International Journal of Petroleum and Geoscience Engineering Volume 05, Issue 01, Pages 1-7, 2017

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1 International Journal of Petroleum and Geoscience Engineering Volume 05, Issue 01, Pages ISSN: Effect of on Cement-formation Bond Using Simulated Wellbore Mtaki Thomas Maagi * Faculty of Earth Resources, China University of Geosciences, Wuhan, , China. * Corresponding author. address: mtaki_maagi@yahoo.com A b s t r a c t Keywords: Cement-formation interface, bond strength, Drilling mud,,. Accepted: 15 Mar 2017 The aim of this experimental study is to investigate the impact of mudcake buildup on wellbore shear bond strength at cement-formation interface, due to mudcake contamination on formation face. prevents strong bond between cement and formation, becoming the primary reason for cement failure in most wells. In this study simulated wellbores with cement bonded with them to simulate cement-formation interface was used. bond strength test was performed when mud was dehydrated on the formation wall with mudcake thickness of 0.7mm and 1.0mm. The shear strength test was also carried out without mudcake, the effect of mud contamination was observed and the difference in shear strength was evaluated. Observations show that mudcake formation affects bond strength. But the effect was found minimum incase of thin mudcake layer (0.7mm). The study observed the increase in shear bond strength by 70.59%, 35.72%, 46.03% and 38.03% when mudcake thickness was reduced from 1.0mm to 0.7mm indicating that when mudcake thickness increases shear bond strength decreases. The results also show that bond strength were found maximum when drilling mud was not involved 2.58, 2.67, 2.89 and 2.95 MPa, than when conventional mud was employed indicating that high cement bond can only be achieved against the unaltered formation. Successful removal of mudcake from the formation wall before injecting cement slurry is vital to ensure effective cementing job and hence improved zonal isolation. Academic Research Online Publisher. All rights reserved. 1. Introduction Although well cementing job in petroleum industry has been developed over the years but its key functions have not changed. The main functions of cementing are to provide zonal isolation, acting as structural support for the casing as well as protecting the casing from attack by corrosive fluids [1]. Lack of success of the cement to provide these functions can result into different problems including; contamination of ground water, hydrocarbon reserves loss in producing wells, loss of injected fluids in injector wells and sustained casing pressure [2]. During drilling mud may change the properties of the formation and can contaminate the cement slurry as well. The filtrate from the drilling fluid invades the formation, leaving solid particles attached to the formation wall resulting into the formation of mudcake at cement-formation interface. This mudcake does not allow cement to form a strong bond at the interface, hence creates the weakest point for cement failure to occur [3]. Therefore drilling fluid must be 1 P a g e

2 removed properly before injecting the cement to fill the annulus. bond strength at cement-formation interface refers to the force required to initiate movement of the cement from the formation or movement of the casing in the cement sheath [4]. This bonding of cement to the interfaces is what normally determines whether there will be fluid communication in the annulus or not. In order to prevent fluid pathways along the interfaces in the well, strong bond is required between cement and formation. However, large number of wells in the world experience leak associated problems particularly at cement-formation interface. In 2014 it was reported that over 18,000 of the wells only in Alberta Canada experienced leak associated issue and over 8,000 wells in the Gulf of Mexico exhibited sustained casing pressure due to gas migration [2, 4, 5]. These problems indicate that fluid channeling occurs at cement-formation interface. Therefore, knowing the nature of contact between cement and formation can be helpful in improving shear bond strength at the interface. Several studies related to shear bond strength at cement-formation interface have already done. Evans and Carter (1962) studied shear bond strength and hydraulic bond strength by treating the rock samples with different drilling mud and cementing them with different cement slurries. Their findings related to CFI was as follows; zonal isolation can be achieved when drilling mud is removed from the borehole, presence of mud cake on cores minimizes the strength of the bond while effective bond strength was obtained when mud cake was removed and the intimate contact of slurry with the formation determines the bond [3, 6]. Ladva et al (2004) observed the bond strength at shale-cement interface and made the following conclusions; when the cement is placed against a mud filter cake, the failure plane is within the mud cake and gas flows through that path. Improved bonding was obtained with filter cake from water based mud compared to oil based filter cakes and cement bond strength was found higher in case of dry samples [3, 7]. The two research teams both agreed that mud cake formation decreases the bonding strength of the CFI. The objective of this study is to investigate the impact of mudcake buildup on wellbore shear bond strength at CFI, occurring due to mud cake contamination on formation face. This experimental work makes the use of simulated wellbores (SWB) to study the cement-formation bonding. Bond strength was observed on samples with mudcake against sample without mudcake. Fig. 1: A schematic diagram of wellbore cementformation interfacial bonding. 2. Experimental Procedures 2.1. Materials Materials used in this experimental study include; water based mud (WBM) from Yumen oilfield with density 1.39g/cm 3, quartz sand, polyvinyl chloride 2 P a g e

