Successful Implementation of Improved Rheological Hierarchy of Mud - Spacer - Cement for Effective Zonal Isolation - A Case Study
Authors: Singh Parvinder , Lal Kishori , Rastogi Ravi , Joshi C. S. , Sinha M P 
Published: April 2017

This paper provides a review of some of the best practices and case study of Agartala located in the northeast end of Indian continent at Tripura Asset and characterized by high pressure well. It elaborates on the design, execution and evaluation of the rheological hierarchy of mud, spacer and cement slurry to improve well integrity. Optimizing the density hierarchy for wellbore fluids has been a routine while achieving a proper rheological hierarchy have been compromised due to tedious testing and sometimes limitations in the field. Establishing appropriate rheological and friction pressure hierarchy prevent fluids (mud-spacer-cement slurry) intermixing especially in deviated and horizontal wells.

The selection of proper spacer and designing formulations with chemical compatibility with drilling fluid and cement slurry is very crucial and challenging. For better mud displacement the down hole forces imposed by the circulating fluids in well have to be sufficient to overcome the yield stress of any vicosified or partially dehydrated drilling fluids in hole. Weighted spacers are between the chemical means buoyancy effect on mud removal. The volume, rate and viscosity of spacer must be sufficient and carefully designed to prevent intermixing.

Rheological modeling can be determined by using fluid friction chart and cementing software. It is not always possible to accomplish the turbulent flow. Therefore, a rheological model was developed to accomplish the ideal viscosity hierarchy by optimizing the spacer formulation design. Optimum rheological hierarchy occurs where the viscosity profile of a spacer system is higher than the viscosity profile of drilling fluid and lower than the cement slurry.

In order to achieve this, An extensive laboratory testing was performed for compatible rheological modeling with mud-spacer-cement slurry (at surface temperature 27 Deg C & 80 Deg C) on rotational viscometer as per the procedure of API RP 10B-2. The water-based weighted spacer system, with density 1.85 g/cc was modeled to temperatures up to 80 Deg C and provided proper suspension properties, plays a significant role in achieving great displacement efficiency, wellbore clean up, effective zonal isolation. The volumetric proportions of the cement slurry/spacer and spacer/mud admixtures were prepared withvarious ratio: 95/5, 75/25, 50/50, 25/75, and 5/95. Rheological compatibility of fluids (cement & spacer and mud & spacer) assessed by calculating the R-Index Value (R). A mathematical modelling was also developed and applied to predict the rheology at elevated temperature by regression analysis/ trendline to assist on job rheological hierarchy maintenance.

Results obtained from field case study show the improvements in CBL-VDL recorded after cementation showed excellent result (02 to 07 mV) against zone of interest and added values such as ideal fluid compatibility, better displacement efficiency, friction pressure hierarchy and effective zonal isolation.