Complex Reservoir Evaluation In Open And Cased Wells
Authors John P. Horkowitz (Schlumberger Wireline & Testing) | Darrel E. Cannon (Schlumberger Wireline and Testing)
Document ID SPWLA-1997-W
Publisher Society of Petrophysicists and Well-Log Analysts
Source SPWLA 38th Annual Logging Symposium, 15-18 June, Houston, Texas
Publication Date 1997
ABSTRACT
Hydrocarbon-bearing formations in West Texas commonly contain radioactive sands and carbonates as well as varying amounts of anhydrite and gypsum, making the distinction between productive and nonproductive intervals difficult, especially through casing. The ability to identify and quantify hydrocarbons in these mixed-lithology, low-porosity reservoirs has changed with the exciting development of spectral tools that accurately determine lithology in both open- and cased holes. The lithology determined is based on elemental concentration logs obtained from induced-neutron gamma ray spectroscopy measurements. When this lithologic information is combined with other measurements, definite improvements in the accuracy of porosity, water saturation and productivity predictions are observed. Stratigraphic interpretation is also significantly enhanced, especially in cased wells. SpectoLith lithology algorithms were developed using a core database of more than 400 samples characterized by dual-range Fourier transform infrared mineralogy and chemical analyses. Total clay dry weight percent is derived from elemental concentrations of silicon, calcium, and iron; anhydrite and gypsum dry weight percents are determined from sulfur and calcium concentrations; and carbonate dry weight percent is determined by calcium concentrations. The remainder is assumed to be composed of the minerals quartz, feldspar and mica. Applications of this technology have successfully 1) quantified clay volumes independent of gamma ray, spontaneous potential and density-neutron, 2) determined permeability and bound fluid based on mineralogy, 3) derived spectral-based foot-by-foot sigma matrix values for accurate sigma based saturation analyses behind pipe, 4) lead to the modification of completion techniques in clastics based on improved clay and carbonate volumes and 5) provided chemostratigraphy information for enhanced geologic correlation and reservoir modeling.
