RU2778361C1 - Method for cementing the casing string of a borehole - Google Patents

Abstract

FIELD: petroleum industry.

SUBSTANCE: invention relates to the field of drilling petroleum and gas boreholes and can be used in cementing casing strings in the steady-state mode. Casing strings are cemented with a shut-in wellhead in the steady-state mode. Before tempering the cement, the annular preventer (PUG) is closed, the hydraulic valve in the working flow line is opened, a cementing unit is connected to the valve block in order to control the volume of the solution exiting the borehole. The back pressure required to ensure the steady-state mode is created in the behind-the-casing space at the wellhead by means of a throttle valve. The back pressure value depending on the position of the lower boundary of the cement solution in the borehole is determined by calculation, with account for the depth of descent of the casing string and the distance from the wellhead to the moving lower interface between the cement solution and the drilling mud in the casing string, respectively, the height of the cement solution column in the casing string at the end of pumping, corresponding to the volume required to cement the casing string; for the injection pressure and back pressure at the wellhead, respectively; the densities of the cement solution and the drilling mud, respectively; the coefficients of hydraulic resistances in the movement of the cement solution and the drilling mud in the casing string and the drilling mud and the cement solution in the annular behind-the-casing space; the speed of the cement solution or drilling mud in the casing string; the diameters of the borehole, the outer and inner casing string.

EFFECT: increase in the quality of cementing and increase in the effectiveness thereof due to the reduction in the borehole construction cost caused by preventing the occurrence of complications and the cost of elimination thereof.

1 cl, 2 dwg

Description

The invention relates to the field of drilling oil and gas wells, and in particular to the technology of cementing casing strings and can be used when cementing casing strings in a steady state.

A known method of cementing wells by pumping through the cementing head plugging and squeezing fluids separated by a separating plug (see E. M. Solovyov Completion of wells. - M.: Nedra, 1989 pp. 165-173). The author argues that in order to prevent a decrease in the level of the cement slurry during pumping into the casing string and a vacuum does not occur in the annular annular space, when the drilling fluid (drilling mud) leaves the well, it is necessary to maintain an appropriate backpressure, while the value of the backpressure he proposes calculated based on the pressure at the end of the cement slurry injection. According to the author, the criterion for the absence of vacuum at the wellhead should be the presence of cement slurry injection pressure. In order to avoid possible lost circulation, it is proposed to control the upward flow rate, i.e. at any moment of cementing, the velocity in the annular annulus should be less than the limiting velocity at which the sum of static and hydrodynamic pressure is less than the absorption pressure. However, the author does not disclose the method of control and regulation of the upstream speed.

The disadvantage of the above technical solution is the low quality of cementing due to the inability to prevent complications that arise.

The closest analogue to the proposed method (prototype) is a method of cementing the casing string (patent RU No. 2199649 E21B 33/14), including the injection of a cement slurry into the casing string, the discharge of a separating plug, by the cement slurry injection process, the minimum value of the cement slurry injection volumetric flow rate is calculated. In this method, breaking the continuity of the cement slurry during its movement in the casing string in a downward flow is proposed to be prevented by intensifying the injection, that is, the injection rate of the cement slurry and the displacement fluid (volumetric velocity) should be greater than the rate of its fall (flow rate). At the same time, the intensity of the flow and the injection rate are determined analytically - by calculation.

The disadvantage of this method is the lack of control over the movement of the cement slurry, both in the casing string and in the annulus. In addition, the intensification of the flow of cement slurry in the casing will cause a proportional increase in the velocity of the upward flow of drilling fluid in the annulus, which will inevitably provoke the collapse of unstable rocks, and as a result, will lead to a large expenditure of time and materials for corrective cementing.

The task to be solved by the present invention is to develop an effective method for cementing the well casing string in steady state, which will provide high-quality filling of the annular space with the formation of a continuous cement shell, excluding the collapse of unstable rocks and the occurrence of other complications.

