RU2719792C2 - Method for predicting drilling mud absorption zones when drilling wells based on three-dimensional geomechanical model and tectonic model of deposit - Google Patents

Abstract

FIELD: soil or rock drilling; mining.

SUBSTANCE: invention relates to methods of investigation of rock massif and can be used for forecast of drilling mud absorption zones during drilling of wells in terrigenous and carbonate reservoirs. Summary: performing complex of geophysical surveys, including seismic exploration operations. Support wells are drilled. Logging data is recorded, coring is selected and analysed for mechanical properties. On the basis of seismic exploration, a cube of seismic attribute Ant-tracking is constructed, which indicates changes in the wave pattern for detection of fractures, cracks and other linear anomalies inside the seismic cube. Exciting disorders are identified. Then one-dimensional geomechanical model and well stability model for support wells is calculated. Resilient-strength properties and main stresses in support wells are calculated. Resilient-strength properties are transferred and propagated in the inter-well space using geostatistics. Three-dimensional geomechanical model is loaded with boundary conditions obtained based on a one-dimensional geomechanical model. Stresses are calculated by finite element method. Result of calculation is full stress tensor in modelling sector, cubes of resilient-strength properties and cubes of main voltages. For isolated surfaces of breaking disruptions Mohr-Coulomb destruction criterion is used, data for calculation of which is taken from calculated cubes of main stresses. For each point on the fracture surface, its state is determined, whether active or inactive. Tectonic model of the deposit is constructed. Discrete network of cracks is built on the basis of tectonic model driver. Conclusion is made on the probability of absorption of drilling fluid based on the intersection of a large number of cracks with the well trajectory, without crossing the trajectory with the fracture. Model of stability of the wellbore of the design well is constructed, and zones of auto-hydraulic fracturing are identified by way of application of the washing program on the model of stability of the design well. Conclusion is made on necessity to correct program of well flushing.

EFFECT: technical result is ensuring trouble-free construction of well, reducing time for construction of well.

1 cl, 5 dwg

Description

The invention relates to the oil and gas industry, and in particular to methods for constructing wells when drilling intervals with zones of complications.

The interest in the possibility of predicting zones of complications during drilling is due to the growing share of complex fields, drilling on which is complicated by lithological and tectonic factors, leading to an extension of the well construction cycle, an increase in unproductive time, additional drilling costs and late commissioning of the well.

In almost all fields, there are problems associated with the absorption of drilling fluid of varying degrees of intensity, both in fractured cavernous deposits and in pore deposits. Therefore, it is necessary to develop methods of both a preventive (anticipatory) nature and measures taking into account the current situation. In addition, when drilling horizontal and shallow wells of long length, there is a problem with cleaning the wellbore, especially at angles of inclination of the wellbore 55-70 °.

There are various technological methods for preventing absorption of drilling fluid during the construction of a well, which include the following techniques:

- drilling with washing with drilling fluids containing mud;

- drilling using aerated foam drilling fluids;

- drilling using aerated afron-containing fluids.

The disadvantages of the known methods of drilling and the prevention of absorption is that after the introduction of drilling fluids, the downhole motors may deteriorate due to clogging of the filters, the telemetry system and the drilling mud cleaning system become practically impossible.

The prior art method for managing a predetermined risk during drilling operations in a well (Application for invention No. 2016110570, based on PCT application US 2013/066856 (10.25.2013) WO 2015/060865 (04.30.2015) The method consists in creating a graphical interface a user to display i) each prediction of a predetermined risk in one of a plurality of risk zones, each of which is associated with a predetermined time increment and risk level, with a predetermined warning interval during drilling operations; and ii) a predefined recommendation to change drilling operations based on one of the predicted risk forecasts;

The technical problem solved by the present invention is the development of a method for predicting potential absorption zones when drilling wells in carbonate and terrigenous reservoirs, characterized by increased accuracy and reliability.

The technical result is to reduce the degree of absorption of the drilling fluid by predicting the zones of absorption of the drilling fluid in space and adjusting the trajectory of the design wells.

Having the opportunity to adjust design wells will ensure trouble-free well construction, a significant reduction in time for well construction.

The technical problem is solved, the technical result is achieved by the fact that the method for predicting the zones of drilling fluid absorption when drilling horizontal, directional, vertical wells in terrigenous and carbonate reservoirs includes a complex of geophysical surveys, including seismic exploration, drilling of reference wells, recording a logging complex in reference wells, core sampling and examination for mechanical properties.

As a result of seismic exploration, a seismic cube of amplitudes of reflected waves is obtained, based on which the main reflecting horizons are extracted.

