RU2223387C1 - Process plugging casing string in well - Google Patents

Abstract

FIELD: development of fields of reservoir fluids by wells, construction of wells. SUBSTANCE: casing string carrying shut-off and by-pass devices is sunk into well. First portion of grouting substance is injected and forced through into string clearance through bottom of casing string, next portions of grouting substance are injected and forced through with some time intervals through holes in shut-off and by-pass devices, starting from lowest device. Column clearance is isolated from string clearance in lower part of casing string. Holes in shut-off and by-pass devices are closed after portions of grouting substance are forced through them into string clearance. Mud solution is utilized as grouting substance. Time interval between injections of portions of mud solution is assumed to be equal to time of clay sedimentation from mud solution in string clearance of each preceding portion. Depth of installation of each shut-off and by-pass device is established by upper boundary of clay sedimentation from corresponding portion of mud solution in string clearance. EFFECT: enhanced reliability of plugging of casing string in well. 3 cl, 2 dwg

Description

The invention relates to the field of downhole development of reservoir fluids and can be used in well construction.

There is a method of plugging a casing string into a well, including lowering the casing string into the well after drilling it to the design depth and replacing the drilling fluid in the annulus with cement by pumping it through the bottom of the casing followed by holding the cement slurry for a while (1).

The disadvantage of this method is the impossibility of its use when plugging casing strings with raising the cement slurry to a high height, since this can lead to hydraulic fracturing due to the creation of large bottomhole pressures in the process of casing string plugging.

There is a method of sealing casing strings, in the implementation of which a clay solution with clay granules with waterproofing properties is used as the plugging material for the period of filling the annular space with plugging material (2).

However, this method of plugging casing strings cannot be used for plugging deep wells, since when crushing a clay solution with clay granules into the annulus, technological complications may arise, for example, as a result of accumulation of clay granules at the bottom of the well, in addition, even in case of successful injection clay material in the annular space may accumulate clay granules mainly in the lower part of the annular space, as a result of which casing string plugging is carried out only in its lower part, and in the upper part in the annular space there may be a clay solution with clay content insufficient for reliable plugging of the exposed rock formations.

Closest to the described method is the so-called method of stepwise plugging of the casing string, which consists in lowering the casing string with a step cementing sleeve (MSC) installed on it, plugging the annular space initially from the bottom of the casing string to the MSC, then, after the time interval required for hardening the first portion of the cement mortar, plugging the annular space above the MSC installation site by forcing through the holes of the second portion of the cement solution. In this case, after the second portion of the cement mortar is pressed through, the openings of the MSC are closed, and sometimes annular packers are used to isolate the column space from the annulus in the lower part of the casing (3).

However, this method of plugging, as well as other methods using cement mortars as a grouting substance, does not provide tightness of the annulus during the operation of the well, since as a result of movements of the casing string and surrounding rocks relative to each other, which are inevitable due to technogenic processes, cracking of cement stone occurs.

The objective of the invention is to increase the reliability of the plugging of the casing string in the well.

The task is achieved in that when implementing the method of plugging a casing string in a well, including lowering the casing string into the well with shut-off devices installed on it, sequential pumping and forcing into the annulus of the first portion of the cement material through the bottom of the casing string and subsequent portions with a time interval through the openings of the ZPU, starting from the lower ZPU, isolating the column space from the annular space in the lower part of the casing and closing the hole th ZPU after forcing through them portions of the grouting substance into the annular space, according to the invention, a clay mortar is used as the grouting material, the time interval between the processes of pumping portions of the grouting material is taken equal to the sedimentation time of clay from the grouting clay solution in the annulus of each previous portion, and the installation depth ZPU is determined depending on the permissible pressures in the annulus and the concentration of the selected grouting clay p alignment. In addition, cement mortar is pumped through the upper ZPU into the annulus to the wellhead, and a destructuring agent is added to the cement slurry and peptizing agents are used in granules with water-proofing properties, and the action of the water-proofing properties of the granules creates at least the time of pumping portions of the grouted clay mortar into the well, forcing it into the annular space and sedimentation from it clay with granules.

In FIG. 1, 2 presents the results of studies using a clay solution as a grouting substance: FIG. 1 - dependence of plastic viscosity (η), dynamic shear stress (τ) and pressure drop (ΔР) required for pumping cement slurry in the annular space on the concentration of clay (K) in the solution; FIG. 2 - dependence of the maximum pressure in the annular space (G) on the lift height of the cement slurry (h) at a depth of its supply into the annular space L = 2000 m.

