A Unique High Temperature, Low Rheology Divalent Brine-Based Reservoir Drilling Fluid Improved Horizontal Drilling Performance in Hard Sandstone Formation

Abstract

This paper describes how a dual function synthetic polymer (DFSP) additive was key to formulating a unique high temperature divalent brine-based reservoir drilling fluid (HT-DB-RDF), deployed to drill a horizontal slim hole interval through a hard sandstone formation. To efficiently drill this horizontal section the HT-DB-RDF was developed with the intent to improve drilling performance. The main parameter targeted for improvement was the rate of penetration (ROP) without compromising wellbore quality or increasing the risk of common drilling problems such as hole cleaning, wellbore integrity, drilling torque and differential sticking. During the well planning phase, different fluid formulations, qualification approaches and field deployments were reviewed from global experience. In certain geologically similar applications, improved ROPs had been achieved using low solids and (in some instances) clear fluid systems. The most notable difference between the planned application and offset experience was the bottom hole temperature (155°C) and fluid density that was required (1.32 SG). A fluids design framework was developed from the offset well analysis and available pre-drill data. Reservoir Drilling Fluid technologies were evaluated, and laboratory work was performed to develop the system. Detailed hydraulics modelling was performed during the planning phase to confirm that the HT-DB-RDF could deliver the required performance under the planned conditions. High flow rates and low rheology were required for turbulent flow in the annulus to transport cuttings, maximizing hydraulic power to the downhole tools and drill bit and enhancing ROP. The success of the HT-DB-RDF was assessed during execution of the well. Downhole fluid properties were used to model drill string hydraulics, equivalent circulating density (ECD) and hole cleaning ahead of the bit under the drilling conditions. Outcomes were compared to pressure-while-drilling data at the rig site to detect trends indicating poor hole cleaning. The results showed a maximum 0.01 SG variance between modelled and measured ECD, and smooth trips illustrated a clean and stable wellbore throughout. Non-fluid related operational issues resulted in a longer time to drill the interval and extended static periods during trips. The HT-DB-RDF exhibited resilience during elevated downhole temperatures. The average ROP recorded during drilling was eight times higher than the previous ROP achieved drilling horizontal offset wells. The key technology that enabled fluid development and execution of the well was the DFSP that delivered both high temperature fluid loss control and hole cleaning capabilities. The DFSP features strongly bonded chemical linkages and cross-linked chains which achieve exceptional performance in divalent brine, facilitating high fluid densities with low solids content to maximize performance.