Method for treating micro-gap of casing-cement sheath interface by local non-uniform expansion of casing

Abstract

The present application belongs to the technical field of oil and gas well drilling and completion, and relates to a method for treating micro-gap at the cementing interface between casing and cement sheath by locally and non-uniformly expanding the casing, and the specific steps are as follows: (1) calculating the thickness of the micro-gap between the cement sheath and the casing according to the cause of the micro-gap; (2) calculating the expansion amount of the casing according to the cause and thickness of the micro-gap; and (3) plugging the micro-gap according to the expansion amount of the casing. The method directly seals the micro-gap by remodeling the interface contact stress through controllable plastic deformation of the casing, thereby breaking through the dependence of traditional repair technology on the size of the gap. Compared with conventional methods, the present application does not need to rely on the permeability of the material, can avoid secondary damage to the wellbore, is suitable for high temperature, high pressure and complex well conditions, and provides an efficient solution for the integrity management of the whole life cycle of oil and gas wells.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas well drilling and completion technology, and relates to a method for treating micro-gap at the casing-cement sheath bonding interface due to local non-uniform expansion of the casing. Background Technology

[0002] Cementing is a crucial step in ensuring wellbore integrity during oil and gas extraction. Its core function is to provide structural support for the wellbore and isolate formation fluids, preventing inter-layer flow, casing corrosion, and wellbore instability. The quality of cementing directly determines the long-term safety, production efficiency, and development costs of oil and gas wells. Defective cementing can lead to abnormal casing pressure, annular flow, and even well control failures. Therefore, high-standard cementing processes are a technical guarantee for the safe and efficient development of oil and gas resources.

[0003] However, even if cementing operations meet design requirements, dynamic fluctuations in downhole pressure and temperature during production and subsequent operations can still lead to cement sheath seal failure. The formation of micro-gap (typically less than 0.1 mm) at the casing-cement sheath bonding interface is one of the main failure modes. The formation mechanism of micro-gap can be clearly divided into two categories: The first category is caused by a decrease in casing pressure, such as during production shutdown or fluid drainage, which causes a sudden drop in casing pressure. Due to elastic contraction, the casing separates from the cement sheath, forming annular micro-gap at the interface. The second category is driven by an increase in casing pressure, such as during production enhancement operations or pressure surges, where the casing pressure rises sharply. Under high pressure, the cement sheath undergoes irreversible plastic deformation. When the pressure recovers, the casing rebounds to its original position due to its elastic properties, while the cement sheath cannot recover due to plastic deformation, ultimately forming residual micro-gap at the interface.

[0004] Currently, conventional techniques for treating micro-gap mainly rely on high-pressure injection of cement grout or chemical sealing materials. However, these methods have significant limitations: First, the micro-gap size is extremely small (typically tens to hundreds of micrometers), making it difficult for conventional materials to effectively penetrate due to their high viscosity and large particle size. Second, high-pressure injection can easily damage the cement ring itself and even induce secondary cracks. Third, the difference in thermal expansion coefficients between the sealing material and the cement ring may cause the interface to re-separate upon temperature changes. Therefore, traditional methods are insufficient for achieving precise and long-term sealing of micro-gap. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a method for treating micro-gap at the casing-cement sheath bonding interface through localized non-uniform expansion of the casing. This method directly seals the micro-gap by reshaping the interfacial contact stress through controllable plastic deformation of the casing, overcoming the dependence of traditional repair techniques on gap size. Compared to conventional methods, this invention does not rely on material permeability, avoids secondary wellbore damage, and is adaptable to high temperature, high pressure, and complex well conditions, providing an efficient solution for the integrity management of oil and gas wells throughout their entire lifecycle.

[0006] Due to incomplete cement bonding and the existence of micro-gaps between the cement and the casing, the casing pressure increases. This invention studies the casing pressure changes based on thick-walled cylinder theory to determine the required wedge-shaped expansion amount and expansion load of the casing. The expansion width is determined through experimental research, and then expansion operations are performed on the corresponding positions of the casing using tools to seal the micro-gaps. The specific steps are as follows:

[0007] (1) Calculate the thickness of the micro-gap between the cement ring and the sleeve based on the cause of the micro-gap;

[0008] (2) Calculate the sleeve expansion based on the cause of the micro-gap and the thickness;

[0009] (3) Seal the micro-gap according to the expansion of the sleeve.

[0010] Depending on the cause of the micro-gap formation, the thickness of the micro-gap in this invention is calculated in different ways.

[0011] When the micro-gap is caused by an increase in pressure inside the sleeve, the calculation steps for the micro-gap thickness are as follows:

[0012] (1-1-1) Prepare cement ring samples that meet the experimental requirements;

[0013] Typically, cement sheath samples need to be adjusted according to the actual downhole conditions, such as cement mix ratio, hardening degree, depth, and temperature, to ensure that the samples can represent cement sheaths in the actual downhole environment.