3 (PVC) pipe and tape water of Wuhan City quality. The cement used was Portland G cement from Gezhouba cement plant. The slurry formulation used is 100 wt.% G Class oil well cement, 44 wt.% tap water, 1.2 wt.% fluid loss agent and 0.5 wt.% dispersant Sample Preparation Simulated wellbore (SWB) was molded by mixing quartz sand, cement and water in TG-3060 Constant speed blender. The mixed material was then poured into a PVC pipe placed on the frame of the YES-300-b Pressure display machine. A simulated borehole was made by pressing the slurry contained in PVC with a steel cylindrical indenter with a pressure of 5KN, and maintained for 3 minutes. The sample was left to dry for 24 hours before taken to the furnace box to get heated. After being heated the PVC pipe became swollen and peeled off to obtain the entire SWB. In this experiment the height of SWB was set to be 5.5 cm while the borehole diameter was set to be 3.3cm and its permeability was 460x10-3 µm 2. About 24 samples were prepared for this study, 8 samples tested the cement bond strength with the absence of mudcake (without drilling mud) while 16 samples tested the bond strength with the presence of mudcake. In order to simulate borehole mudcake the SWB was sealed on one side using glass plate with butter. Mud was injected into the borehole until its volume was fully and maintained for 4 hours before poured out. A layer of mud cake was found attached on the borehole wall. Using a glass rod, the formed cake was scraped on the borehole wall to obtain the required thickness set for this study which is 0.7mm and 1.0mm. The cement paste was then properly mixed in a blender at water-cement ratio of 0.44 and poured into the borehole to create a cement bond. 1.2% fluid loss agent and 0.5% dispersant were also added to the slurry to ensure it has good consistency. The mixing procedure was performed according to API specification 10B. The samples were taken into a curing device for a certain period. The curing temperature was set to be 90 while curing was 2, 7, 15 and 30 days. Fig. 2: Simulated wellbore preparation. Fig. 3: A sample after being cured Testing Force and Calculating After taking the samples out of water bath curing device and cooled at room temperature, shear force was tested using WDW - Y10A electronic universal testing machine. Push out testing of the cement-formation bond was done with downward moving steel cylinder. When the force on cement paste reached a certain value, the cement-formation interface was destroyed. Carefully shear force was 3 P a g e

4 recorded, sample height was measured and then the Table 1: Summary of the experimental setup. shear bond strength at cement-formation interface was calculated by dividing shear force by the interfacial area. 3. Results and discussion Table 2 and Table 3 show shear strength results for Fig. 4: A schematic diagram for shear force testing at cement-formation interface. strength at cement-formation interface (P) is calculated based on the force (F), sample cement-formation interface when mudcake thickness was 1.0mm and 0.7mm respectively, while Table 4 shows percentage increase in shear strength between the two tests. thickness (h) and borehole diameter (D). The bond strength calculation formula is as follows: 10F P hd (1)[13] Where; Table 2: strength test results for cement-formation interface with mudcake 1.0mm. Experimental conditions Average P - The shear strength at cement-formation interface, unit MPa. temperature F - force, unit KN h - Sample height, unit cm thickness 1.0mm D Borehole diameter, 3.3 cm. The 10 in 0.54 equation (1) is a result of unit conversion P a g e

5 Table 3: strength test results for cement-formation interface with mudcake 0.7mm. Experimental conditions temperature 90 thickness 0.7mm Average and 38.03% when mudcake thickness was reduced from 1.0mm to 0.7mm. This indicates that thick mudcake layer affects the bond strength at cementformation interface more than thin mudcake layer. Similar experiment was conducted to observe the effect of mudcake buildup on wellbore shear strength at cement-formation interface without involving drilling mud. Only dry samples were used in this case. Table 5 shows the results obtained when the experiment was performed in the absence of mudcake while Table 6 shows shear strength results comparison for 1.0mm and 0.7mm mudcake thickness and when drilling mud was not involved. Table 4: strength increase in percentage at cementformation interface for 1.0mm and 0.7mm mudcake thickness test results. 1.0mm 0.7mm Percentage increase in (%) Table 5: strength test results for cement-formation interface without drilling mud. Experimental conditions temperature 90 Without mud Average The shear bond strength growth rate shows a significant increasing phenomenon as the curing increases in both cases. Under the same curing, the shear bond strength values obtained when mudcake thickness was 0.7mm are higher than the values obtained when mudcake thickness was 1.00mm As shown in Table 4 there is an increase in shear bond strength values by 70.79%, 35.72%, 46.03% 5 P a g e

