Pre-Drilling Pore Pressure Prediction Technique Based on High-Quality OBN Seismic Velocity and its Application in K Oilfield

Abstract

Abstract Pre-drilling pore pressure prediction based on seismic velocity is a critical step in the oil and gas industry to ensure drilling safety and optimize well construction. However, due to the limit resolution of the seismic data, the pore pressure prediction result is not good enough for well drilling. With the increasing quality of new seismic data, this study proposed a new pre-drilling pore pressure prediction technique based on the high quality OBN seismic velocity. This new technique consists 6 steps: ①Abnormal high pore pressure identification based on the pore pressure test, mud weight, well logging velocity and seismic velocity. ②Mechanism analysis based on the lithology, sequence stratigraphic and tectonic movement. ③Feasibility analysis based on the comparing the OBN seismic velocity with abnormal high-pressure interval. ④Geological consistency OBN seismic velocity optimization, based on high-quality logging velocity. ⑤Pore pressure prediction based on the petrophysical analysis and pressure prediction parameters optimization. This new technique has been successfully applied in the K oilfield in Central Asia. Integrated pressure testing data, mud weight, and seismic velocity can clearly identify high pore pressure in middle to deep formation of the study area. The abnormal pressure is formed due to under-compaction, where the mudstone of the maximum flooding surface acts as a barrier to the outflow of pore fluids in the deep layers, resulting in abnormally high pore pressure. Abnormally high pressure can be clearly identified by the OBN seismic velocity spectrum, with different pressure gradient intervals having different OBN seismic velocity gradients. Geological consistency velocity optimization methods can enhance the accuracy of OBN seismic velocity interpretation and more matched with well log velocity, thereby improving pressure prediction accuracy. Modified Fillippone method was used for pore pressure prediction, and more accurate Vmax and Vmin can be obtained through rock physics analysis, further enhancing pressure prediction accuracy. From the initial seismic velocity spectrum and pressure test results from nearby drilled wells QC, the results were matched with pore pressure prediction results, with a high correlation (up to 92%). The new pre-drilling pore pressure technique integrated the advantages of pore pressure testing, high resolution of well log velocity, and high-quality of OBN seismic velocity. It enhances the accuracy of OBN seismic velocities through geological consistency seismic velocity optimization and improves the accuracy of pressure prediction parameters (Vmax and Vmin) through rock physics analysis, and ultimately improving the accuracy of pore pressure prediction. This method is applicable to both clastic layers and carbonate layers.