Advanced Well-Site Geochemistry While Drilling: Improved Wellbore Positioning and Formation Evaluation of Unconventional Reservoirs

Abstract

Abstract

Instrumentation for rigsite elemental analysis, developed during the 2000s for conventional reservoirs, was expanded in 2009 to include additional technologies to enable a more complete characterization of shale reservoirs. Since then, hundreds of shale wells have benefitted from combined XRF, XRD, and programmed pyrolysis to determine chemostratigraphic zonation, mineralogical composition, and key organic parameters (S₁, S₂, TOC, and T_max) while drilling. To further enhance characterization, critical fluid data such as alkanes (C₁-C₈) and aromatics (benzene and toluene) distribution can be obtained from advanced mud gas analysis, helping to characterize fluid type, density, and mobility. A workflow combining cuttings-based mineralogy and geochemistry, gas analysis, and open-hole log data from vertical pilot wells enables rapid and effective formation evaluation for selection of target zones for laterals.

Integrated wellsite geochemical analyses were used to verify the reservoir quality of a target zone while drilling a vertical pilot hole in the Woodford Shale in Hughes County, Oklahoma. Results indicated:

1. Total Organic Carbon (TOC) of 3.8 to 7%, Total Hydrocarbon Content (THC) up to 10 times the background gas levels, and enrichments in redox-sensitive elements associated with organic matter (V, Ni, Cu, As, Mo and U).
2. Fluid type, quality, and thermal maturity for Type II/III kerogen as shown by T_max (442-455 °C; oil window), C₁/THC indicative of light oil, higher fluid mobility and relatively low amounts of water associated with hydrocarbon indicated by fluid saturation curve (FS), and fluid mobility (FM), and water saturation (S_w) curves.
3. Variations in mechanical properties as estimated by the mineralogy-derived Relative Brittleness Index (RBI), and by C₁/ROP gas data.

The landing point(s) in the Woodford was chosen using TOC values, fluid type and quality, and mechanical properties necessary for an optimal completion design. A robust three-unit chemostratigraphic zonation was developed for the cherty Woodford from the vertical well. This section was characterized by higher SiO₂, lower detrital-affinity elements (TiO₂, Zr, Nb, and Th), and enrichments in the redox-sensitive trace elements. Elemental data acquired from cuttings on the lateral well was tied back to this zonation while drilling. This verified stratigraphic position for the 5200-foot lateral, and assisted in keeping the wellbore in the target zone despite penetrating several faults.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016