A Novel Approach for Tailored Cement Job Design and Placement to Mitigate Losses
Authors: Jandhyala Siva Rama Krishna , Yerubandi Krishna , Singh John Paul Bir , Morgan Ronnie G , Cuello Jimenez Walmy 
Published: March 2023

Abstract
The current approach to designing a lost circulation treatment for drilling or cementing applications is primarily based on workflows designed by project teams using experience and knowledge from historical wells. However, the effectiveness of this approach depends on targeting operations with well conditions similar to historical jobs and the collective experience of the project team. Consequently, there is less flexibility to apply these workflows globally or to a job with different fluids placement strategies and/or different fluid treatments. In addition, dynamics of the wellbore during losses are influenced by multiple factors such as wellbore geometry, downhole fluid properties, free-fall (during cementing), details of the unknown loss zone geometry, as well as the features of the loss circulation material (LCM) (i.e. material, particle size distribution, density etc.). Thus, there is a need to create a robust and effective workflow based on engineering models that provide a platform to systematically design cement jobs to mitigate losses. Similar workflows may be adapted for controlling losses during drilling.
This work presents a complete coupling of a wellbore hydraulics model with loss zone dynamics including LCM particle bridging and packing at the loss zone. The loss zone dynamics use circulating pressures from the hydraulic model, estimated geometry of the loss zone and characteristics of LCM laden fluids to determine the loss rate and the filter cake buildup. As fluid systems with different densities, rheology and LCM packages enter the loss zone during circulating and/or cementing, the extent of losses is governed by solving for a coupled problem of flow through (i) the wellbore and (ii) the loss zone with filter cake buildup. The first step in the analysis is to characterize the loss zone by calibrating the hydraulics model to match circulation pressures and loss rates observed prior to the addition of any LCM material in the wellbore fluids such as pre-job circulation before cementing.
The integrated model described in this study was successfully verified and validated based on multiple field cases. These cases covered three types of loss zones – induced, natural fractures, and permeable zones. Pre-job circulation loss rates ranged from 30 to 240 bbl/hr. LCM designs include single and composite systems with multi-modal particle size distributions. LCMs were loaded in a wide range of fluid systems involving, but not limited to, spacer, lead and tail cement slurries. In all cases, the predictions from the model matched operational loss control within an accuracy of 10%. Furthermore, post job evaluations indicated that real time pumping pressures aligned with model predictions, indicating that the hydraulic response of the model matched that of the wellbore.
The evidence gathered during the verification process proves the ability of the tool to tailor and design cement jobs to mitigate losses. Additionally, due to the integrated framework, the novel tool allows for tailoring of multiple operationally relevant variables such as flow rate, density, volume, rheology, and fluid recipe including LCM type and concentration across different loss zones. The framework also provides an estimated top of cement (TOC) which is one of the most important cement job objectives.