa;

σ _H is the maximum horizontal stress, Pa;

ν is the Poisson's ratio;

σ _v - geostatic stress, Pa;

α is the bio poroelasticity coefficient (~ 1);

P - reservoir pressure, Pa;

σ _t - tectonic stress, Pa.

The values of the components of the stress tensor obtained in the interval of the target coal seam characterize its natural VAT before the construction of a horizontal well. So, according to the results of one-dimensional geomechanical modeling of the studied coal seam (Fig. 1), the vertical geostatic stress was 19.195 MPa, the minimum horizontal stress was 9.269 MPa, and the maximum horizontal stress was 11.771 MPa.

At the next stage, for various options of a horizontal methane-gas well (Fig. 2-4) in specialized software (for example, FLAC, VISAGE, ANSYS, etc.), two-dimensional numerical geomechanical modeling is performed in the studied planes perpendicular to the axis of the horizontal section, with a step of 0 , 5 m to the limit distance at which there is no significant change in VAT. The components of the natural VAT tensor, determined by the one-dimensional geomechanical model, are used as the boundary conditions of the two-dimensional model. Moreover, taking into account that the model is flat, one of the components of the horizontal stress (perpendicular to the plane under consideration) is taken equal to zero. The results obtained are two-dimensional patterns of the distribution of stresses in the studied planes (Fig. 5).

Then, for the obtained distributions, on the basis of the well-known formula, an assessment is made of the change in the permeability of the target formation over the entire horizontal section for each of the selected horizontal well designs (Seidle J. Fundamentals of Coalbed Methane Reservoir Engineering. 1nd ed. Tulsa, USA: PennWell Corporation, 2011. - 401 p.):

,

,

where k / k ₀ is the ratio of the permeability of the coal seam at the current voltage to the initial permeability of the coal seam;

α is the coefficient of sensitivity of the coal seam to stress;

C _f is the compressibility factor of the coal seam, MPa ^-1 );

σ is the current stress acting on the coal seam, MPa;

σ ₀ is the initial stress obtained by the results of one-dimensional geomechanical modeling.

According to the calculation results, zones (Fig. 6) are distinguished in the stress distribution patterns in which there is no change in permeability (k / k ₀ = 1), in which permeability decreases (k / k ₀ <1) and zones in which permeability is observed coal seam (k / k ₀ > 1).

At the final stage of the proposed method, an analysis of the array of permeability assessment results is performed to select from the considered horizontal methane-coal well designs that option during construction of which the largest volume of zones of increased permeability of the target coal seam is observed. So, for the considered coal seam, the largest volume of zones of increased permeability was obtained as a result of modeling a multilateral well.

Thus, the claimed method allows by using geomechanical modeling to increase the accuracy of determining the most effective for the existing geological conditions of the design of a horizontal methane-coal well without the need for additional downhole research at the field.

Claims (1)

A method for selecting a horizontal methane-coal well design, including obtaining information about the stresses acting in a rock mass using geomechanical modeling, assessing the permeability of a coal seam and selecting the optimal well design, characterized in that the preparation of a one-dimensional geomechanical model is carried out according to studies in a draining vertical well or in the nearest vertical structural and exploratory wells, which consists of plotting the depth of the reservoir pressure P according to the results of hydrodynamic studies, plotting the geostatic stress curve σ _v according to the density logging data, plotting the horizontal stress curves σ _h and σ _H according to the acoustic logging data obtaining, in the interval of the target coal seam, the values of the stress tensor components characterizing its natural stress-strain state before the construction of a horizontal well, the construction of two-dimensional geomechanical models for various options for the construction of a horizontal well in planes perpendicular to the axis of the horizontal section, using the stress tensor components as boundary conditions determined from the prepared one-dimensional geomechanical model, an assessment of the change in the permeability of the coal seam for various options for the design of the horizontal well, and the choice of the option that causes the greatest increase in the permeability of the coal seam.

Images