1. Studies on the Impacts from Different Activators and Inorganic Components of Curing Agents on Performance of Solidifiable Plugging Fluid
Author: Xiaoyang Guo,Zhizhong Deng,Zaoyuan Li
Public Speaker：Zhizhong Deng
Unit’s name:Southwest Petroleum University

2. Contents
Reach Background
Experiment Measured
Conclusion

3. Reach Background The issues are raised in the context of how come ?
To solve the problems occurring during the treatment process in the case of any malignant lost circulation, we studied a type of new solidifiable gel plugging fluid system.
What are the aspects of mutual concern still need improving to handle the lost circulation?
a. Permeable loss
b. Natural fractures loss
The door sealed phenomenon is caused during the addition of the blocking materials to the drilling fluid, when the weak gel fails to significantly improve the stratum bearing capacity, causing the gel plugging agent to be easily eroded; this erosion results in a lack of strength in the later period.
c.induced fractures loss
d.Cavern loss
Fig. 1 the various types of Lost Circulation

4. Relevant theories Focus
The theories are now the “tail sealing theory”, “stress cage” and “fracture closure stress theory”, all of which require the formation of an effective packing zone supported by a certain strength. The engineering demands set specific requirements on the material properties。
Leakage Channel
a. Door Sealed
b. Erosion
C. Form a Packing Zone
Plugging Agent
Wellbore Bore
Formation
Weak Gel or Cement
Solidifiable Plugging Fluid
Focus
Most of the reports focus on the impact of the additives on the cementing material; in contrast, the impact of the percentage change of cementing material components on the rheology and strength is rarely reported.
Fig. 2 Sketch of the Plugging Effect

5. Materials SiO2 Al2O3 Fe2O3 CaO MgO Other 27.15% 19.16% 2.12% 31.37%
Primary components of the solidifier: CaO, SiO2, Al2O3 and MgO, with an average particle size at the surface of μm.
Table 1 Main Solidifier Components and Contents
SiO2
Al2O3
Fe2O3
CaO
MgO
Other
27.15%
19.16%
2.12%
31.37%
8.64%
11.56%
Particle Size (μm)
Surface Area (%)
Fig.3 Distribution of the Particle Size of the Solidifier

6. Hydration Mechanism of the Solidifier
The determinant of the solidifier hydration rate is the activity of the reactionless vitreum on the particle surface.
The vitreum in the solidifier is a kind of microheterogeneous substance and its basic structural unit are the [SiO4]4- and [AlO4]5- tetrahedron composed by network particles, while the Ca2+, Mg2+ and the six-coordinated Al3+ etc. particles that may alter the network will be outside the network chain, however, these particles will also be distributed in the network structure in the form with certain coordination .
In order to activate the activity of the solidifier, the solidifier must be in the solutions of polar molecules or OH- ions; these polar molecules or OH- ions can break down the ionic bond and covalent bond and the solidifier will break up into [SiO4]4- and [AlO4]5- as well as Ca2+, Ca(OH)+ etc. cations; within the solution supersaturated with hydration products dispersed and decomposed from the solidifier particles, the hydration products continuously nucleated and developed, and final products will overlapped between each other to form the jelling network structure.

7. Without adding the bentonite
Impact of the Bentonite on the Base Mud
Bentonite is the major component of drilling fluid. The main purpose of adding bentonite to base mud is to adjust the sedimentation stability, shear thinning behavior, and compatibility to prepare the mud for a stable performance.
Bentonite is an important component of the drilling fluid. While meeting the performance of the plugging fluid, bentonite is sufficiently compatible with the drilling fluid. This compatibility indicates that the performance change of the plugging fluid is not caused by the difference of the components of the solidification products of the plugging fluid, but by the change of the microstructure of the solidified material.
Adding the bentonite
Without adding the bentonite
Fig. 4 XRD Analysis after Adding Bentonite

8. The SEM microscopic analysis of the test samples with bentonite
a. Base Mud without Bentonite
b. Base Mud with Bentonite
Fig5 SEM analysis
The SEM microscopic analysis of the test samples with bentonite indicates that the microstructure of the plugging fluid solidification product without the addition of bentonite (Fig. 5a) is looser and that there are fewer filling materials between the different structures. As a solid phase particle, bentonite is effectively distributed among the different particles (Fig. 5b) to realize close packing, which helps the strength intensification and shear force increase after the solidification of the plugging fluid.

9. Impact of the Bentonite on the Base Mud Rheology
Shear Force Pa
Viscosity mPa.s
Shear Rate S-1
Shear Rate s-1
Fig. 6 Shearing Force at Different Shear Rate
Fig. 7 Effective viscosity
A fraction of the water molecules are adsorbed onto the surface of lattices to reduce the free water.
The end-end structure becomes the end-face structure, which effectively forms the spatial network in the base mud, thus causing the increase of viscosity.
in general, the viscosity increases with the increase of the amount of bentonite, and there is almost no viscosity difference for both no added bentonite and the addition of 1%.