To identify discontinuous faults based on the seismic cube of the amplitudes of the reflected waves, a cube of the seismic attribute Ant-tracking is constructed, which indicates changes in the wave pattern for detecting faults, cracks and other linear anomalies inside the seismic cube of the amplitudes of the reflected waves. The resulting cube of the seismic attribute Ant-tracking emphasizes all potential discontinuous disturbances (faults, cracks).

The next step is to calculate a one-dimensional geomechanical model and a wellbore stability model for reference wells.

For this, it is necessary that a certain set of logs was recorded on the reference wells: gamma-ray logs, density logs, acoustic logs and caliper. The calculation of elastic-strength properties is carried out according to standard formulas and calibrated using data obtained from core studies for mechanical properties, caliper, as well as FIT and LOT data.

The next step is the transfer and distribution of elastic-strength properties in the interwell space using geostatistical methods. As a trend for the propagation of elastic-strength properties, cubes of elastic-strength properties obtained as a result of seismic inversion can be used. The resulting cubes of elastic-strength properties, subsequently, are used in the calculation by the finite element method, the result of which is a three-dimensional geomechanical model. The three-dimensional geomechanical model includes a full stress tensor, cubes of elastic-strength properties and cubes of basic stresses.

Then, a tectonic model of the field is built on the basis of previously identified fracturing faults and information from the reference wells. After constructing the tectonic model, the driver of the tectonic model is calculated, on the basis of which a discrete network of fractures is constructed within the modeling sector, where the probability of drilling fluid absorption is inferred based on the intersection of a large number of fractures with the project well trajectory (if the well trajectory does not intersect with a break).

At each point on the surface of discontinuous faults, the Mohr-Coulomb fracture criterion is calculated. For the calculation, previously calculated cubes of basic stresses and cubes of elastic-strength properties are used. According to the calculation results, the elements of discontinuous violations can be divided into 2 groups: active and inactive. The definition of the critically loaded state (active state) of an element of a discontinuous violation is based on the fact that this element, according to the Mohr-Coulomb criterion, must overcome the brittle fracture band in order to become active (potentially fluid-conducting).

The conclusion about the presence of the absorption zone is made on the basis of the well trajectory falling into the “active” fault zone, characterized by lower effective stresses and proximity to the maximum horizontal stress azimuth and / or the well trajectory falling into the highly fractured region, determined on the basis of the constructed discrete network of fractures and / or falling of the project well trajectory into the extension region calculated on the basis of the tectonic model of the field and / or the presence of intervals with a low gradient Г P, which leads to auto-PIU provided incorrectly matched mud weight.

The calculation of the zones of tension or compression based on existing stresses and paleostresses, where it is concluded that a particular area is predisposed to absorption of the drilling fluid during drilling.

A model of the stability of the well of the project well is identified and auto-hydraulic fracturing zones are identified by superimposing a flushing program on the model of stability of the well of the well, where it is concluded that it is necessary to adjust the well wash program.

The invention is illustrated by drawings and photographs, where in FIG. Figure 1 shows a diagram of the Mohr-Coulomb fracture criterion, where the following notation is introduced: the abscissa axis shows the effective normal stresses (in MPa), the ordinate axis shows the tangential stresses (in MPa), the diagonal line corresponds to the rock strength, red dots located above the rock strength show the destruction of the rock and, therefore, the active fracture region — the region through which fluid filtration is possible and gray dots showing the inactive fracture region. Considering the fact that the azimuth and the angle of inclination of the fault can vary along its axis, some areas on it become active, that is, capable of transmitting fluid, in FIG. Figure 2 shows a general view of the faults using the Mora-Coulomb fracture criterion, where inactive fault zones correspond to areas with green fill, and active fault areas correspond to red fill; wells that fall into the red “active” areas are characterized by catastrophic mud losses, long well construction cycles and their high cost, in FIG. 3 shows a model of a discrete network of fractures built on the basis of a tectonic model driver, where a well trajectory entering a highly fractured zone is accompanied by mud absorption (without crossing faults), FIG. 4 - shows the tension and compression zones obtained on the basis of the stress-strain state of the medium and the tectonic model of the field, where absorption during drilling is most typical for tension zones, and tightening and landing during drilling for compression zones, FIG. 5 - shows the calculation of the wellbore stability model for the design trajectory. The red window shows the interval of auto-hydraulic fracturing at the current (indicated by a green dotted line) density of the drilling fluid.

The proposed method is implemented as follows.

A complex of geophysical research studies is being carried out at the field, including seismic surveys, drilling of reference wells, recording a complex of logs on reference wells, sampling and core testing for mechanical properties.

Based on the results of seismic exploration, the main reflecting horizons are dived and the discontinuous faults are highlighted.

