Research on the Prediction Method of Formation Pore Pressure Based on Ensemble Methods

Abstract

ABSTRACT: Accurately predicting overpressure can effectively guide oil and gas exploration and development. However, the parameterization method is limited by the model form, and the accuracy is low. In this study, a new method to predict pore pressure using ensemble method was proposed. For the first time, acoustic velocity, porosity, mud content, and density were used as input variables and effective stress as output variable, and then accurate prediction of pore pressure was achieved based on the effective stress theorem. Compared with the predictions of traditional models, the performance of the developed models was verified. The results show that compared with the traditional models, the developed method has the advantages of high accuracy, and its absolute average percentage deviation (AAPD), root mean square error (RMSE) are 0.76 % and 0.030 g/cm3, and the coefficient of determination (D2) is 0.992. In addition, the Random Forest (RF) is superior to the Gradient Boosting Machine (GBM) in goodness of fit, generalization and prediction accuracy, which AAPD and RMSE are only 0.13 % and 0.013 g/cm3, and D2 is 0.993. The model developed in this study is not affected by the mechanical depositional environment and is applicable to sandy mudstone formations, such that it can be a useful and highly accurate alternative to the traditional prediction methods with fixed parameter forms. 1. INTRODUCTION Formation pore pressure, refers to the pressure possessed by the fluid (oil, gas, or water) in the empty space of the formation. During the drilling process, if the pore pressure is accurately predicted, complex downhole accidents such as blowouts and drilling fluid leaks will be effectively reduced and drilling costs will be lowered (Zoback, 2007; Tingay et al., 2009). When the pore pressure is less than the hydrostatic pressure, it is called anomalous low pressure; and when the pore pressure is greater than the hydrostatic pressure, it is called anomalous high pressure (Fig. 1). More than 47.7% oil or gas reservoirs are classified as anomalous high-pressure reservoirs around the world. The mechanisms of anomalous high-pressure genesis include, but are not limited to, the effects of disequilibrium compaction, hydrothermal pressurization, clay mineral transformation, permeation, hydrocarbon generation, hydrocarbon cracking, and pressure transfer. Among these mechanisms, disequilibrium compaction is one of the most dominant loading mechanisms for overpressure formation, and fluid expansion (hydrothermal pressurization, hydrocarbon generation, and hydrocarbon cracking are all part of fluid expansion) is one of the most dominant unloading mechanisms (Vernik. 2016).