The invention consists in the following.

After drilling the well to the design depth and carrying out the necessary preparatory work with the wellbore, the casing, equipped with the required number and at the calculated ZPU distances, is lowered into the casing last. After lowering the casing string, they begin to plug its annular space. Initially, the first portion of the cement slurry is pumped through the bottom of the colony into the annulus, after which the tube space is isolated from the annulus by putting the casing packer installed on the column in the lower part into working condition, or by pumping cement slurry after the portion of the cement slurry with the calculation of the rise it in the annular space approximately to the height of one pipe (primitive cementing). In the case of plugging a casing running down to the bottom of the reservoir, the cement slurry in the annulus is raised to the roof level of the reservoir.

After the time interval necessary for sedimentation of clay from the injected portion of the grouted clay slurry, the next portion of the grouted clay is pumped into the well and pushed into the annulus through the openings of the mud installed at a distance from the bottom of the column equal to the height of the clay sediment from the first portion of the grouted clay solution. After the second portion of the grouted clay is fed into the annulus, the openings of the ZPU are closed and the time required for sedimentation of clay from this portion of the grouted clay is maintained. Then, in the same sequence, the drilling fluid and water sludge are replaced from the previous portion of the grouted clay mud in the annulus to the next portion of the grouted clay mud through the openings above the located ZPU. At the same time, the casing is arranged in such a way that the distances between the i-th and i + 1-th ZPUs are equal to the amount of clay sediment from the i-th portion of the cement slurry.

To determine the distances between the ZPU carry out the following preliminary studies.

It is known that with an increase in the clay content in a clay solution, its viscosity grows, and therefore the necessary pressure for pushing it into the annular space when used as a plugging substance. Therefore, studies are conducted of the dependence of the rheological properties used to plug the clay solution on the clay concentration in it, by which the necessary pressure drops are determined for pumping such solutions into the annulus. Based on the technical capabilities (power of pumping units, pipe strength limits) and geological conditions (hydraulic fracturing pressure, the presence of highly permeable formations in the rock interval, cased by a column, etc.), an acceptable clay concentration is selected for each portion of the grouting clay, according to which the height is determined clay sedimentation sediment and the distance between ZPU according to the formula

where Li is the installation depth of the i-th ZPU (starting from the bottom), m;

N - casing string length, m;

hoc.i - the height of the clay sediment from the cement slurry of the i-th portion in the annulus, m

If the sedimentation time of clay from cement slurry is significant, then to accelerate this process, destructuring agents (coagulants) - salts of divalent and trivalent metals (Ca ²⁺ , Mg ²⁺ , Fe ³⁺ , etc.) can be added to the solution which occurs electromagnetic coagulation of clay particles. And when there are increased requirements for the insulating properties of a clay sediment, grouting slurry is added in granules (ampoules) with water-resistant properties of substances with dispersing cations, which contribute to the peptization of clay, thereby increasing ductility, stickiness, swelling of clay, and its water permeability decreases. As peptizers, alkalis (NaOH, KaOH, NH OH, etc.) can be used. At the same time, the action of the waterproofing properties of the granules creates (for example, by coating them with films of substances hardly soluble in water) at least the time of pumping portions of the grouted clay solution into the well, forcing it into the annulus and sedimenting clay and granules from it.

To increase the stability of the estuarine part of the casing string, its upper part through the last (upper) ZPU can be plugged with cement mortar.

An example implementation of the method.

It is necessary to plug the casing with an external diameter of d _h = 0.168 m, a length of L = 2000 m in a well with a diameter of d _c = 0.214 m. As a plugging substance, we use a clay solution prepared from clay previously dried and crushed.

Studies were carried out of the rheological properties of a clay solution with various concentrations of clay in it. As a result of the studies, the dependences of plastic viscosity (η), dynamic shear stress (τ) and pressure drop (ΔР), necessary for pumping the clay solution in the annular space of the well to a height of 1 m, depending on the concentration of clay in the solution (K) (Fig. .1). The pressure drop was calculated according to the well-known formula

where p is the dimensionless shear stress, determined by the graphic dependence on the Saint-Venant parameter (S), which in turn is determined by the formula

where Fк = π (Dc ² -Dк ² ) / 4,

Q is the rate of pumping of the solution, m ³ / s;

η is the plastic viscosity of the solution, Pa · s.

In the calculations, Q = 0.002 m ³ / s was taken.

To simplify the calculations, we neglect the pressure loss on the movement of the drilling fluid in the annular annular space above the cement slurry and in the pipe space (the drilling fluid is used to push portions of the cement slurry into the annular space).