10. Impact of Bentonite on the Suspension Stability
Bentonite can improve the viscosity of the base fluid and realize a stable suspension of the solidifier in the base mud. According to the Stoke Law:
Velocity mm/s
Pressure Gradient Pa/mm
Fig. 8 Pressure Gradient for Different Velocities
Given that the solidifier particle size is taken as µm according to the laser particle size analyzer, the density difference is 1.65 g/cm3 and the viscosity is 30 mPa.s, the settling velocity is calculated as 1×10-3 cm/s according to the Stoke Law, and the time is 990 s for settling or subsiding for 1 cm to occur.

11. Impact of CaO on the Compressive Strength of the Plugging Fluid
Adding the CaO
Without adding the CaO
CaO content %
Strength Mpa
Fig. 9 XRD Analysis after the Addition of CaO
Fig. 10 Compressive Strength for Different CaO Percentages
The compressive strength of the plugging fluid is the highest when the CaO percentage in the solidifier reaches 33%.
After the dissolution of CaO, a saturated Ca(OH)2 solution is rapidly generated on the surface of the solidifier vitreum, which promotes the early strength agent to destroy the vitreum on the solidifier surface to cause the hydration reaction of the activated calcium-rich phase and silicon-rich phase of the solidifiers and to ensure a shorter setting time for the plugging fluid and higher early strength.

12. a. 33% of CaO Percentage in the Solidifier
b. Over 33% of CaO Percentage in the Solidifier
Fig.11 SEM analysis
Various structures are connected by the coarser fabric tissues, and the gaps among the structures are well filled. As a result, the compressive strength of the plugging fluid solidification is higher when the CaO percentage is 33% in the solidifier.
The microstructure of the solidified plugging fluid is looser when the CaO percentage is over 33% in the solidifier (Fig. 11b), and the structures are cross-linked by fine fibrous materials, so its compressive strength is lower.

13. Impact of the MgO Content on the Solidifier Performance
Adding the MgO
Without adding the MgO
MgO Percentage %
Strength MPa
Fig. 13 Compressive Strength for Different Percentages of MgO
Fig. 12 XRD Analysis after the Addition of MgO
The studies indicated that, when MgO is maintained at 50℃, there is no strength until the MgO percentage reached 18% and that the strength is higher than 8.5 MPa. The strength increased with the increase of MgO percentage. When MgO is maintained at 90℃, the strength increased with the increase of the MgO percentage. The maintenance period is prolonged for 24 h, and the strength increases, on average, by 0.85 MPa.

14. MgO Percentage %
Strength MPa
Fig. 15 SEM Analysis Sketch for the Addition of MgO
Fig. 14 Compressive Strength for Different Percentages of MgO (with added CaO)
The microstructure of the solidified plugging fluid is very compact when the percentage of MgO in the active solidifier is 16%, with the gaps among the structures being well filled, and the structures are cross-linked（Fig. 15）. Therefore, the solidified plugging fluid presents a higher compressive strength.
The results indicate that the overall trend of the strength first increases and then decreases with the increase of the MgO percentage (Fig. 14). The plugging fluid strength is the highest when the MgO percentage is in the range of 12 to 16%.

15. Impact of the Al2O3 Percentage on the Solidifier Performance
Strength MPa
AL2O3 Percentage %
Strength MPa
Fig. 16 Compressive Strength for Different Percentages of Al2O3
Fig. 17 Compressive Strength for Different Percentages of Al2O3 (with added CaO)
The results indicated that the strength of the plugging fluid is the highest for no added Al2O3 (Fig. 17). The compressive strength decreased with the increase of the amount of added Al2O3, and the strength degradation is more obvious with the amount of added Al2O3 less than 27%.
There is no strength in the plugging fluid after maintaining Al2O3 for 48 h at 50℃ (Fig. 16), and its strength reaches the maximum value when the percentage of Al2O3 is 27% at 90℃; the strength decreases for percentages of Al2O3 greater than 27%.

16. Impact of the SiO2 Percentage on the Solidifier Performance
Strength MPa
Fig. 18 Compressive Strength for Different Percentages of SiO2 (with added CaO)
SiO2 is an inert substance. SiO2 can realize the effective filling in the components and plays an important role in the strength degradation of the test samples in the later stage. The studies indicate (Fig. 18) that there is no obvious strength change when the amount of added SiO2 is less than 31%; with a further increase above 31%, the strength decreases significantly.

17. Conclusion
no new substance is generated after the addition of bentonite.
The bentonite forms the spatial network structure through hydration, which can effectively change the rheology and sedimentation stability of the plugging fluid.
In the single component test of MgO, the compressive strength is at a maximum when its percentage reaches 26%; in the tests when CaO is added in respective mixtures with MgO, Al2O3 and SiO2 , the compressive strengths are all greater than 3 MPa.
During field construction, the actual working conditions can be used to adjust the flow profile index, viscosity coefficient and solidification strength to meet the requirements of the plugging construction.

18. Thank you!