Permanent Borehole Seismic in Ultra Deep Offshore Appraisal Wells
Authors: Taylor Scott , Hiner Michael 
Published: October 2011

Abstract
Incomplete reservoir characterization is a leading concern in estimating and predicting the reservoir recovery factor and thus the commercial viability of a discovery. The importance of reservoir characterization is amplified for both offshore and ultra deep water offshore asset classes where huge investment decisions are based upon an inequality of initial information from appraisal well tests. Characterization and monitoring of the reservoir between wells still depends largely on data from either towed streamers or ocean bottom cabled or node based sensor networks. The available seismic data is typically of very limited bandwidth, giving similarly limited spatial resolution, and so consequently uncertainties related to reservoir content and continuity (compartmentalization) remain high. In principle, this risk can be mitigated by well tests and by obtaining higher-resolution seismic data, such as 3D VSP's or even cross well tomography data; however, standard operational procedures and current workflows currently make these solutions both costly and risky activities. Offshore appraisal wells are an excellent potential source of dynamic borehole seismic data, in addition to distributed pressure and temperature measurements. The sensors can be installed closer to the reservoir horizon and most importantly below salt layers, yielding significant enhancements in resolution from time lapse active (VSP/cross-well) and passive (microseismic) seismic surveillance. Additionally, permanent seismic sensors are a field proven approach to providing operators with a dynamic and deterministic source of casing and cement bond integrity data in real-time (Smith, et.al, 1998, 2001, 2002, 2010). The acquisition of seismic data from appraisal wells is not currently a standard operating procedure or available option for operators due to a number of prevailing technical issues. The authors present an overview to instrument offshore appraisal wells during both temporary and permanent abandonment procedures and describe state-of-the-art approaches to meeting current challenges.

Introduction
"Deepwater" oil and gas assets are simply conventional reserves in an unconventional setting. They constitute an asset class of their own largely due to the fact that they share a common set of technical challenges over the entire exploration and production lifecycle: from their identification, development, and production??. Primary deepwater regions are located in the Gulf of Mexico (GOM), Brazil, and West Africa. Approximately 70% of Brazil's current portfolio of recoverable hydrocarbon reserves are located in deepwater basins. Although Brazil has some of the world's most prolific deepwater fields, as an investment target Brazil must compete with West Africa, where the deepwater exploration success rates run as high as 80% in some areas, and with the Gulf of Mexico (GOM), where discoveries are smaller but more frequent (Figueiredo, 2006). One key investment driver is the ability of an operator to optimize an assets recovery factor. This consideration is of more significance for deepwater assets.