Automating Pore-Pressure Prediction in Subsalt Sediments
Authors: Rizvi AiJaz , Saleh Saad T , Williams Ken 
Published: May 2013

Predicting pore pressure in subsalt sediment has been, and continues to be, challenging for the well planner. Subsalt sediments belong to a wide range of depositional settings and have been affected by complex geologic and tectonic events, making it difficult to accurately predict pore pressure.

A joint industry project (JIP) acquired, analyzed, and documented more than 100 subsalt wells in the Gulf of Mexico from shelf to deepwater (2007-2009). The result of this unique and major study culminated in an unprecedented, comprehensive database, which includes geophysical, petrophysical, and drilling data. This database was used to develop best practices for pore-pressure estimation below salt.

In this work, both conventional and unconventional pore-pressure prediction methods—including acoustic and resistivity methods—were applied, tested, ranked, and evaluated against known pore-pressure indicators, such as, MDT/RFT, kicks, and gas shows. The existence, thickness, and effect of a gouge (rubble) zone adjacent to the salt body was recognized by a set of criteria, and its potential effects on pore pressure and fracture gradient in the vicinity of salt have been addressed.

This paper addresses the different pore-pressure regimes present in subsalt sediments, pore pressure variations in different deepwater settings, and the key findings from applying more than 10 different pore-pressure prediction methods. Variations of compressional/shear velocity pore-pressure prediction methods are briefly addressed.

The goal of this work is to automate the pore-prediction methods to make it less " user dependent" and to base it on rock physics and geologic settings. Resistivity-based pore-pressure prediction methods are reviewed with emphasis on salinity normalization to negate salinity variations around a salt body. With more than 95 wells tested with these different methods, pore-pressure prediction methodologies have been proven successful, with an average error of less than 0.5 ppg.

Introduction

The drilling of subsalt sediments is often challenged by the presence of a narrow drilling window, as well as wellbore instability in rubbelized sediments. The operators attempt to exit the salt with a good drilling strategy based on the available data from pre-drill to real-time data. The pre-drill pore-pressure estimation is marred by the poor quality of seismic velocity estimation below salt. If the velocity data becomes more definitive and accurate in the light of acquisition of a VSP log, which may be run just prior to exiting the salt, the need still exists today to use a robust pore-pressure estimation model.

The subsalt joint-industry project was active during 2006 and 2007 with eleven active participants, including major operators and service companies with the objective of developing best practices for subsalt pore-pressure prediction. A total of 107 subsalt wells were analyzed in this project. These wells provided the database for the other studies that were done for this project: the Vp/Vs, resistivity normalization, subsalt gouge-zone analysis, and others. Pore pressure for all of the wells was estimated from petrophysical analysis of log data. A number of different pressure-analysis models were tested on each of the wells. The results were compared to variations in geology and region to attempt to derive correlations between subsalt pressure and geographic location. The additional project objectives included:Determining the state-of-the-art in subsalt planning and drilling,Acquiring and analyzing data from 50-100 subsalt wells and document pressure and drilling data,Examining seismic for those locations for structural effects on pore pressure and for imaging at salt entry/exit,Building a set of geologic models and examining the influence on seismic imaging,Developing and documenting best practices.