Mechanical Expansion of Steel Tubing as a Solution to Leaky Wellbores

Podsumowanie

This article reports on a laboratory scale investigation of an existing field procedure and its adaptation for sealing of leaky wellbores. It consists of mechanical expansion of metal pipe, which results in an improved metal/cement bond, ultimate sealing of hydraulic pathways and prevention of gas leaks caused by the presence of a microannular channel.

Streszczenie

Wellbore cement, a procedural component of wellbore completion operations, primarily provides zonal isolation and mechanical support of the metal pipe (casing), and protects metal components from corrosive fluids. These are essential for uncompromised wellbore integrity. Cements can undergo multiple forms of failure, such as debonding at the cement/rock and cement/metal interfaces, fracturing, and defects within the cement matrix. Failures and defects within the cement will ultimately lead to fluid migration, resulting in inter-zonal fluid migration and premature well abandonment. Currently, there are over 1.8 million operating wells worldwide and over one third of these wells have leak related problems defined as Sustained Casing Pressure (SCP)¹.

The focus of this research was to develop an experimental setup at bench-scale to explore the effect of mechanical manipulation of wellbore casing-cement composite samples as a potential technology for the remediation of gas leaks.

The experimental methodology utilized in this study enabled formation of an impermeable seal at the pipe/cement interface in a simulated wellbore system. Successful nitrogen gas flow-through measurements demonstrated that an existing microannulus was sealed at laboratory experimental conditions and fluid flow prevented by mechanical manipulation of the metal/cement composite sample. Furthermore, this methodology can be applied not only for the remediation of leaky wellbores, but also in plugging and abandonment procedures as well as wellbore completions technology, and potentially preventing negative impacts of wellbores on subsurface and surface environments.

Wprowadzenie

The reported experimental procedure has two main components that are critical: composite cylinders that simulate wellbores and the expansion fixture that is used to carry out mechanical manipulation of the cement.

Wellbores are the main gateway for production of subsurface fluids (water, oil, gas, or steam) as well as injection of various fluids. Regardless of its function, the wellbore is required to provide a controlled flow of produced/injected fluids. Wellbore construction has two distinct operations: drilling and completion. Wellbore cement, part of the completions procedure, primarily provides zonal isolation, mechanical support of the metal pipe (casing), and protection of metal components from corrosive fluids. These are essential elements of uncompromised, fully functioning wellbores. The integrity of the wellbore cement sheath is a function of the chemical and physical properties of the hydrated cement, the geometry of the cased well, and the properties of the surrounding formation/formation fluids^2,3. Incomplete removal of drilling fluid will result in poor zonal isolation since it prevents formation of strong bonds at interfaces with rock and/or metal. Cement sheaths can be subjected to many types of failure during the life of a well. Pressure and temperature oscillations caused by completion and production operations contribute to the development of fractures within the cement matrix; debonding is caused by pressure and/or temperature changes and cement hydration shrinkage^4,5,6. The result is almost always presence of microannular fluid flow, although its occurrence can be detected early or after years of service life.

Heathman and Beck (2006) created a model of cemented casing subjected to over 100 pressure and temperature cyclic loads, which showed visible debonding, initiation of cement cracks which can pose preferential pathways for migrating fluid⁷. In the field, the expansion and contraction of metal components of a wellbore will not coincide with those of cement and rock, causing interfacial debonding and formation of a microannulus, leading to an increase in permeability of the cement sheath. An additional casing loading can cause the propagation of radial cracks in the cement matrix once the tensile stresses exceed the tensile strength of the material⁸. All of the aforementioned cement failures can result in micro-channeling, which leads to gas migration, the occurrence of SCP, and long-term environmental risks.

A considerable number of producing and abandoned wells with SCP constitute a potentially new source of continuous natural gas emission⁹. The analysis conducted by Watson and Bachu (2009) of 315,000 oil, gas, and injection wells in Alberta, Canada also showed that wellbore deviation, well type, abandonment method, and the quality of cement are key factors contributing to potential well leakage in the shallower part of the well¹⁰. The existing remedial operations are costly and unsuccessful; the squeeze cementing, one of the most commonly used remedial techniques, has a success rate of just 50%¹¹.