[0014] (1-1-2) Triaxial loading and unloading experiments were conducted on the cement ring sample, and the stress-strain curve of the cement ring was recorded to obtain the plastic strain.

[0015] In triaxial tests, the formation stress borne by the cement sheath is simulated by applying axial and radial pressures (or lateral stresses) to the cement sheath sample. The deformation response of the cement sheath sample is recorded under different loading rates, stress paths, and environmental conditions.

[0016] Furthermore, the stress-strain curves of the cement ring samples were recorded to obtain the elastic deformation stage and the plastic deformation stage of the cement. The onset of the plastic deformation stage marks the permanent deformation of the cement ring, usually accompanied by stress exceeding its yield strength.

[0017] Furthermore, unloading tests were conducted, gradually reducing the applied external stress, and the strain recovery of the cement ring sample was observed. Ideally, if the deformation of the cement ring is elastic, the strain will be fully recovered after unloading; if it is plastic deformation, there will be unrecoverable plastic strain.

[0018] Furthermore, the plastic strain can be calculated from the residual deformation of the sample after unloading. The residual strain represents the amount of permanent deformation of the cement ring.

[0019] (1-1-3) The thickness of the micro-gap is calculated as follows:

[0020] Δr1=ε 塑 ×h (1)

[0021] Where h is the height of the cement ring sample in the triaxial experiment, in meters; ε 塑 Δr1 represents the plastic strain; Δr1 represents the thickness of the micro-gap caused by the plastic deformation of the cement ring, in meters.

[0022] When the micro-gap is caused by a decrease in pressure inside the casing, and localized non-uniform expansion is used to fill the micro-gap, external pressure needs to be applied to induce plastic deformation of the casing. This process is mainly based on the theory of open thick-walled cylinders. Therefore, the calculation steps for the micro-gap thickness are as follows:

[0023]

[0024] In the formula, Δr2 is the radial displacement (deformation) at any radius r of the cylinder, representing the degree of expansion or compression of the cylinder in the radial direction due to the internal pressure p, in meters (m); p represents the internal pressure of the sleeve, the pressure applied to the inner wall of the cylinder, with the external pressure being negative, usually assuming uniform distribution of the internal pressure, in Pa; r1 and r2 are the inner and outer radii of the sleeve, respectively, in meters (m); r is the radial position of any point in the thick-walled cylinder, satisfying r1≤r≤r2, used to calculate the radial displacement at that point, and r=r2, in meters (m); E is the elastic modulus, in Pa; μ is Poisson's ratio, dimensionless.

[0025] After calculating the micro-gap thickness, it is also necessary to determine the expansion amount of the sleeve.

[0026] When the micro-gap is caused by an increase in pressure inside the casing, the formula for calculating the casing expansion is:

[0027] Casing expansion = ε 塑 ×d+Δr1 (3)

[0028] Where, ε 塑 Δr1 represents the plastic strain, d represents the thickness of the cement ring in meters, and Δr1 represents the thickness of the micro-gap caused by the plastic deformation of the cement ring in meters.

[0029] When the micro-gap is caused by a decrease in pressure inside the casing, the specific process for obtaining the casing expansion is as follows:

[0030] (1-2-1) Prepare cement ring samples that meet the experimental requirements;

[0031] (1-2-2) Triaxial tests were performed on cement ring samples to obtain stress-strain curves. The strain ε corresponding to 75% of the maximum axial stress was selected.

[0032] (1-2-3) The casing expansion amount is calculated as follows:

[0033] Casing expansion = ε × d + Δr² (4)

[0034] In the formula, ε is the strain corresponding to 75% of the maximum axial stress, in %; d is the thickness of the cement ring, in meters; and Δr2 is the thickness of the micro-gap caused by the pressure reduction, in meters.

[0035] After determining the micro-gap thickness and the sleeve expansion amount, this invention uses an expansion device to force the sleeve to expand by squeezing it, forming a certain deformation and precisely sealing the micro-gap in the annular space. The expansion device used in this invention is a hydraulic sleeve local non-uniform expansion device or an energy-based sleeve local non-uniform expansion device commonly used in the art.

[0036] Compared with the prior art, the present invention has the following advantages:

[0037] (1) Precisely seal micro-gaps to improve cementing sealing performance

[0038] Traditional methods primarily rely on high-pressure injection of cement or other materials to fill micro-gaps, but these methods are ineffective at effectively penetrating and sealing extremely small gaps. This invention utilizes localized, non-uniform expansion to induce controllable plastic deformation in the casing, effectively filling micro-gaps and thus improving the sealing effect.

