# Method for determining profile of thermal conductivity of mine rocks in well

FIELD: mining.

SUBSTANCE: according to the method, a casing string with temperature sensors attached to its outer surface is lowered to a well and cement is pumped to an annular gap between the casing string and well walls. During a cement filling and hardening process, temperature measurements are made and thermal conductivity of surrounding mine rocks of the well is determined as per the measured relationship between temperature and time.

EFFECT: possibility of simultaneous reception of information on properties of a relatively thick layer of rocks around the well and information on thermal conductivity of rocks for the whole cemented interval of depths.

3 cl, 2 dwg, 1 tbl

The invention relates to geophysical research wells and can be used to determine thermal properties of the rock formations surrounding the well.

Knowledge of thermal properties and, in particular, thermal conductivity of rocks is necessary for modeling and optimization of oil and gas, especially for optimization of thermal methods of heavy oil production. Thermal properties of rocks is usually measured in the laboratory on core samples extracted from the well. The results of measuring the heat capacity of the rock is quite applicable for modeling field temperature oil reservoir, and the results of the measurement of thermal conductivity of the core may be substantially different from thermal conductivity of the blocks of rocks in situ. This is due to:

(1) change in rock properties of the core during drilling,

(2) the difference of laboratory RT conditions from the reservoir,

(3) the influence of reservoir fluids, which do not always take into account when conducting laboratory measurements.

One of the major problems is the representative results of laboratory measurements. Usually the core recovery is significantly below 100% and laboratory studies do not give information about the properties of fractured layers and weakly consolidated rocks (where core recovery is low) that may significantly affect the value of thermal conductivity bol is the oldest blocks of rocks, used for reservoir simulation. Therefore, in addition to laboratory studies on the core for many years conducting experiments to determine thermal properties of the rocks in situ in the well, but so far not developed suitable for the practical use of the method or device.

It was proposed many different approaches to the determination of thermal conductivity of rocks in situ. For example, it was proposed to use for this purpose the recovery process of the unperturbed temperature of the array after drilling or after washing the wells (see Dehnow C. N., Deacons D. I. Thermal study wells. Moscow, HUNTINGTN, 1952, 128 S.). The disadvantage of this method is the strong dependence of the results of measurements of flow and free thermal convection of a fluid in the borehole, from the radius of the well and the position of the temperature sensor in the borehole. In addition, it is difficult to accurately simulate thermal excitation of the array during drilling or well wash that is necessary for quantitative interpretation of the measured temperature and evaluation of thermal properties of rocks.

Most of the work on determination of thermal conductivity of rocks in situ based on theory of linear heat source. A well placed long enough (3-5 m) of the heated probe and record the rate of increase of temperature of the probe, which is th depends on thermal properties of the surrounding materials (see, for example, Huenges, E., Burhardt, H., and Erbas, K., 1990. Thermal conductivity profile of the KTB pilot corehole. Scientific Drilling, 1, 224-230). The main disadvantages of this method are a great time (about 12 hours) required for the measurement of thermal properties at each site wells, the distortions associated with free thermal convection of a fluid in the borehole, and the need to supply the downhole sonde significant electrical power.

Some methods use a small heated probes, which are pressed against the borehole wall (see Kiyohashi H., Okumura K., K. Sakaguchi, and K. Matsuki, 2000). Development of direct measurement method for thermophysical properties of reservoir rocks in situ by well logging. Proceedings World Geothermal Congress 2000, Kyushu-Tohoku, Japan, May 28 - June 10, 2000. These methods allow to reduce the duration of measurements, however, they require smooth walls of the borehole, sophisticated equipment, a complex numerical model to determine thermal properties of rocks on the results of the temperature measurement probe and allow us to estimate thermal properties of only a very thin (1-3 cm) layer of rock near the borehole walls. This layer was subjected to mechanical stresses during drilling, may have induced fractures, pores in the rock filled with mud, and not a reservoir fluid, therefore, thermal properties of this layer can vary significantly from rock properties away from the well.

There are also known methods using moveable the e probes. The heat source is at the head of the probe, the temperature sensor is on the end of the probe (see, for example, U.S. patent 3,892,128). These methods allow you to quickly assess thermal properties of rocks at a considerable depth interval, however, as in the previous case, they provide information about the properties of only a very thin layer of rocks around the well.

The technical result achieved by the invention is to provide the opportunity to simultaneously obtain information about the properties of relatively thick (about 1 m) layer of rocks around the well and information about thermal conductivity of rocks for the entire case hardening depth interval; in addition, it does not require to supply in a borehole electrical energy.

