A New Mineralogy Cuttings Analysis Workflow for Optimized Horizontal Fracture-Stage Placement in Organic Shale Reservoirs
Authors: Buller Dan , Scheibe Christian , Stringer Charles , Carpenter Glenn 
Published: October 2014

The dominant methods of geosteering and horizontal formation evaluation in most organic source rock reservoirs has been limited to the use of logging-while-drilling (LWD) gamma ray and conventional mud logging. This limitation is primarily attributable to cost constraints and the historical preference for geometric fracture-stage placement. This process has resulted in varied performance between closely spaced wells thought to be drilled in similar stratigraphic positions and like rock.

Very little new vertical well data is typically acquired in the development phases of most of these plays to document any changing physical rock properties that may contribute to the variable performance between wells. The perceived cost of additional pilot wells or additional horizontal LWD, open hole, or cased-hole measurements restricts most operational teams to a situation in which best practices may be recognized but are rarely implemented. To address this issue, this paper proposes and presents a cost effective cuttings analysis workflow, using a new combination of available technologies that is calibrated to vertical and horizontal petrophysical and mechanical properties. An automated fracture-stage and cluster placement method using this analysis workflow is applied to validate well treatment and post-fracture performance.

In recent years, several tools have been developed to analyze drill cuttings from oil and gas wells. The most commonly used tools include X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX), bulk density, and pyrolysis. Although each of these tools can be used to develop a limited determination of the in-situ rock character, the combination of three of these tools (XRF, SEM/EDX, and pyrolysis) can provide a more comprehensive picture of formation properties.

The combination of XRF analysis with the SEM/EDX analysis is the key to the cuttings workflow. The exact location within the borehole can be determined and a robust mineralogy developed that is independent of normative mineralogy (typical XRF) or operator-interpretive mineralogy (XRD). Additional outputs include relative brittleness index, bulk density, lithology, fractional and textural relationships, total organic carbon (TOC) proxy, and a new porosity index. Trace and major elemental ratios are also available for precise stratigraphic placement. The addition of cuttings pyrolysis enables hydrocarbon typing, producible hydrocarbons, TOC, and total inorganic carbon (TIC) within each sample to be established.

In this paper, outputs from the XRF-SEM/EDX-pyrolysis analysis of two vertically cored wells are benchmarked against complete vertical log suites for the modeling of petrophysical and mechanical properties. Subsequent horizontal cuttings properties for the two area examples, Marcellus and Eagle Ford Shales respectively, are presented and analyzed with the vertical modeling applied. In addition, the Eagle Ford horizontal cuttings analysis results are compared and contrasted with a through-casing pulsed neutron log (PNL) for potential upscaling of the sample frequency for continuous physical properties evaluation, including effective porosity. The exact stratigraphic placement from only a cuttings analysis is also demonstrated.

Finally, the calibrated Eagle Ford and Marcellus horizontal cuttings analyses are used as inputs for an optimized fracture-stage and perforation cluster placement design for each of the wells. For validation, individual fracture-stage pumping performance is compared to the predicted formation properties from the Eagle Ford cuttings analysis example.