[0039] (2) Non-destructive repair to avoid damage to the cannula.

[0040] Existing repair methods, such as cutting the casing or re-cementing, may affect wellbore integrity. This invention applies a controlled external force to induce localized plastic deformation in the casing without affecting its overall strength, thus avoiding the risks associated with cutting or re-cementing.

[0041] (3) Controllable expansion design ensures operational accuracy

[0042] This invention can precisely control the expansion of the casing, ensuring that micro-gaps can be sealed without causing excessive expansion. At the same time, by optimizing the spacing of the expansion sections (2-3m), it avoids casing damage caused by stress concentration, thereby improving the reliability and uniformity of the operation.

[0043] (4) Adapt to complex well conditions and improve operational adaptability

[0044] Traditional cement repair methods are poorly adaptable to different formations and complex well conditions, while this invention is applicable to different well depths, temperatures and pressures, and can be widely used for the repair of high-temperature and high-pressure wells, deep wells and old wells. Attached Figure Description

[0045] Figure 1This is a schematic diagram of axial stress-strain during plastic deformation in a triaxial unloading experiment.

[0046] Figure 2 This is a schematic diagram of the stress-strain curve of cement under triaxial testing. Detailed Implementation

[0047] Example 1: Micro-gap plugging in high-temperature and high-pressure gas wells

[0048] A high-temperature, high-pressure gas well has a depth of 4500m and a bottom-hole temperature of 180℃. The internal pressure of the casing fluctuates periodically (30-70MPa). Well logging revealed a micro-gap of approximately 0.08mm thickness at the casing-cement sheath interface, causing the wellhead casing pressure to abnormally increase from 15MPa to 22MPa, posing a risk of gas channeling. Analysis indicated that the micro-gap was primarily caused by plastic deformation of the cement sheath due to the increased internal pressure of the casing.

[0049] To address this issue, this embodiment employs localized non-uniform expansion of the casing to treat the micro-gap at the casing-cement ring bonding interface. The steps are as follows:

[0050] (1) A downhole environment (temperature 180℃, confining pressure 50MPa) was simulated by triaxial loading-unloading experiments. Cement ring samples with a height H = 0.1m were prepared using G-grade cement (mixture ratio: 35% silica fume + retarder). Triaxial loading and unloading experiments were conducted on the cement ring samples, and the yield strength of the cement ring was measured to be 45MPa, and the plastic strain εplastic = 0.08% ( Figure 1 The micro-gap thickness was calculated to be 0.08 mm according to formula (1), which is consistent with the measured value. Based on the thick-walled cylinder theory, the expansion amount of the sleeve (d is the thickness of the cement ring of 0.05 m) was determined, and the designed expansion amount was 0.12 mm (covering the micro-gap and reserving a safety margin δ, which is usually taken as 0.01–0.02 mm). The expansion shape (arc shape) was optimized by finite element simulation, and it was determined that the expansion pressure needs to be ≥500 MPa (matching the yield strength of P110 sleeve of 550 MPa). The expansion width covers the micro-gap area and extends to 50 mm on each side, with a total width of 300 mm.

[0051] (2) Using acoustic imaging logging (frequency 20kHz), the micro-gap was accurately located at a depth of 4485-4490m. An energy-type expander was lowered to the target formation at 4488m using drill pipe. The locator (made of spring steel) had a calibration error ≤0.1m, and a stability test was performed (vibration amplitude <2mm). The expander was then activated to seal the micro-gap. After repair, the wellhead casing pressure dropped to 15.2MPa (normal range 15±0.5MPa), and the acoustic amplitude decreased from 35dB to 12dB (threshold <15dB), significantly improving the interface bonding quality. Periodic pressure monitoring (30 days) showed that the casing pressure stabilized at 15.2-15.5MPa with no leakage.

[0052] This example achieves precise sealing of micro-gaps (100% coverage), with local plastic deformation of the casing less than 0.15%, without affecting the overall strength (internal pressure resistance remains at 550MPa). The operation takes only 2 hours, which is a significant improvement in efficiency compared to traditional cementing methods, verifying the high efficiency and reliability of this invention under complex well conditions with high temperature and high pressure.

[0053] Example 2: Micro-gap sealing caused by pressure drop inside shallow well casing

[0054] A shallow well with a depth of 1500m and a bottomhole temperature of 80℃ experienced a sudden drop in casing pressure from an initial 25MPa to 10MPa due to periodic fluid drainage operations. This caused elastic contraction of the casing, resulting in annular micro-gaps (approximately 0.04mm thick) at the casing-cement sheath interface. Well logging showed an abnormal increase in wellhead casing pressure from 5MPa to 7.5MPa, indicating a risk of formation fluid cross-flow. Analysis revealed that the micro-gaps were primarily caused by elastic contraction due to the decrease in casing pressure.