This technical result is achieved by the fact that, in accordance with the proposed method of determining the profile of thermal conductivity of rocks is lowered into the well casing attached to its outer surface temperature sensors, pump cement into the annulus between the casing and the borehole wall during injection and curing of the cement carry out temperature measurements to determine thermal conductivity surrounding the borehole rocks by the formula

where λ(z) is thermal conductivity of the rocks at depth z, Q_{c}is the heat of hydration of cement, V_{a}(z) - volume of the annular gap, per meter of length of the borehole at the depth z, C(z) is the coefficient determined by the method of linear regression when the approximation of dependence measured in borehole temperature T(z,t) from the inverse time t^{-1}asymptotic formula

T(z,t)=T_{ƒ}(z)+C(z)·t^{-1},

where T_{ƒ}(z) is the temperature of the rocks at depth z.

As temperature sensors can be used in fiber-optic sensor.

The invention is illustrated by drawings, where Fig.1 shows the geometry is cylindrically symmetric model, which was used in the calculations of Fig.2 - the results of numerical modeling of the temperature dependence of the cement from the inverse of the time elapsed after the start of hydration for two values of thermal conductivity of rocks.

As shown in Fig.1, in accordance with the invention for the temperature monitoring of the injection process, and hardening (hydration) of the cement and the subsequent temperature monitoring oil/gas or injection of fluid 1 in the hole, surrounded by rock 4, vacation is t casing 2 is attached to fiber cable 5 meter temperature.

During the hydration of cement 3 pumped into the annulus between the casing 2 and the borehole wall, there is a considerable amount of heat release (Q_{c}=100÷200 MJ per 1 m^{3}cement mortar). The maximum temperature increase during hardening of cement is approximately 20-50°C. the Main stage of cement hydration (and heat) lasts 30-50 hours, after which the radius of the area with increased temperature increases and the temperature in the hole relaxes to the undisturbed temperature of the rocks at this depth.

The relaxation rate of the temperature depends on the amount of excess thermal energy Q per 1 m length of the borehole, and thermal properties of the rocks surrounding the borehole. Excess heat energy Q can be found as the product measured in the laboratory of heat of hydration of cement Q_{c}and volume of the annular gap, which is defined by the outer radius of the casing r_{co}and measured with the help of caliper radius of the well, depending on depth z: r_{w}(z). Thus, the recovery rate of the temperature in the borehole after cementation is determined solely by thermal properties of the surrounding rocks.

Below is a theoretical model that is used as the basis for the determination of the heat its the government of rocks measured in the borehole depending on the temperature from time to time.

Known solution to a cylindrically symmetric tasks conductive conductivity on the evolution in time of an arbitrary initial temperature distribution in a homogeneous medium (see, for example, Carslaw,, Eger D. thermal Conductivity of the solid tel. M.: Nauka, 1964, S. 88). In particular, if the initial temperature distribution having the form of a cylinder

the time dependence of the temperature in the center of this cylinder has the form

where r_{0}is the radius of the cylinder, '*and*' is thermal diffusivity of the medium.

At sufficiently large times after the beginning of the relaxation temperature (

This formula can be written in the General form of the energy conservation law (by multiplying the numerator and denominator of (3) to the multiplier π·ρ):

where

Numerical experiments show that the generalized asymptotic formula (4) are valid for any initial temperature distribution. Thus r_{0}there is a characteristic size of the region in which the initial temperature is substantially different from the ambient temperature, and the required condition is met:

Formula (4) shows that if the initial thermal perturbation in a cylindrically symmetric problem set as the number of excess heat in a homogeneous environment, the asymptotic behavior of the temperature is determined solely by thermal conductivity of the environment.

In this case, the environment is heterogeneous (Fig.1): borehole fluid (0<r<r_{ci}, r_{ci}the inner radius of the casing), casing (r_{ci}<r<r_{co}, r_{co}- the outer radius of the casing), cement (r_{co}<r<r_{w}, r_{w}is the radius of the borehole) and rock (r_{w}<r), has a significantly different thermal properties. However, as shown by numerical calculations, the asymptotic who ormula (4) accurately describes the change in borehole temperature over time.
This is due to the fact that at large times the increase in the radius of the heated zone is determined solely by thermal conductivity of the rocks, and the radial temperature variations near the well small.

In this case, the excess thermal energy Q is determined by the product of heat of hydration of cement Q_{c}(J/m^{3}and the volume of the annular gap V_{a}(m^{3}one meter length of the borehole)

where L is the depth interval used for averaging volume of the ring is the first gap. A typical value of this parameter L=2÷3 m, it gives a vertical resolution of the proposed method. The value of L is determined by the smoothing effect of the vertical conductive heat transfer in the rock and the typical time scale of the measurements.

