Ganer, B.L., Union Texas Petroleum Corp. Member SPE-AIME
Accurate porosity and water saturation determination of Cotton Valley sandstone is difficult since porosities below 12% are typically encountered. In this low porosity range the errors introduced in the process of obtaining log derived porosity and water saturation values process of obtaining log derived porosity and water saturation values can cause significant misinterpretation of productive potential. Such errors can be due to the presence of secondary matrix mineralization, parameters selected to calculate porosity and water saturation, and parameters selected to calculate porosity and water saturation, and calibration inaccuracies.
In the case of Cotton Valley sandstone, which is predominantly a sand-shale sequence, the occurrence of secondary minerals can be significant. The Cotton Valley sands are a complex intermingling of various minerals including quartz, chert, pyrite, mica, illite, chlorite and an assortment of carbonates. Carbonates in isolated intervals can account for as much as 50% of the rock matrix. The heavy minerals, when not accounted for, can result in pessimistic log derived porosity and hence overlooked commercial potential. Additionally, calibration inaccuracies can result in either optimistic or pessimistic porosity analysis further confounding commercial evaluation. Variations in grain size coupled with the mineral composition complicate permeability determination. The variables mentioned above account for the difficulty in predicting reserves experienced in Cotton Valley sand exploration.
This paper addresses the porosity analysis, using log measurements, of Cotton Valley sands in nineteen wells of the Terryville Field operated by Union Texas Petroleum. For a listing of the wells analyzed please refer to Table 1. Terryville Field is located in Lincoln Parish, North-central Louisiana (Figure 1). Cotton Valley sand in Terryville Field are primarily a sand-shale sequence found at depths between 8,500' to 11,000'. They are Upper Jurassic in age bounded above by the Knowles Limestone and below by the Bossier shale (Figure 2). The sands generally have low porosity, low permeability, and calcareous cementation. Due to the low porosity and low permeability of Cotton Valley sand, massive hydraulic fracture treatments are typically performed in completion work.
The Terryville Field deep Cotton Valley sand potential was discovered in 1978. In the course of the past four years several problems have arisen involving evaluation of the sand and reserve estimation. Log analysis was not adequately consistent with subsequent production data for thirty-four (34) completions in the field. At the onset of the study the capabilities of the log measurements to evaluate the Cotton Valley sands were in doubt. Alignment of core analysis and log analysis was fair. Water saturation and porosity productive limits for the sand were uncertain. Concurrent with porosity productive limits for the sand were uncertain. Concurrent with the initiation of the study, newly acquired computer software, was set up to process lag interpretations. This allowed for more rapid sampling and modeling of log data to provide accurate evaluation of log calibrations and reservoir characteristics as indicated by log measurement.
Sands of the Cotton Valley sand-shale series are identified as the C, D, Bodcaw, Vaughn, McCrary, McFearin-Davis, Bollinger, Schuler, Sexton and Taylor. A typical Cotton Valley log section with correlative sand tops is shown in Figure 3. Completion attempts have been made in most of the sands. The C and D sands appear to be better developed and have been produced in older wells. produced in older wells. The objectives of the study were:
Recognize logging tool miscalibration and make correction
Determine petrophysical parameters for the Cotton Valley sands
Reinterpret the field using new petrophysical parameters
Determine productive limits for log derived porosity and watersaturation to evaluate the productive potential of the CottonValley sands.