The technical result, to which the present invention is directed, is to improve the quality of cementing and increase its efficiency by reducing the cost of well construction by preventing the occurrence of complications and the cost of their elimination.

The specified technical result is achieved due to the fact that in the method of cementing the well casing string in steady state, which includes sealing the annular annular space with a preventer, connecting the flow line of the annular annular space through a throttle with a measuring tank designed to control the volume of drilling fluid leaving the well, pumping cement slurry into the casing string, control of the pressure in the measuring tank during the said injection of cement slurry, when the pressure in the measuring tank drops to zero, a counterpressure is created using a throttle in the annular annular space, which ensures the movement of cement and drilling slurries at a constant speed in a steady state, at the same time, the calculation of the value of the mentioned back pressure, which ensures the movement of cement and drilling fluids at a constant speed in the steady state in the annular annular space, is carried out according to the formula:

for

for

где

where

- глубина спуска обсадной колонны и расстояние от устья скважины до движущейся нижней границы раздела между цементным и буровым растворами в обсадной колонне, соответственно;

- the depth of the casing string and the distance from the wellhead to the moving lower interface between the cement and drilling fluids in the casing string, respectively;

h ₀ is the height of the cement slurry column in the casing at the moment of completion of injection, corresponding to the volume required for cementing the casing;

P _n and P _y - injection pressure and back pressure at the wellhead, respectively;

ρ _c and ρ _p - density of cement and drilling fluids, respectively;

λ _ct , λ _rt and λ _rk , λ _ck - coefficients of hydraulic resistance during the movement of cement slurry and drilling fluid in the casing, and drilling fluid and cement slurry in the annular annulus, respectively;

v - the speed of movement of cement or drilling fluid in the casing;

d _c , d _n and d _b - diameters: wells, outer and inner casing, respectively;

g is the free fall acceleration.

With an increase in the volume of cement slurry pumped into the casing, the pressure on the cementing unit (for example, TsA-320) gradually decreases to atmospheric pressure, after which a part of the cement slurry column breaks off under the action of forces due to the difference in density of drilling and cement slurries. Then there is a spontaneous uncontrolled increase in the speed of the cement slurry in the casing, which in turn causes a proportional increase in the speed of the drilling fluid in the annular space of the well.

An increase in the velocity of the cement slurry in the casing, accompanied by a corresponding increase in the velocity of the upward flow of the drilling fluid in the annulus, leads to the loss of stability of clay rocks, the removal of cuttings from the caverns and the occurrence of such complications as premature thickening of the cement slurry, errors in determining the volume of the mixing zones of drilling and cement slurry. The elimination of complications of this kind requires a lot of time and materials, and sometimes leads to the abandonment of the well.

In addition, at the upper boundary of the interface between the cement and drilling mud, an air “plug” with a pressure below atmospheric is formed under the separating plug. Calculations show that in order to catch up with the escaping column of cement mortar, it is necessary to pump it at a speed of more than 4 m/s, which is technically possible, but not economically feasible. The resulting vacuum causes the locking pins to be difficult to retract during the preparatory work before starting the separating plug, to the extent that the cock on the cementing head has to be opened to compensate for the pressure. Since the column of cement slurry in this case moves at maximum critical speeds, the resulting air lock will occupy a significant initial volume. This volume will not decrease until the slurry column moves into the annulus and equalizes the pressure in the casing string. The final volume of the air plug will depend on the pressure formed in the string at the moment the separation plug is seated in the support ring (stop ring) after the entire column of cement slurry has been forced into the annulus.

Calculations and practice show that the air plug, even in the final compressed state, can occupy a significant volume, which will complicate when cementing shallow wells, as well as wells where there is a danger of cement slurry absorption, determining the “stop” moment and can lead to exposure of the column shoe.

