Reservoir Prediction and Development Challenges in Turbidite Slope Channels
Authors: Mayall Mike , O&apos;Byrne Ciaran 
Published: October 2008

The paper discusses the challenges facing development of deepwater reservoirs deposited in turbidite slope channels. Consideration of the reservoir facies and stacking patterns shows the pitfalls and also the potential for mitigating the effect of flow barriers and baffles within reservoirs. This can impact many aspects of the development including reservoir modeling, well designs and development concept selection.

Large Erosional Turbidite Channels are dominated by 4 major facies; Basal lags (good but generally thin reservoirs), slump and debris flows (poor or non-reservoir), high N:Gsandy channels (good reservoir), and sinuous channel levees (thin sands difficult to develop). The development of these components and their stacking pattern can be recognized on seismic but there are significant pitfalls. Considering the detailed geometry of the reservoir facies may be important when assessing how easily wells from a fixed platform versus a subsea development can access the reservoir.

Low-Relief Channel Levees comprise both sand and mudfilled axis with sandy levees. The muddy axis, heterogeneities in the levees and vertical stacking of channels give rise to development challenges which may require a compromise between the numbers of wells required to mitigate flow barriers and the risk of utilizing complex multiple completion or multilateral wells to minimize well numbers.

The high expense of developments in deep water requires a thorough assessment of the subsurface risks. Understanding the potential pitfalls and having appropriate mitigation plans is critical to a successful economic development.

Introduction

The development of turbidite reservoirs in deep water is most advanced in areas such as the Gulf of Mexico and offshore Brazil (1,2,3,4). In both of these areas the reservoirs are generally dominated by sheet-like geometries of varying thickness. In the Gulf of Mexico in particular, the vertical stacking of the reservoirs, which increases the reserve density (bbls/Km2), is an important aspect in assuring the economic success of the deepwater developments.

The massive costs of developments in deepwater, usually $2-4billion, is typically split between wells 25-30%, subsea 20-30% and platform/FPSO 50-30%. The fundamental development concept choice of Dry Tree vs. Subsea Development is based on a number of complex factors, costs, water depth, well numbers, reserves, corporate experience and expertise, contracting strategy etc. However this initial choice of scheme can have a critical influence on the development well designs, particularly, length of wells, complexity of drilling, completion difficulty, ease of maintenance and data collection. Consideration of development well 'drillability' at this stage in the project decision-making is often reduced to a consideration of drilling radius vs. depth below mud-line. However targeting important elements of the reservoir and mitigating risks of flow barriers in the reservoir may require very specific azimuths and trajectories in the development wells.

More recently, slope turbidite channels have emerged as major reservoir type in West Africa (5,6) with a number of giant fields having commenced production or being readied for development.