In this paper we report on the evaluation of the Expandable Casing Technology (ECT) as a new remediation technique for leaky wellbores^12,13. ECT can be applied in new or existing wells¹⁴. The first commercial installation of this technology was performed by Chevron on a well in shallow waters of the Gulf of Mexico in November 1999 ¹⁵. The current operating envelope for expandable tubulars encapsulates an inclination of 100° from vertical, temperature up to 205 °C, mud weight to 2.37 g/cm³, a depth of 8,763 m, hydrostatic pressure of 160.6 GPa and a tubular length 2,092 m ¹⁶. A typical expansion rate for solid expandable tubulars is approximately 2.4 m/min ¹⁷.

This study offers a unique approach to the adaptation of ECT technology as a new remediation operation for SCP. The expansion of the steel pipe compresses the cement which would result in closure of the gas flow at the interface and seal the gas leak. It is important to mention that the focus of this study is the sealing of an existing microannular gas flow, therefore we only focused on that as a possible cause of leaky wellbores. In order to test the effectiveness of newly adapted technology for this purpose, we designed a wellbore model with an existing microannular flow. This is obtained by rotating the inner pipe during cement hydration. This is not to simulate any field operations, but simply to fast-forward what would happen after decades of thermal and pressure loading in a wellbore.

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Protokół

1. Composite Sample (Figure 1)

NOTE: Most cement jobs in the Gulf of Mexico (USA) are done using Class H cement¹⁸, therefore, the same type of cement was used to perform the lab experiments to simulate field-like conditions, the potential applicability of this technology for SCP remediation in the Gulf of Mexico.

1. Sample preparation
   NOTE: The 61-cm long sample consists of two grade B electrically resisted welded (ERW) carbon steel pipes (Figure 1). The inner pipe is 61 cm long and has a 6 cm outside diameter (OD) with 2.8 mm wall thickness. The outer pipe is 59.7 cm long, has 10 cm OD and a wall thickness of 5.7 mm. Yield strength and tensile strength of the pipes are 241 MPa and 414 MPa, respectively.
   1. Drill 12 holes of 2.4 mm on the outer pipe to provide the relief of pressure during expansion and mimic porosity of rocks in field conditions. Drill eight 8.6 mm holes next on the outer pipe, 90° apart with four holes 13 cm from the top and four holes 53 cm from the top.
   2. Thread these holes with 3.2 mm NPT (National Pipe Thread) threading tip to allow connection with pipe fittings and nylon tubing manifold assembly on the bottom (inlet) and top (outlet) side of the sample. Ensure that the inlet and outlet ports are 40.64 cm apart and are used for running of pre- and post-expansion multi-rate gas flow-through experiments.
   3. Coat outer pipe with anti-corrosion spray to prevent corrosion during the curing period which could interfere with the experiments due to formation of iron hydroxide and corrosion products could cause microfracturing of cement.
      NOTE: This scenario will be tested in the future experiments as corrosion of metal is often present in wellbore systems.
   4. Machine out the welding bead on the inside wall of the inner pipe.
   5. Cut custom-made steel coupling to a length of 4.5 cm, from 6.35 cm OD pipe. Thread the piece on the inside wall and weld it to the 0.63 cm thick steel plate ring (Figure 2). Thread the bottom part of the inner pipe on the outside wall in length of 4.5 cm to allow connection with the welded coupling, as shown in Figure 2.
   6. Weld the outer pipe to the steel plate ring.
   7. Lubricate the inner pipe’s outside wall with petroleum jelly and baking spray along its entire length. Screw the inner pipe into the coupling to finish the composite sample assembly.
   8. Cement the volume between inner and outer pipes with 1.57 g/cm³ cement slurry, 0.87 w/c ratio.
   9. Cure samples in a water bath at ambient conditions for a minimum period of 28 days. Keep the pH of the water bath between 12 and 13 by adding Ca(OH)₂ to the water to maintain high pH environment.
2. Preparation of 13.1 lb/gal cement slurry (for volume of 2.2 L)
   1. Pour 1,350 g of water into the 4 L, 3.75 horsepower laboratory blender and pre-hydrate 30 g (2% by weight of cement) of bentonite for 5 min on low speed (30,000 x g).
   2. After 5 min, pour 5 ml of defoaming agent and 1,500 g of cement powder into the blender and shear for 40 sec on high speed of 51,755 x g. Pour the cement slurry into the annulus of the pipe assembly and cover with a wet cloth and plastic wrap to avoid exposure to air and prevent carbonation of cement.
   3. Six hours after the cement slurry is poured between the pipes, rotate the inner pipe a quarter-turn back and forth every 15 min for the next 20 hr of cement hydration to prevent cement bonding with the inner pipe and create a microchannel (required for microannular gas flow).
   4. Place the cemented composite sample horizontally in the water bath for a minimum period of 28 days. Ensure that the water bath has a pH value of around 13 which is achieved by adding 100 g of Ca(OH)₂ into 20 L of water.

