Updating the Geomechanical Model and Calibrating Pore Pressure From 3D Seismic Using Data from the Gnu-1 Well, Dampier Sub-basin, Australia

Summary

A post-mortem analysis of the Gnu-1 well was conducted to help us to understand drilling experiences in the context of the pore-pressure and stress profiles. The post-mortem involved a review of the drilling experiences and an analysis of CAST image data, wireline-log data, and the logging-while-drilling (LWD) logs. This information was used to refine and verify a geomechanical model (in-situ stress, pore pressure, and rock-mechanical properties) in the vicinity of the Gnu-1 well. Of prime concern was the verification of the predrill pore-pressure prediction previously undertaken using 3D-seismic-velocity data and offset-well data. Wellbore-failure and natural-fracture analyses were integral parts of the post-mortem. Wellbore breakouts seen in the image data allowed the pore pressure in the 8.5-in. hole section of Well Gnu-1 to be constrained. Modeling using image data collected in the Athol formation indicates that the pore pressure does not increase as rapidly as was estimated in the predrill study. Pore pressures in the North Rankin formation and below were consistent with the predrill study. The geomechanical model was able to explain the losses seen in the Athol formation in Well Gnu-1 when using the mud weights experienced by the open hole at the time of drilling.

Introduction

The Gnu prospect is situated in the northern portion of Block WA-209-P in the Dampier subbasin, Australian northwest shelf (Fig. 1). The prospect is located within the Reindeer gas field. A number of offset wells exist in the region, the closest wells being Well Reindeer-1 (approximately 1.5 km to the northeast) and Well Caribou-1 (2 km to the southeast).

Well Gnu-1 was designed as an exploration well. The anticipated overburden stratigraphy at the location of Well Gnu-1 consists of Tertiary and Upper Cretaceous carbonates, marls and siltstones that overlie Cretaceous claystones, siltstones and minor sandstones, and greensands. The primary aim was to drill vertically to intersect the Muderongia australis glauconitic sandstone and then to build angle and continue drilling a deviated hole through the main Reindeer field gas appraisal within the Legendre formation and into the North Rankin, Brigadier, and Mungaroo formations.