[0055] To address this issue, this embodiment employs localized non-uniform expansion of the casing to treat the micro-gap at the casing-cement ring bonding interface. The steps are as follows:

[0056] (1) The micro-gap thickness during the pressure drop in the casing was calculated using the thick-walled cylinder theory: inner radius r1 = 0.1 m, outer radius r2 = 0.12 m; elastic modulus E = 200 GPa, Poisson's ratio μ = 0.3; pressure drop Δp = 15 MPa, plastic strain εplastic = 0.08%, cement ring thickness 0.05 m. The radial displacement was calculated using formula (2):

[0057]

[0058] Casing expansion = ε × d + Δr² = 0.08% × 0.05 × 10³ + 0.0409 = 0.0809 mm

[0059] The measured value of 0.08 mm is consistent with the actual value, and the designed expansion amount is 0.08 mm (including safety margin).

[0060] Finite element simulation verification was conducted. The casing expansion shape was designed as an arc, and the expansion pressure was required to be ≥300MPa (matching the yield strength of J55 casing 350MPa). The expansion width covered the micro-gap area and extended to 20mm on each side, with a total width of 140mm.

[0061] (2) Ultrasonic imaging logging (frequency 18kHz) was used to accurately locate the micro-gap at a depth of 1490-1495m. A hydraulic expander was lowered to the target formation at 1492m using drill pipe. The fiber optic gyroscope calibration error was ≤0.03m, and a pressure sealing test was performed (leakage rate <0.1mL / min). The expander was then activated to seal the micro-gap. The wellhead casing pressure decreased from 7.5MPa to 5.1MPa (normal range 5±0.2MPa); the acoustic amplitude decreased from 25dB to 8dB (threshold <10dB), and the interface bonding quality met the standards; the local plastic deformation of the casing was <0.08%, and the internal pressure resistance remained at 350MPa. Periodic pressure testing (30 days) showed that the casing pressure stabilized at 5.1-5.3MPa with no signs of crossflow, verifying the long-term effectiveness of the repair.

Claims

1. A method for addressing micro-gap at the casing-cement ring bonding interface due to localized non-uniform expansion of the casing, characterized in that, The specific steps are as follows: (1) Calculate the thickness of the micro-gap between the cement ring and the sleeve based on the cause of the micro-gap; (2) Calculate the sleeve expansion based on the cause of the micro-gap and the thickness; (3) Seal the micro-gap according to the expansion of the sleeve; When the micro-gap is caused by an increase in pressure inside the sleeve, the calculation steps for the micro-gap thickness are as follows: (1-1-1) Prepare cement ring samples that meet the experimental requirements; (1-1-2) Triaxial loading and unloading experiments were conducted on the cement ring sample, and the stress-strain curve of the cement ring was recorded to obtain the plastic strain. (1-1-3) The thickness of the micro-gap is calculated as follows: Δr1=ε 塑 ×h; Where h is the height of the cement sample in the triaxial experiment, in meters; ε 塑 Δr1 represents the plastic strain; Δr1 is the thickness of the micro-gap caused by the plastic deformation of the cement ring, in meters. When the micro-gap is caused by an increase in pressure inside the casing, the formula for calculating the casing expansion is: Casing expansion = ε 塑 ×d+Δr1; Where, ε 塑 Δr1 represents the plastic strain; d represents the thickness of the cement ring in meters; Δr1 represents the thickness of the micro-gap caused by the plastic deformation of the cement ring in meters. When the micro-gap is caused by a decrease in pressure inside the sleeve, the calculation steps for the micro-gap thickness are as follows: ； In the formula, Δr2 is the radial displacement at any radius r of the casing, in meters; p represents the internal pressure of the casing, in Pa; r1 and r2 are the inner and outer radii of the casing, respectively, in meters; r is the radial position of any point inside the casing, satisfying r1≤r≤r2, and the micro-gap thickness is obtained when r=r2, in meters; E is the elastic modulus, in Pa. Poisson's ratio is dimensionless. When the micro-gap is caused by a decrease in pressure inside the casing, the specific process for obtaining the casing expansion is as follows: (1-2-1) Prepare cement ring samples that meet the experimental requirements; (1-2-2) Triaxial tests were performed on the cement ring samples to obtain stress-strain curves. The strain corresponding to 75% of the maximum axial stress was selected. ; (1-2-3) The casing expansion amount is calculated as follows: Casing expansion = ε × d + Δr²; In the formula, Δr2 represents the strain corresponding to 75% of the maximum axial stress, in %; d represents the thickness of the cement ring, in meters; Δr2 represents the thickness of the micro-gap caused by the pressure reduction, in meters.

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