In the proposed method of well casing cementing, by controlling and adjusting the back pressure in the annular space at the wellhead, the necessary constant speed of movement of cement and drilling fluids, that is, a steady state, is ensured throughout the entire process. The drilling fluid is generally used as the displacement fluid.

In hydrodynamics, steady motion is understood as motion in which the speed and pressure at each given point in space filled with a moving fluid remain constant all the time (but can change when moving from one point in space to another), that is, motion that is constant in time, changing in a space in which the velocity and pressure depend only on the coordinates of the moving fluid particle, i.e. from its position in space, and do not depend on time. Thus, with steady motion

υ \u003d f ₁ (x, y, z)

p \u003d f ₂ (x, y, z)

In other words, in the steady state, exactly as much fluid comes out of the well as was pumped into it. At the same time, the continuity of the flow of cement slurry is ensured, there is no inertial component, since uniform movement occurs without acceleration, hydrodynamics is minimal, which will preserve unstable formations, eliminate absorption and other complications.

The invention is illustrated by drawings.

In FIG. 1 shows a wellhead piping scheme.

In FIG. 2. shows the dependence of the difference in injection pressure and back pressure at the wellhead on the volume of cement slurry or displacement fluid pumped into the string.

In the drawing, the elements are indicated by the following positions: cementing head 1, universal preventer (PUG) 2, valve block 3, throttle 4, measuring tank 5 of the cementing unit, casing 6, drilling fluid 7, cement mortar 8, annulus 9.

The method of cementing wells in unstable rocks in steady state is carried out as follows.

In accordance with the proposed method in a well, for example, a gas well or an underground gas storage (UGS) well, casing strings 6 are cemented with a sealed mouth. The sealing of the mouth is carried out with a universal preventer (PUG) 2. Before mixing the cement, the universal preventer (PUG) 2 is closed, the throttle 4 is opened in the valve block 3 on the working flow line. A measuring tank 5 of the cementing unit is connected to the block of valves 3, designed to control the volume of drilling mud 7 coming out of the well. to the surface.

In the absence of a cementing control station (CCS), monitoring the volume of drilling mud 7 leaving the annulus 9 of the well allows obtaining information about the volume of the injected cement slurry 8, that is, the location of its lower boundary in the casing string 6 at the current moment. The rest of the grouting technique is tied with the cementing head 1 and installed in a standard way.

The necessary backpressure is created in the annular space 9 at the wellhead using a throttle 4. In the process of cementing, the necessary backpressure is created in the annulus 9 in accordance with the calculation by smoothly adjusting the degree of opening of the throttle 4 on the valve block 3.

Immediately after the cessation of pumping the estimated volume of cement slurry 8, the throttle 4 is closed. The wellhead is completely sealed for the period of preparatory work before starting the separating plug.

The injection pressure and the pressure in the annulus 9 (at the wellhead) are monitored throughout the cementing process. The pressure in the annulus 9 at the wellhead is regulated from the moment the cement slurry is pumped and is controlled as it moves along the entire length of the casing 6. In this regard, the calculation is performed for each position of the cement slurry column 8 in the casing 6, that is, the distance from it the upper boundary to the wellhead with an interval of 10 meters. Each calculated value corresponds to a certain volume of cement slurry 8 (when cement is mixed) or displacement fluid (during displacement).

The backpressure in the annular space 9 of the well is controlled as follows.

From the moment the cement slurry is pumped into the string, the operator of the cementing unit records the volume of the incoming drilling fluid 7 into a measuring tank 5 (for example, a measuring tank TsA-320M), tied with a block of valves 3. Since the motion mode is steady, the amount of drilling fluid leaving the well 7 coincides with the amount of cement slurry pumped into the string 8. Information about the amount of outgoing drilling fluid from the operator of the cementing unit goes to the technologist, who is located at the valve block 3 with a pressure gauge installed on it. The technologist has a calculation made on a personal computer. This calculation serves as an annex to the standard work plan.

