De-risking Shallow Hazards through Pore Pressure Prediction and Special Studies — A Case Study from KG-Offshore Basin

Abstract

Shallow hazards in drilling are caused due to water flow from an over pressured shallow sand encountered at shallow depths below the mudline in deepwater regions of the world. High sedimentation rates and an impermeable mud or clay seal from a condensed section are the main factors contributing to overpressures in shallow water flow sands. If isolated sand bodies are in this shale or mudstone, water from such bodies will not escape easily due to the presence of low-permeability sediments around them. In addition, the high rate of sedimentation from the overburden exerts an enormous pressure on these sediments, causing these isolated bodies containing large amounts of water to be over pressured. The shallow hazards can cause extensive damage to the borehole and pose a great threat to drilling. Their identification/delineation prior to drilling is therefore of utmost importance in reducing risk. The study area has a highly promising Miocene roll structure beneath the shallow hazard zones which warrants probing. Negotiating to this structure through hazardous shallow anomaly already encountered during drilling necessitated for detailed shallow hazard analysis. The area has very limited well data. This limitation in the input data prompted the authors to look for suitable methods to address the issue. An innovative approach to generate pseudo logs from freshly picked seismic velocities was adopted. The pseudo logs were used for carrying out post stack model based inversion. This approach could overcome the limitations of non availability of well data significantly and the generated impedance volume was of optimum quality. The impedance volume could delineate the zones of shallow hazards. The volume was calibrated at the well location indicating low impedance at high pressure zones. This paper describes this innovative/unconventional approach adopted for delineation of these shallow hazards.