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Cleaning Functional Spacer for Improving Sealing Integrity and Zonal Isolation of Cement Sheath in Shale Gas Wells: Laboratory Study and Field Application


 Maintaining sealing integrity of cement sheath is essential to oil and gas well completion. The development of large-scale hydraulic fracturing reservoir stimulation technology in recent years has put forward higher requirements for the sealing integrity of cement sheath. For horizontal shale gas wells, it is often necessary to clean the oil-based mud (OBM) before pumping cement slurry to ensure sufficient contact and bonding of cement slurry to casing and formation. Therefore, the removal efficiency of OBM determines the cementation quality of cement slurry with casing and formation.
 The present work implements and evaluates a novel cleaning functional spacer (CFS) to overcome OBM removal difficulty through laboratory experiments. CFS consists of weighted spacer (CFS I) and chemical preflush (CFS II). CFS I and CFS II both contain chemical washing agent. OBM removal efficiency test results suggest that with the increase of the concentration of chemical washing agent, the removal efficiency of OBM increases, and the increase of temperature will promote the removal of OBM. Also, it can be found that OBM that sticks to the casing is easier to remove than OBM that sticks to the simulated permeable formations. The water drop on an oil-wet surface gives a contact angle of 66.15°, as nonwetting behavior, while it is reduced to almost zero with the treatment of CFS II on OBM surface. Electrical stability test results show that the demulsification voltages of OBM and CFS II mixture fluids decreased as the percentage of the chemical preflush increased. CFS II will cause demulsification of OBM, reducing the demulsification voltage of the mixture and thus facilitating its removal. The viscosity measurement results demonstrate high compatibility of CFS I with OBM at different volume ratio combinations, which shows its pumpability is safe in field application. The shear bonding strength test indicates that the bonding strength increased by more than 3,212.8% with the use of CFS. Also tight cementation formed at the casing cement sheath interface (CCI), cement sheath formation interface (CFI), and no microannulus appearance. A case history of its use in a shale gas well in the field demonstrates the successful application of CFS to prevent sustained casing pressure (SCP) and maintain the sealing integrity of cement sheath.