If the unperturbed temperature T_{ƒ}(z) breeds on the depth z is known, thermal conductivity of the rocks λ(z) is determined by the value of the function F(z,t) at large times (t>t_{0}):

Time t_{m}must be greater than the duration of the main phase hydration of the cement and the time at which it becomes applicable asymptotic formula (4). A typical value of t_{m}=100-150 hour the century

Usually unperturbed temperature rocks T_{ƒ}(z) is unknown and thermal conductivity of the rocks are encouraged to identify as follows.

The measured temperature values at t>t_{m}approximate the asymptotic formula (at the time of hydration over 100 hours)

In this method of linear regression to find the parameter C(z) and the temperature of the rocks T_{ƒ}(z), which is not used in the subsequent calculation of thermal conductivity.

The parameter C is used to calculate thermal conductivity of rocks by the formula:

The proposed method of determination of thermal conductivity of rocks has been tested on synthetic cases, prepared using a commercial simulator Comsol. The geometry is cylindrically symmetric model, which was used in the calculations are shown in Fig.1.

The inner and outer radii of the casing is equal to r_{ci},=0.1 m, r_{co}=0.11 m, the radius of the hole r_{w}=0.18 m, the outer radius of the computational region is STI r_{
e}=20 m Used in the calculations of thermal properties of the borehole fluid (the effective thermal conductivity, taking into account the free thermal fluid), casing, cement and rock are given in the table.

TC W/(m·K) | ρ, kg/m^{3} | C, j/(kg·K) | |

Fluid | 3 (effective value) | 1000 | 4000 |

Column | 30 | 7800 | 500 |

Cement | 0.8 | 2600 | 900 |

Rock | 1 and 2 | 2700 | 1000 |

We used the following analytical formula for the power of the heat of hydration of cement q(t):

Calculations were performed for the following parameters characterizing the heat generated by cement hydration: Q_{c}=1.5·10^{8}J/m^{3}, t_{0}=6 h, t_{1}=8 hours.

In Fig.2 shows the calculated dependence of the temperature in the annular gap at a distance of 0.13 m from the axis of the borehole from the inverse time t^{-1}with^{-1}(the time interval 300-100 hours from the beginning of the hydration of cement) for two values of thermal conductivity of rocks: λ=1 and 2 W/m·K. the regression equations and the white lines correspond to the linear approximation of the numerical simulation results. The initial temperature was taken equal to zero. In the time interval calculated dependences are well described by straight lines (9). Shown in the figure, the regression equations have close to zero free members (0.0283 and 0.0473), which corresponds to zero initial temperature, and substitution in equation (10) coefficients of the regression equation (1 W/m·K)=703030 and(2 W/m·K)=387772) gives the following values TopLop is bednesti rocks: 1.07 and 1.96 W/m·K.

You can increase the accuracy of determination of thermal conductivity of rocks and significantly reduce the required duration of temperature measurement, if the solution of the inverse problem is to use numerical modeling of the hydration process of the cement in the well.

1. The method for determining the profile of thermal conductivity of rocks in the borehole, in accordance with which it is lowered into the well casing attached to its outer surface temperature sensors, pump the cement slurry in the annulus between the casing and the borehole wall during injection and curing of the cement carry out temperature measurements to determine thermal conductivity surrounding the borehole rocks by the formula

where λ(z) is thermal conductivity of the rocks at depth z, Q_{c}is the heat of hydration of cement, V_{a}(z) - volume of the annular gap, per meter of length of the borehole at the depth z, C(z) is the coefficient determined by the method of linear regression when approximating the dependence of the measured downhole temperature Tz,t) from the inverse time t^{
-1}asymptotic formula

T(z,t)=T_{f}(z)+C(z)·t^{-1},

where T_{f}(z) is the temperature of the rocks at depth z.

2. The method according to p. 1, whereby as the temperature sensors use fiber-optic sensor.

3. The method according to p. 1, according to which to determine thermal conductivity of rocks using numerical simulation of the hydration process of the cement in the well.

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EFFECT: increased yield.

16 cl, 4 dwg

FIELD: oil and gas industry.

SUBSTANCE: device includes a mechanical oscillating system with constant magnets fixed on it and a converter of mechanical oscillations to electrical ones. A mechanical oscillating system is made in the form of a cylindrical bimetallic spiral, one end of which is rigidly fixed, and the other one is free, and the converter of mechanical oscillations to electrical ones is made in the form of a system of interacting electromagnetic fields of constant magnets rigidly fixed on a cylindrical bimetallic spiral and coils of a drive and pickup of oscillations providing for transverse oscillations of the cylindrical bimetallic spiral.

EFFECT: enhancing reliability of a device and improving its design.

2 dwg