2. Pre-expansion Flow-through Experiments

1. Screw 3.2 mm fittings into four inlet and outlet ports on the outer pipe of the sample. Connect inlet and outlet manifolds with pressure transducers to the fittings (Figure 5).
2. Pressurize gas cylinder to initial inlet pressure of 50 kPa. Turn on computer software to record pressures.
3. Open the flow meter and begin the flow-through test. Monitor inlet and outlet pressures on the screen for 1 min, as shown in Figure 6.
4. Pressurize gas cylinder to inlet pressure of 172 kPa and monitor the pressure for another 2 min.
5. End flow-through experiment and pressure recording. Close the gas cylinder and vent the remaining gas into the atmosphere. Dismantle the manifolds and cover top of the sample with wet cloth while powering the expansion unit, to prevent carbonation and drying of cement.
6. Coat the inside wall of the inner pipe with lubricant for smooth running of the expansion cone and the sample is ready for expansion.

3. Expansion Setup and Expansion Procedure

1. Fully retain the expansion mandrel from the lower housing by the hydraulic cylinder, as shown in Figure 4a. Place the composite sample with hydrated cement in the lower sample housing of the fixture through the opening at the top (Figure 4b).
2. Fully elongate the expansion mandrel through the sample after which the expansion cone with desired expansion ratio (Figure 3) is slipped onto it, as shown in Figure 4c. Screw the retaining mandrel onto expansion mandrel, then screw the retaining mandrel guide onto the lower connector of the lower housing. The sample is ready for expansion.
3. Power the hydraulic unit to an optimum pressure of 10.3 MPa, and turn on the computer software for axial force recording.
4. Activate the control switch to retract the expansion mandrel and pull the expansion through the inner pipe of the sample, thus expanding the pipe and compressing the cement sheath. Expand samples to the length of 40.64 cm (Figure 4d) and then elongate the expansion mandrel into original position. Stop recording of axial forces.
5. Unscrew the retaining mandrel guide and remove the retaining mandrel. Take off the expansion cone from the expansion mandrel and fully retract the mandrel in order to remove the sample form the lower housing.
6. After the sample is removed, prepare it for post-expansion multi-rate gas flow-through experiments.

4. Post-expansion Multi-rate Flow-through Experiments

1. Clean inlet and outlet ports from any excess of squeezed cement paste.
2. Screw pipe fittings into four inlet and outlet ports on the outer pipe of the sample. Connect inlet and outlet manifolds to the fittings, as shown in Figure 5.
3. Pressure the gas cylinder to initial inlet pressure of 172 kPa. Turn on computer software to record pressures.
4. Open the flow meter and begin the flow-through test. Monitor inlet and outlet pressures on the screen (Figure 6).
5. After 5 min, pressurize the gas cylinder to inlet pressure of 345 kPa and monitor the pressures for another 5 min.
6. After 5 min increase the inlet pressure to 517 kPa.
7. After 5 min increase the inlet pressure to final inlet pressure of 690 kPa for another 5 min.
8. End the flow-through experiment and pressure recording. Close the gas cylinder and vent the remaining gas into the atmosphere. Dismantle manifolds from the sample.

5. Calculations of the Effective Permeability of the Microannulus

NOTE: The main purpose of this study was to provide qualitative information regarding existence of gas flow before and after expansion. The experimental design does not possess sophisticated components to be able to measure the width of the channel and flow rate accuracy. During these preliminary experiments sealing of gas flow was the main focus. Therefore, any of permeability calculations shown here are more semi-quantitative and not main goal of the study.