Upon receipt of information about the pressure drop to zero on the pressure gauge, which is installed on the cementing head 1, that is, the discharge pressure has decreased to atmospheric pressure, the cement slurry 8 is pumped in the volume q', while by controlling the injection of the cement slurry with the help of the throttle 4, a back pressure is created in the enclosed space. As the volume of the cement slurry increases with the help of the throttle 4, the value of the backpressure increases, which reaches its maximum P _{y max} after the entire volume of the cement slurry q ₄ is pumped into the casing. At this point, the cement plug is started. At the time of preparatory work for the launch of the plug, the wellhead is completely sealed (drilling fluid does not move along the annulus 9 of the well).

At the moment of starting the cementing plug with the help of the throttle 4, the pressure P _{y max} is set, which is maintained constant until the cement slurry reaches the column shoe.

The release of the cement slurry into the annular space is accompanied by a decrease in back pressure, which, after the volume q ^{11 of} the cement slurry and the displacement fluid are pumped into the well, becomes equal to zero, that is, the control valve and the PUG are fully opened. Further, the cementing process is carried out according to standard technology.

The dependence of the difference in injection pressure P _n and pressure in the annular space P _y (back pressure) on the volume of liquid pumped into the column q with adjustable back pressure at the mouth is shown in Fig. 2.

On FIG. 2, the following designations are adopted:

Р ₀ - pressure at the beginning of cement slurry injection;

P _k - pressure at the end of the displacement of the cement mortar by the displacement fluid;

P _{y max} - maximum back pressure at the mouth in the annular space, equal to the pressure at the end of the displacement;

y _o - the height of the injected cement slurry with a volume q ¹ at which the discharge pressure becomes equal to zero, after which an adjustable back pressure is created in the annular space; y ₀ ¹ - corresponds to the volume of displacement fluid q ¹¹ , after pumping which it is necessary to fully open the valve in the annulus;

q ¹¹ is the total volume of cement slurry and displacement fluid, after which the pressure in the casing string and behind the casing string is equalized;

- соответствует объему продавочной жидкости q_пp, после закачки которого цементный раствор достигает башмака колонны;h _o - the height of the cement slurry column after pumping the estimated volume q _c of the cement slurry;

- corresponds to the volume of the squeezing liquid q _pr , after which the cement slurry reaches the column shoe;

q ₀ - the total volume of the displacement fluid and cement slurry used in cementing, (q ₀ =q _pp +q _c ).

The calculation of the required backpressure value is carried out according to the formulas:

for

at

where

- глубина спуска обсадной колонны и расстояние от устья скважины до движущейся нижней границы раздела между цементным и буровым растворами в обсадной колонне, соответственно;

- the depth of the casing string and the distance from the wellhead to the moving lower interface between the cement and drilling fluids in the casing string, respectively;

h ₀ is the height of the cement slurry column in the casing at the moment of completion of injection, corresponding to the volume required for cementing the casing;

P _n and P _y - injection pressure and back pressure at the wellhead, respectively;

ρ _n and ρ _p - density of cement and drilling fluids, respectively;

_λct , λ _rt and λ _rk , λ _ck - coefficients of hydraulic resistance during the movement of cement slurry and drilling fluid in the casing, and drilling fluid and cement slurry in the annulus, respectively;

v is the speed of the fluid flow (cement or drilling fluid used as a displacement fluid) in the casing string;

d _c , d _n and d _b - diameters: wells, outer and inner casing, respectively;

g is the free fall acceleration.

An example of the implementation of the method.

The production casing was run to a depth of ISO meters in a wellbore with a diameter of 215.9 mm. The previous column with a diameter of 245 mm was lowered to a depth of 950 meters. Drilling was carried out with flushing with a drilling fluid with a density of 1200 kg/m ³ , conditional viscosity 35-40 seconds, water loss 8 cm ³ / 30 min., crust 1 mm. In accordance with the project and the work plan, it was planned to shut and pump 15 tons of cement for fastening with its rise up to 500 m from the mouth.