For geological reasons, the pressure in the annulus (annulus) space over the entire section of the well should not exceed 3 hydrostatic pressures. This condition can be written as the following equation:

where ρ _ch. , ρ _b.p. - the relative specific gravity (with respect to water) of the cement slurry and mud, respectively;

L is the depth of the grouting clay mortar into the annulus;

h is the lifting height of the cement slurry in the annulus;

ΔР - pressure drop determined by the formula (2);

100 - conversion factor.

In FIG. 2 shows the results of calculations of the maximum pressures (G) created in the annular space when the cement slurry is pushed, depending on the height of its rise in the annular space (h) for 50% clay concentrations in the solution (ρ _{gl. Р.} ⁼ 1.8), 60% (ρ _{gl. R.} = 1.96) and 70% (ρ _{gl. R.} ⁼ 2.12), while taking ρ _b.r. = 1.1, and ΔP was determined according to the schedule in figure 1. Calculation of G was carried out according to the modified formula (4)

From the results of the calculations it is clear that for the studied clay, for our conditions, the concentration of 50 wt.% Is the most acceptable, while the admissible rise in the cement slurry (first portion) in the annular space is 1000 m.

Studies have shown that sedimentation of clay from a solution with such a concentration occurs during the day and the height of the clay sediment is 70% of the initial volume of the solution. So the installation depth of the 1st ZPU will be L ₁ = 2000 - 0.7. 1000 = 1300 m.

We solve equation (4) with respect to h and determine the permissible lifting height of the second portion of the cement slurry (with the same properties as the first portion) in the annulus

We take h = 650 m, then the installation depth of the second ZPU will be

Similarly, we determine the installation depth of the remaining ZPU

The volume of portions of the cement slurry (Vi) is determined by multiplying the area of the annular annulus (fk) by the height of the solution (h):

In our example, V ₁ = 13.8 m ³ , V ₂ = 9.1 m ³ , V ₃ = 6 m ³ , V ₄ = 5.2 m ³ , V ₅ = 2 m ³ , V ₆ = 1.3 m ³ , V ₇ = 0.53 m ³ .

The concentration of the created clay sedimentation sediment in the annulus in our example will be about 70% by mass. For such a concentration, the pressure drop necessary for pumping the solution in the annular space to a height of 1 m is about 0.5 MPa / m (see Fig. 1), with a casing string lowering depth of 2000 m, the total pressure drop will be 1000 MPa. Such is the tamponing ability of the clay sediment created in the annulus from the clay solution.

The described technical solution allows using well-known technological operations and means to carry out reliable plugging of the casing, since the occurrence of man-caused processes during the operation of the well does not violate the tightness of the annular space, because the plugging material does not have a rigid connection with the surrounding rocks and casing, fluid-conducting cracks cannot form in it, and the casing and surrounding rocks can undergo some relative movements.

Literature

1. Sereda N.G., Soloviev E.M. Drilling oil and gas wells. - M .: Nedra, 1981, p. 264-268.

2. RF patent N 2046926 "Method for sealing casing strings in wells", priority dated April 29, 1991.

3. Bulatov A.I., Avetisov A.G. Handbook of a drilling engineer. T.2. - M .: Nedra, 1973, p. 52-55.

Claims (4)

1. A method of plugging a casing string into a well, including lowering the casing into the well with shut-off devices installed on it — ZPU, sequentially pumping the first portion of the grouting substance into the annulus through the bottom of the casing and subsequent portions with a time interval of ZPU openings, starting from the lower ZPU, isolation of the column space from the annular space at the bottom of the casing and closing of the ZPU openings after forcing portions there of cementing material into the annular space, characterized in that a clay mortar is used as cementing material, the time interval between the processes of pumping portions of the clay solution is taken equal to the sedimentation time of clay from its solution in the annular space of each previous portion, and the installation depth of each ZPU is determined by the upper boundary sedimentary clay sediment from the corresponding portion of the clay solution in the annulus. 2. The method according to claim 1, characterized in that cement mortar is pumped into the annulus to the wellhead through the upper ZPU. 3. The method according to claim 1 or 2, characterized in that a destructuring agent is added to the clay solution. 4. The method according to one of claims 1 to 3, characterized in that peptizers are added to the clay solution in granules with water-proofing properties, and the action of water-proofing properties creates at least the time of pumping portions of the clay solution into the well, forcing it into the annulus and sedimentation from it clay with granules.

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