The selection of the main reflecting horizons is carried out according to the standard method. To identify discontinuous faults, a cube of the seismic attribute Ant-tracking is constructed, which emphasizes changes in the wave pattern to detect faults, cracks and other linear anomalies within the seismic cube. For a more clear and contrasting identification of discontinuous faults, the construction of the cube of the seismic attribute Ant-tracking is carried out in a certain way: from the original seismic cube of the amplitudes of the reflected waves in the deep domain, the construction of the cube of the seismic attribute Chaos, an attribute showing a measure of the seismic signal randomness. The window parameters for calculating (x, y, z) are set to 1.5, 1.5, 1.5. Then, from the Chaos seismic attribute cube, the Ant-tracking seismic attribute cube is built in Passive mode. The resulting seismic cube emphasizes all potential faults and heterogeneities. To reduce the noise level of the signal, the resulting cube of the seismic attribute Ant-tracking Passive is fed to the input of the Ant-tracking algorithm, but this time in Aggressive mode. This mode emphasizes the most likely tectonic disturbances and heterogeneities. The resulting cube of the seismic attribute Ant-tracking Aggressive is again fed to the input of the Ant-tracking algorithm in Aggressive mode, resulting in the cube of the seismic attribute Ant-tracking Aggressive II. This manipulation is necessary for a more contrasting emphasis of tectonic disturbances and noise reduction. Dive bursting faults, depending on their complexity and geometry, is carried out in manual or automatic mode.

The next step is to calculate the one-dimensional geomechanical model and the wellbore stability model for reference wells. For this, it is necessary that a certain set of logs was recorded on the reference wells: gamma-ray logs, density logs, acoustic logs and caliper. The calculation of elastic-strength properties is carried out according to standard formulas and calibrated using data obtained from core studies on mechanical properties.

Next, elastic-strength properties are distributed in the interwell space using geostatistical methods, while leaving one or more wells not involved in the distribution of properties to check the quality of the upscaling of logs and the quality of the distribution of properties. The preferred aspect ratio of the modeling sector is 1: 1. Also included in the model are previously identified tectonic disturbances.

Then, the three-dimensional geomechanical model is loaded with the boundary conditions obtained on the basis of the one-dimensional geomechanical model and the stress calculation by the finite element method. The calculation result is a complete stress tensor in the modeling sector, as well as cubes of elastic-strength properties and cubes of basic stresses.

Over geological time, a gap can enter both the active and passive phases of tectonic development. The process of activation of discontinuous violations means its transition to a critical stress state, overcoming the brittle fracture band, that is, when the shear stresses acting at a specific point on the surface of a fault (rupture) exceed the effective normal stresses.

The emergence of new and activation of existing discontinuous violations is interpreted according to the Mohr-Coulomb law and graphically displayed using a three-dimensional diagram. After creating a new crack, the shear stresses are reset to the fracture plane, and the stress state also changes after the crack is activated. (Fig. 1). Each point on the fault surface is calculated based on the Mohr-Coulomb fracture criterion, which takes into account the azimuth, zenith angle, angle of internal friction, cohesion, azimuths of maximum and minimum stresses, pore pressure, as well as the calculated stress tensor obtained by the finite element method.

Below are two formulas: (1) - for the limit state at the time of the occurrence of the fault, (2) - for the limit state at the time of the activation of the fault.

- касательное напряжение на площадки n;

- shear stress on the site n;

- нормальное напряжение на площадки n;

- normal voltage at the site n;

τ _ƒ is the adhesion strength of the sample (cohesion);

K _ƒ - coefficient of internal friction of the sample;

K _s - coefficient of dry friction;

p _ƒl is the fluid pressure;

To calculate the stresses acting on a fault, it is necessary to know the values of the principal stresses and the direction cosines of the normal to the crack in the space of principal stresses. (3); (4)

l, n - the first and third direction cosines of the normal to the crack in the space of principal stresses;

In the analysis of the critical stress state, it is also necessary to take into account the kinematics of the discontinuous disturbance and its orientation in the relative azimuth of the maximum horizontal stress and the azimuth of the well.

Active fault zones are capable of transmitting fluid and cause mud uptake. Thus, fault areas are identified that are characterized by potential absorption of the drilling fluid and the project path is corrected.

