Anomalous Pore Pressure and its Relation with In-Situ Stress Regime in Deepwater Play

Abstract

Abstract Drilling is more challenging when the pore pressure is anomalous because it is higher or lower than the normal hydrostatic pressure for a depth of interest. This study is based on a review of pore-pressure analysis of more than 100 deepwater wells in the Gulf of Mexico (GOM). The intent in reviewing the data was to define what area/depth and conditions create the greatest risk of anomalous pore pressure. Both geological and geomechanical factors, such as salt structure and in-situ stress regime, were considered to determine the underlying cause. Overcompaction from tectonic forces or salt structures was identified as a key factor in generating anomalous pore pressure. By studying the combination of different sets of log measurements, such as gamma ray (GR) sonic travel time (DT), resistivity (RES), and density (RHOB), each anomaly related to pore-pressure distribution was examined in more detail, reviewing the circumstances under which the anomalous pore pressure occurred and, whenever possible, identifying the underlying cause. Through a better understanding of the relationship between pore pressure and in-situ stress, an improved methodology for predrill pore-pressure and fracture-gradient prediction for deepwater wells has been developed. Normalized stress data trends, including both pore pressure and in-situ stresses from different GOM regions, were plotted as a tool to compare the relationship between the pore pressure and in-situ stress regime. The coupling effects between pore pressure and in-situ stress outline best-practice methodology for defining anomalous pore pressure across the GOM. The solution can lead to an effective approach to detecting anomalous pore-pressure distribution, which is particularly important when deepwater drilling because the operating safety margin decreases as the water depth increases, and there is also an urgent need in stimulation to have more knowledge and control of the prediction process for pore pressure.