6 Table 6: strength results comparison at cementformation interface for 1.0mm and 0.7mm mudcake thickness and when drilling mud was not involved mm mud cake mm Mud cake without mud As shown in Table 6, the values of shear bond strength are higher when drilling fluid was not employed in the experiment than when drilling fluid was used with mudcake thickness of 1.0mm and 0.7mm which shows that the existence of mud cake affects the bond strength at the cementformation interface. It was also observed that the values of bond strength are higher for the case of 0.7mm than 1.0mm cake thickness indicating that as mudcake thickness increases shear bond strength at cement-formation interface decreases. Visual observation of the interface was done after the shear strength test by studying the fractured samples as shown in Figure Conclusions From this study the following conclusions can be made; i. buildup on formation wall prevents strong bond between cement and formation, becoming one among the primary reasons for cement failure to occur. But this effect was found minimum incase of thin mud cake (0.7mm). ii. The study observed the increase in strength by 70.59%, 35.72%, 46.03% and 38.03% when mudcake thickness was reduced from 1.00mm to 0.7mm. This indicates that when mudcake thickness increases shear bond strength decreases. In case of thick mudcake layer, spacers and preflush additives have to be used to thin mudcake and facilitate solidification at cement-formation interface leading into improved shear strength. iii. The maximum shear bond strength was found in dry samples when drilling fluid was not involved, 2.58, 2.67, 2.89 and 2.95 MPa indicating that high cement bonding can only be achieved against the unaltered formation. Successful removal of mudcake from the formation wall before injecting cement slurry is vital to ensure effective cementing job. iv. The shear bond strength increases with the increase of curing, this indicates that hardness of the cement paste increases with. v. Effective long term zonal isolation depends on successful removal of drilling fluid from the wellbore before cementing job. Fig. 5: Photograph of the broken samples after shear bond testing, (a) the formation face has no mud cake (b) there is mudcake buildup at the interface. Acknowledgments I extend my appreciation to the Department of Petroleum Engineering, CUG for funding this experimental study. I am also thankful to Prof. Gu 6 P a g e

7 Jun for his advice, support and encouragement during this project. Finally, I am grateful to my Lab mates. They always offered support and encouragement during this study. Thank you so much. References [1] Brandl, A., Cutler, J., Seholm, A., Sansil, M., and Braun, G., Cementing Solutions for Corrosive Well Environments, SPE , SPE Drilling & Completion, 2: , [2] Agbasimalo, N.C., Experimental Study of the Effect of Drilling Fluid Contamination on the Integrity of Cement-formation Interface, MSc Thesis, Department of Petroleum Engineering, Louisiana State University, USA, [3] Mushtaq, W., Experimental Study of Cement- Formation Bonding, MSc Thesis, Department of Petroleum Engineering and Applied Geophysics, Norwegian University of Science and Technology, Norway, [4] Radonjic, M., and Oyibo, A., Experimental Evaluation of Wellbore Cement- Formation Bond in Presence of Drilling Fluid Contamination, 5th International Conference on Porous Media and Their Applications in Science, Engineering and Industry, Eds, ECI Symposium Series, [5] Annular Casing Pressure Management for Offshore wells, A Rule by the Minerals Pressure Management, Federal Register, Page 23582, [6] Evans, G.W., and Carter, L., Gregory, Bonding Studies of Cementing Compositions to Pipe and Formations, API Drilling and Production Practice, 72, [7] Ladva, H.K.J., Craster, B., Jones, T.G.J., Goldsmith, G., Scott, D., The Cement-to-Formation Interface in Zonal Isolation. IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition; Kuala Lumpur, Malaysia, September [8] Khandka, R.K., Leakage behind Casing. TPG 4920 Drilling Specialization Master Thesis Work, NTNU, Norway, June [9] Parcevaux, P.A., and Sault, P.H., Cement shrinkage and elasticity: a new approach for a good zonal isolation, SPE Annual Technical Conference and Exhibition. Society of Petroleum Engineers, [10] Peterson, B., Bond of cement compositions for cementing wells, 6th World Petroleum Congress, [11] Huang, H.F., Huan. B.U., Tian, H., Wang, R.H., Bonding strength experiment of mud to cement fluid on the second interface. Journal of China University of Petroleum (Natural Science Edition), 30: 46-50, [12] El-Sayed, A.A.H., Effect of Drilling Mud Contamination on Cement Slurry Properties, Journal of King AbdulAziz University (JKAU), Fourth Saudi Engineering Conference, held in Jeddah, , [13] Jun, G., Lai, P., Huang, J., Sun, D., He, J., Wen, G., and et al, Evaluation and Application of Blast Furnace Slag as Modifier, China University of Geosciences, [14] Nelson, E.B., Well Cementing, a book produced by Schlumberger Educational Services, Houston, Texas, [15] Yong, M., Rong, C.M., Yang, G., Qing, S., Li, L., How to Evaluate the Effect of Mud Cake on Cement Bond Quality of Second Interface?, Proceedings- SPE/IADC Middle East Drilling and Technology Conference, Cairo, Egypt. October 22-24, [16] API RP 10B, Recommended Practice for Testing Well Cements, 22nd edition, Washington, DC, API, P a g e