Producibility of Turbidite Reservoirs: What Factors are Important?

Cossey, S.P.J., Cossey & Associates Inc., Houston, TX

Abstract

One goal of field development is to place high rate, productive wells in "sweet spots" or areas where pay is thickest. However, thick pay does not necessarily mean high flow rates. Producibility (normalized flow rate) of a reservoir should be used in con junction with pay thickness to locate "sweet spots". Data from 159 completions in deep-water clastic reservoirs from 12 Gulf of Mexico (GOM) and North Sea fields have been analyzed to determine the geological and engineering factors that might contribute to a higher normalized flow rate or Specific Productivity Index (SPI), here defined as BOPD/psi of drawdown/per foot perforated. SPI has been compared to tubing size, architectural element, viscosity, net-to-gross, log shape, and permeability. The highe st SPI's in the North Sea are from reservoirs with net/gross >85%, although 60% is a critical cut-off in the Gulf of Mexico. There is a good correlation between average reservoir permeability and maximum SPI in unconsolidated GOM reservoirs and between a verage permeability and minimum SPI in consolidated North Sea reservoirs. The highest SPI's are also found in completions with 4.5" tubing.

A simple classification of log shape was used in the study to determine the range of uncertainty of SPI for each reservoir type. Reservoirs with blocky log shapes had the highest SPI's, although reservoirs with funnel or bell log shapes had very similar ranges of uncertainty in SPI's. Hilterman (1998) showed that acoustic impedance varies with log shape in the GOM, indicating that some aspects of producibility might be mappable from seismic data.