1. For calculation of the effective permeability, use the constant nitrogen flow rate of approximately q=1.42 cm³/sec upon pressure stabilization. The gas deviation factor for nitrogen at ambient conditions is Z=1 and viscosity μ=0.018 cP. Conduct all the flow-through tests at ambient conditions of T=535 ºR.
2. Calculate the area of the cemented annular space by taking inner radius of the outer pipe, r_Oinn=4.6 cm, and outer radius of the inner pipe, r_Iout=3.05 cm. The distance between inlet and outlet ports (ΔL) is 40.64 cm. Pressure differential (P_inlet-P_outlet), recorded by inlet and outlet pressure transducers, is the only variable used in calculations of effective permeability of the pre-manufactured microannulus (K_ef)¹⁹:
     Eq. 1
   q – nitrogen flow rate [cm³/sec]    K_ef – effective perm. of microannulus [mD]
   r _Iout – ID of outer pipe [cm]          r_Oinn – OD of inner pipe [cm]
   µ – gas viscosity [cP]                   Z – gas deviation factor
   T – temperature [ºR]                    ΔL – distance between pressure transducers [cm]
   P_inlet – inlet pressure [atm]         P_outlet – outlet pressure [atm]
3. Substitute all of the above values into the Equation 1 and calculate the effective permeability as shown below in Example 1. The inlet pressure recorded during pre-expansion flow-through experiment was P_inlet=12 kPa (0.12 atm) while the outlet pressure transducer was P_outlet=0.4 kPa (0.004 atm).
   Example 1:

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Wyniki

Pre-expansion gas flow-through tests on the composite sample showed pressure recording on the outlet pressure transducer, confirming gas flow through the pre-manufactured microannulus (Figures 7 and 8). Initial conditions were kept the same where initial inlet pressure was 103 kPa and the gas flow rate was kept at 85 ml/min for that period. The time lag in pressure recording between the inlet and outlet pressure transducers was 7.5 seconds, while the highest pressures recorded after incr...

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Dyskusje

The reported experimental procedure has two main components that are critical: composite cylinders that simulate wellbores and the expansion fixture that is used to carry out mechanical manipulation of cement. When designing wellbore models (cement/pipe composite cylinders), it is critical to choose adequate cement density, store samples under total humidity conditions (100% RH) and establish pipe-cement debonding before cement slurry completely sets. Failing to achieve this would make the entire gas flow experiment impo...

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Materiały

Name	Company	Catalog Number	Comments
ASTM A53 Grade B ERW Schedule 40 Steel pipe - OD=10.16 cm, ID=10.04 cm, L=59.7 cm	Baker Sales	BPE-4.00BB40
ASTM A53 Grade B ERW Schedule 10 Steel pipe - OD=6 cm, ID=5.94 cm, L=61 cm	Service Steel	n/a
Expansion Cones - AISI D2 grade alloy steel (60 RC hardness)	Shell	Custom-made
Pipe coupling - OD=6.35 cm, ID=6 cm, L=4.4 cm	LSU	Custom-made
Steel plate ring - OD=10.16 cm, ID=5.76 cm, thickness=6.35 mm	Louisiana Cutting	Custom-made
Class H Cement	LaFarge	04-16-12 / 14-18
Defoaming agent - D-Air 3000L	Halliburton	n/a
Bentonite clay	LSU	n/a
Calcium hydroxide	LSU	n/a
Expansion Fixture	Shell	Custom-made
Pressure transducers	Omega	PX480A-200GV
Teflon tubing	Swagelok	PB0754100
Union tee	Swagelok	SS-400-3
Elbow union	Swagelok	SS-400-9
Female elbow	Swagelok	SS-400-8-8
Port connector	Swagelok	SS-401-PC
Forged body valve	Swagelok	SS-1RS4
Tube adapter	Swagelok	SS-4-TA-1-2
Pipe lubricant	E.F. Houghoton & Co.	71323998
Instant Galvanize Zinc Coating	CRC	78254184128