Within the modeling sector, a tectonic model of the field is constructed using the semiclassical method, namely, by constructing an inverse stress model. To build a tectonic model, all tectonic disturbances within the modeling sector are used. The conditionality of the tectonic model and the calculated driver of the tectonic model is checked by comparing the obtained geometric characteristics of the cracks (angle and dip azimuth) with the characteristics of the cracks recorded by special geophysical instruments - imagers. On the basis of tectonic modeling and probabilistic assessment of cracks, the driver of the tectonic model is identified and the necessary parameters are obtained for constructing a tectonic model of fracturing, namely, the 3D trend of the density, angles and azimuths of cracks. In areas with high fracturing conclude the potential absorption zones of the drilling fluid (Fig. 3).

Based on the tectonic model, the relative displacements and deformations of rocks are calculated. In this case, displacements and deformations are estimated in three directions: x, y, z. In areas where there is multidirectional movement of rocks, tension zones are formed, respectively, in areas where the movement is directed towards each other, compression zones are formed. Due to the tectonic movement of rocks, a redistribution of the void space in the rocks occurs, which leads to the formation of areas prone to tool failure during drilling and mud losses (Fig. 4).

Using the calculated cubes of elastic-strength properties and cubes of basic stresses from a three-dimensional geomechanical model, the properties and stresses are transferred to the planned trajectories of the wells. For planned well trajectories, a wellbore stability model calculation is made (Fig. 5) and a conclusion is made about the need to adjust the well wash program.

The effectiveness of the proposed method was tested at 45 wells Tsarichansky + Filatovsky license area of the Orenburg region. For the study, wells were taken both vertical, so directionally and horizontally. The analysis was carried out both in areas with discontinuous violations and in areas remote from faults. Studies on historical drilling data have shown that the absorption was caused by one (or several) absorption mechanisms. Further drilling was carried out taking into account adjustments to the design trajectories of the wells, which allowed to significantly reduce the time of drilling the well and reduce its cost. This once again confirms the validity of the assumption that it is the tectonic component that makes a high contribution to the distribution of the areas of potential absorption of the drilling fluid, while timely adjustment of the well path avoids drilling problems. In addition, a noticeable role was played by the quality of the initial seismic cube of the amplitudes of the reflected waves - its low quality makes it difficult to isolate and trace tectonic disturbances, leads to underestimation or overestimation of tectonic disturbances.

The results of the studies showed that the simultaneous use of four mechanisms to reduce the risk of absorption during drilling leads to a high synergistic effect that allows you to effectively deal with the absorption of the drilling fluid, adjusting the trajectories of the design wells and making recommendations on the density of the drilling fluid for each drilling interval.

Using the proposed method can significantly reduce the time of well construction by reducing the time to deal with absorption, such as the installation of cement bridges, pumping blocking compounds, the excess of a portion of the wellbore to bypass the absorption zone.

Thus, wells drilled without absorption of the drilling fluid (or with minimal absorption of the drilling fluid) were drilled in inactive areas of the faults (or outside the faults), in areas with minimal fracturing and in static areas (not in the zones of extension or compression), where the flushing program individually selected for each well, and the most emergency wells met one or more mechanisms of absorption of the drilling fluid.

Claims (2)

1. A method for predicting mud absorption zones during drilling of horizontal, directional, vertical wells in terrigenous and carbonate reservoirs, characterized by a complex of geophysical surveys, including seismic surveys, drilling of reference wells, logging, core sampling and examination for mechanical properties, construction based on seismic exploration of the cube of the seismic attribute Ant-tracking, which indicates changes in the wave pattern to detect faults, cracks and other x linear anomalies inside the seismic cube, and the identification of discontinuous faults, the next step is to calculate a one-dimensional geomechanical model and model of wellbore stability for reference wells, then calculate the elastic-strength properties and the main stresses in the reference wells, transfer and propagate the elastic-strength properties in the interwell space using geostatistical methods carry out the loading of the three-dimensional geomechanical model with boundary conditions obtained on the basis of the one-dimensional geomechanical mode and, and they calculate stresses by the finite element method, the result of the calculation is the full stress tensor in the modeling sector, cubes of elastic-strength properties and cubes of basic stresses, the Mohr-Coulomb fracture criterion is applied to the selected surfaces of discontinuous faults, the data for the calculation of which are taken from the calculated cubes of basic stresses , and determine for each point on the fault surface its state - active or inactive, then construct a tectonic model of the field, then producing the construction of a discrete network of fractures based on the tectonic model driver, the probability of drilling fluid absorption is inferred based on the intersection of a large number of fractures with the well’s trajectory, without intersecting the trajectory with a fracture, and a model of the wellbore stability is constructed and auto-fracturing zones are identified by applying a flushing program on the stability model of the well of the project well, where they conclude that it is necessary to adjust the well wash program. 2. The method according to p. 1, characterized in that when constructing a tectonic model, tensile or compression zones are calculated, where they conclude that a particular area is predisposed to absorption during drilling.

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