Unlocking Shallow Reserves by Improving Zonal Isolation in Unconsolidated Sands: A Successful Approach to Shallow Gas Development

Abstract

Shallow gas development in the Triton Field has historically been limited by recurring zonal isolation failures at shallow depth, often resulting in Sustained Casing Pressure (SCP), stemming from severe hole washouts in unconsolidated sand formations that weaken cement bonding and jeopardize well integrity. As the field operator transitions toward developing both deep and shallow reserves within a single wellbore to reduce overall cost and environmental impact, achieving reliable zonal isolation at shallow depth becomes essential. A cross-disciplinary technical workflow was established to investigate and quantify the root cause of shallow isolation failures, confirming that excessive washout during top-hole drilling significantly deteriorated cement bonding capability. To mitigate this, drilling flow rate and Rate of Penetration (ROP) were carefully maintained to minimize formation erosion, while a high-density cement slurry system and updated job design practices including controlled flow regime were implemented to enhance cement bonding in enlarged boreholes. Additional measures such as deployment of turbolizers and reduced pump rate in the washout zone were introduced to improve mud removal and optimize cement placement efficiency. Post-job evaluations across four wells (TNT-B1, TNT-B2, TNT-B4, and TNT-A3) demonstrated a substantial improvement in cement bonding at shallow intervals compared to three pre-innovation reference wells (TNT-A1, TNT-A2, and TNT-F1). Prior to the innovation, Cement Bond Log (CBL) values in the shallow section consistently ranged from 20 to 80 mV, indicating poor-to-marginal bonding from the casing shoe to depths of 270 to 450 m. Following implementation of the integrated approach, three of the four post-innovation wells exhibited low CBL values below 20 mV beginning at approximately 135 m measured depth (MD), with TNT-A3 achieving excellent values below 10 mV almost immediately below the conductor pipe shoe. TNT-B4 demonstrated a partial improvement, with medium-to-high CBL values above 315 m attributed to excessive hole enlargement in that interval, consistent with playback analysis indicating greater slurry losses. Cement bond logs consistently confirmed enhanced zonal isolation, enabling safe perforation of shallow gas zones without inducing SCP or compromising well integrity. The novelty of this work lies in the integration of drilling optimization, engineered high-density slurry design, controlled flow regime, and mechanical displacement enhancement to directly address the root cause of zonal isolation failures, transforming previously uneconomic shallow gas targets into reliably producible resources. The field-proven methodology can be adopted in similar shallow unconsolidated environments, offering substantial technical, operational, and economic value.