As an addition to the standard work plan, the required backpressure was calculated depending on the depth of the lower boundary of the cement slurry, the volume of which was controlled using a measuring tank of the cementing unit (mernik). Cementing was carried out with the wellhead sealed with a universal preventer. The preventer was closed before the start of work, the throttle is fully open, since at the beginning of cementing, in accordance with the calculation, no back pressure is created. We started mixing cement and pumping it into the casing string. The discharge pressure gradually began to decrease, and after pumping 1.25 m ^{3 of} cement slurry, which was noted in the measuring tank of the cementing unit, dropped to zero. From that moment in the annular space of the well, using a throttle, they began to maintain the calculated back pressure, depending on the amount of injected cement slurry. In the pressure range of 0-3.0 MPa, the pressure at the throttle increased by 0.4 MPa on average every 1 m ³ . After pumping the planned volume of cement slurry 9.6 m ³ , the throttle was closed completely, the movement of the cement slurry in the casing stopped. The pressure on the throttle was 3.0 MPa, which corresponded to the calculated one. After turning off the locking screws of the cementing head, which turned away without effort, since there was no vacuum in the casing string, a separating cementing plug was launched with simultaneous opening of the throttle and releasing pressure to 2.8 MPa.

Further, 10 m ^{3 of} squeezing fluid was pumped at a constant back pressure, which corresponded to the achievement of the lower boundary of the cement slurry of the column shoe. When the cement slurry began to move into the annulus, we started to reduce the pressure on the throttle. After pumping another 2.5 m ^{3 of} the displacement fluid, which was used as a drilling fluid, the injection pressure began to increase, and the preventer was opened. Further cementing was carried out with an increase in injection pressure, as in standard technology. In the process of squeezing, there were no water hammers, which usually correspond to the moment when the drilling fluid reaches the upper boundary of the cement mortar. After squeezing was completed, 150 liters of drilling fluid were used in excess of the estimated volume of squeezing fluid to determine the “stop” moment, while up to 1.5 m ^{3 of} squeezing fluid is pumped according to standard technology. When determining the quality of cementing by acoustic cement logging (ACC), the presence of 79% of solid cement stone, solid + partial - 10% was noted, while in the base wells of solid and partial cement stone, no more than 40% is recorded.

The claimed method of cementing the casing string of a well provides, in unstable rocks in steady state, an increase in the quality of cementing and an increase in its efficiency by reducing the cost of well construction by preventing the occurrence of complications and the cost of their elimination.

Claims (14)

A method for cementing a well casing in a steady state, including sealing the annular annular space with a preventer, connecting the flow line of the annular annular space through a throttle with a measuring tank designed to control the volume of drilling fluid leaving the well, pumping cement slurry into the casing string, controlling the pressure in the measuring tank during the said injection of cement slurry, and when the pressure in the measuring tank drops to zero, backpressure is created using a throttle in the annular annular space, which ensures the movement of cement and drilling slurries at a constant speed in the steady state, while calculating the value of the said backpressure, which ensures the movement of cement and drilling fluids at a constant speed in steady state in the annular annular space is carried out according to the formula: for

for

where

and y are the depth of the casing string and the distance from the wellhead to the moving lower interface between the cement and drilling fluids in the casing string, respectively; h ₀ is the height of the cement slurry column in the casing at the moment of completion of injection, corresponding to the volume required for cementing the casing; P _n and P _y - injection pressure and back pressure at the wellhead, respectively; ρ _c and ρ _p - density of cement and drilling fluids, respectively; λ _ct , λ _rt and λ _rk , λ _ck - coefficients of hydraulic resistance during the movement of cement slurry and drilling fluid in the casing string and drilling fluid and cement slurry in the annulus, respectively; v - the speed of movement of cement or drilling fluid in the casing; d _c , d _n and d _b - diameters: wells, outer and inner casing, respectively; g is the free fall acceleration.

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