Laboratory construction of a novel elastic self-sealing cement slurry

The elastic self-sealing cement slurry is a novel slurry system, which is capable of facilitating self-repairs of cement sheath in the extreme wellbore conditions, but it involves seldom approached. In this work, a novel elastic self-sealing cement slurry is designed and preliminarily established, by means of screening the functional additives. The main compositions of elastic self-sealing cement slurry are determined, which will provide important information for the development of high performance self-sealing cement slurry systems.


Introduction
The main objective of oilwell cement is to offer zonal isolation and well integrity, and to protect the casing and tubing. In most cases, conventional cement systems can meet these requirments; however, coventional cement is not always the effective way for more challenging reservoirs. For example, during the productive phase of the wellbore, set cement behind casing are subjected to stressed due to pressure and temperature changes in the wellbore. Not surprising, the cement sheath may develop cracks internally or debond at the casing or the wellbore creating flow pathways for unwanted fluid migration. Therefore, extreme conditions demand the development of specific cementing technologies.
Elastic self-sealing cement slurry is one of novel slurry systems, which introduces a self-repairing cementitious materials into compositions, allowing the cement to set, wherein upon a loss of structural integrity the set cement self-seals. As a result, the potential pathways of fluid migration in cement stones can be obstructed. Cavanagh et al [1] showed that for a self-sealing cement slurry, self-repair materials are fundamental to contribute to the self-sealing property. On the basis of such design, Dry et al [2] has successfully prepared an oil & gas activated self-sealing cement sheath, wherein a key swellable material is added to perform seal-repair by cold flow.
Previous studies disclosed that the elastometic materials are one of crucial compostions to eatablish elastic self-sealing cement slurry system. The elastomeric material may be semi-crystalline, amorphous, partically amorphous, or the combinations described above [3][4][5]. In spite of various investigation on elastic self-sealing cement slurry system, the previous achievements appear to be limited in constructing such novel cement slurry. It is thus necessary to understand the design process of elastic self-sealing cement slurry, which will provide key informaiton on development of high performance cement slurry suitable for different foramtions. As an ongoing study on high performance cement slurry, in the present work, a latex cement slurry with elastic self-sealing property has been specially constrcuted, and the main compostions have been explored and analyzed in combination to the property requirements of cement slurry.

Laboratory tests
To determine the appropriate compositions of elastic self-sealing cement slurry system, the main additives linked with applied performance have been selected by means of laboratory tests. Previous studied showed that together with the elastomer, filtration reducer, retarder, dispersant, and toughener are indispensible to construct the elastic self-sealing cement slurry. So the basic fomula of the elastic self-sealing cemeng slurry is: G class cement + 60wt.% fresh water + 30wt.% NaCl + 5wt.% filtration reducer + (0.3~1)wt.% dispersant + 0.5wt.% retarder + toughening agent + elastic material. The detailed type of addtives will be investiated and defined.
Typical applied performance of cement slurry such as rheology, sedimental staiblity, filtrate loss, compressive strength, and thickening property have been systemaically tested to design the elastic selfsealing cement slurry. Rheological data of the designed cement slurry were eamined at 300, 200, 100, 6, and 3rpm using a Fann 35 viscometer. Filtration tests were performed at 60℃ and 7MPa by a filter tester, wherein the testing temperature was defined in terms of the wellbore environments. The thickening data of cement slurry were measured under the designed conditions with a thickening apparatus.

Selection of filtration reducer
Filtration reducer is an important composition for the elastic self-sealing cement slurry system, which has a positive effect in controlling the solution loss. In order to determined effective type of filtration reducer, a set of additives involving CG88L, CG82L, CG81L, SY626L, and SY616L have been examined and the results are given in Table 1.  Table 1 tabulates the filtration loss, Φ300 dial readings, Δρ of the elastic self-sealing cement slurry with different filtration reducers. It can be easily observed from Table 1 that, the values of filtrate loss are in the range of 26~80ml, and CG88L has the minimum value, indicating the best effect of filtration loss control. Meanwhile, in the considered series, SY626L and SY616L, no testing value are obtained, suggesting much bad rheological property for the slurry systems. While for CG81L, CG81L, and CG88L, these specimen exhibit comparable rheoligy control capability, and Φ300 readings vary from 233 to 243. A further comparison of Δρ discloses that CG88L and SY626L has the strongest sedimental stability, whose Δρ is as low as 0.01 g/cm 3 . Therefore, CG88L are recommended as the filtration reducer on the basis of the comparison of performance data.

