A 4-D Case Study: Rock Matrix Sensitivity Test on a Field in Deep Water, Nigeria

Abstract

A feasibility study (FS) is often done before planning and acquiring 4D seismic to determine if it will positively impact the economic viability of the field. A 4D FS was done on an AVO Class 2p turbidite deep water field in Nigeria (F1) based on core measurements. The results of the 4D modeling illustrated that changes of the fluid fronts are readily seen but the 4D changes from pressure are hard to see with the current expected change in pressure. More 4D modeling was undertaken with core measurements from analog fields to analyze the potential variance of the 4D signal. This was done by substituting the dry rock frame and acoustic impedance (AI) contrast of analog fields into F1 with saturations and pressures being held constant. The analog fields were chosen based on the closest depth below mud line, depositional environment and AI contrast to F1. Additionally, the analogs were selected based on a nonlinear method of defining the dry rock frame that relates differential pressure (Pd), porosity, dry and bulk modulus. Based on a plot of Pd and velocity (Vp), a low Vp (F2) and high Vp (F3) analog field were chosen. F2 and F3 were selected because they were both deep water fields deposited as turbidites that are AVO Class 3. The 4D modeling with F2 and F3 rock parameters, like F1, confirm that fluid fronts are seen. The main difference between F2 and F3 from F1 is the increased strength of the amplitude differences highlighting the fronts caused by the larger AI contrast of AVO Class 3 rock. Again, the expected change in pressure is not easily interpreted on the modeled 4D seismic. Another suite of 4D modeling was commenced by changing the pore pressure (Pp) to investigate 4D pressure effect. F1's 4D response still saw fluid fronts with some pressure influence. The pressure change dominated F2's 4D response, making it much more difficult to interpret fluid fronts. The fluid fronts on F3's 4D response were still easily interpreted. The change in pressure caused noticeable 4D seismic differences between the three fields. It was found that the AI contrast between the reservoir matrix and surrounding rock is influential to detectable 4D signal. Secondarily, the location within the pressure regime of the rock physics models is important because the relationship between Vp and Pd is not linear. This is why the increase of the Pp influenced the results. Because of this, the choice of analog field and its dry rock frame is an important component of a 4D FS.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015