Optimization and application of KCl polymer drilling fluid balancing wellbore stability and logging response accuracy

In Dagang Oilfield in China, the utilization of the KCl polymer water-based drilling fluid (WBDF) in mid-deep exploration/appraisal wells presents a challenge in simultaneously optimizing resistivity logging accuracy and wellbore stability. To address this, it is necessary to conduct geology-engineering integration studies. Based on the formation resistivity, an analytical model was developed to assess the impact of KCl concentration in the WBDF on array induction logging response accuracy. The maximum permissible KCl concentration for the target formations was determined, and technical strategies were proposed to maintain wellbore stability at a reduced KCl concentration. After that, considering the inhibitory, encapsulating, and plugging effects, a low-KCl-concentration WBDF was optimized and applied. Model calculations demonstrate that increasing KCl concentration in the WBDF decreases resistivity, thereby reducing logging accuracy. To maintain a logging accuracy of ≥ 80%, the upper limits for KCl concentration in the WBDF are 4.8%, 4.2%, and 3.6% for the 3rd Member of the Dongying Formation, the 1st and 2nd members of the Shahejie Formation, respectively. Cuttings recovery experiments revealed that a minimum KCl concentration of 3% is required to ensure basic shale inhibition. A combination of 3% KCl with 1% polyamine inhibitor yielded cuttings recovery and shale stability index comparable to those achieved with 7% KCl alone, and the shale inhibition performance was further enhanced with the addition of an encapsulator. The optimized WBDF has been successfully deployed in exploration/appraisal wells across multiple blocks within Dagang Oilfield, resulting in superior wellbore stability during operations. Furthermore, the electric logging interpretation coincidence rate improved from 68.1% to 89.9%, providing robust technical support for high-quality drilling and accurate reservoir evaluation in exploration/appraisal wells.