Selection of retarder
Thickening performance of cement slurry is of importance to ensure the pumping capacity upon cementing, and such performance is generally controlled by the retarder. Herein, several types of  The obtained thickening data and the related description are summarized in Table 2. According to the data in Table 2, one can find that in the considered dosages, all of the thickening times are increased with exception of DPSC. In the referred systems, H21L-M, CTR, HPAA, and MPAA exhibit better thickening capability, and the thickening times exceed 260min. Moreover, analysis on thickening curve shows that the cement slurry with retarder H21L-M has a right-angle-set, meaning a short liquid-solid conversion time, which is able to meet the application requirments. Hence H21L-M is selected as the retarder for the designed cement slurry.

Selection of dispersant
Dispersant is also fundamental to the cement slurry, which can adjust the rheolgical property by weakening the chemical association of cement particles. Herein 6 dispersants have been evaluated by examining the rheological data, filtration loss, and gel strength. The charateristic parameters of Φ300 readings, filtration loss, gel strength values of the investigated sytems are collected in Table 3. In the investigated specimen, CF series present suitable Φ300 readings, wherein CF44L has the strongest filtration loss propety than others. In addition, its initial consistency is equal to 18 BC, which means the proper rheological property for the designed cement slurry. A further comparison of gel strength tested shows that for the cement slurry with CF44L, the gel strength values are tested to be 7Pa and 25Pa at 10s and 10min, respectively, which are less than those tested in other systems, suggesting excellent pumping property. In combination of rheology and filration loss, CF44L are finally recommended as the despersant for the studied cement slurry.

Evaluaton on toughening agent
Toughening agent is paramount to the comprehensive property of the cement slurry, which is, to some degree, able to determine, together with toughening capability, the rheological and filtration loss peroperty. Previous studies showed that latex can perform well as toughener and, thus, latex is selected as toughener for the construction of elastic self-sealing cement slurry system [6,7]. As the unnique liquuis toughener, latex functions are summarized as follows:  Reducing the permeability of cement sheath;  Improving the densify conversion performance of cement slurry;  Reducing the elastic modulus of cementing bond;  Improving the cementing quality between cement and pipe column, cement and formation interface, and preventing oil, gas and water channeling;  Reducing the filtration of cement slurry.

Evaluation on elastometic agentss
Elastometic agent is the most key composition in the elastic self-sealing cement slurry, which contributes mainly to the self-sealing behavior. In view of the early results, an elastometic material, RES, has been selected as the elastometic agent to construct the cement slurry system.  Figure 1 presents the recovery experiments of RES under a certain compression. It can be readily found that within the condition of strong compression and deformation, RES can recover the initial shape without cracks and damage, verifying its strong elasticity, which can be helpful for curing the potential damage of cement sheath.  Figure 2 futher shows the RES microstructures before and after recovery tests. As can be seen in Figure 2, the surface of RES before recovery test appears to be very rough, and fine gel particles can be observed in the surface. After the recovery test, however, the RES surface becomes more compact and smooth than that before tests. Apparently, the compression load can has a large impact on the internal structure of RES material, which can facilitate the redistribution of gel particles in the material. It can

Conclusions
In the present work, a novel elastic self-sealing cement slurry has been preliminarily established by a suite of laboratory tests. The main functional additives such as elastomer, filtration reducer, retarder, dispersant, and toughener have been determined by examining the rheolgical, filtration loss, and microstructure. These conclusions should be instructive to further design and develop high performance cement slurry.