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.
Our objective is to obtain — while drilling, in real time — a complete petrophysical characterization from drill cuttings and the return mud-stream. Currently such rock and fluid direct characterization is achievable only, with significant time delay and at a considerable cost, from measurements on cores. The approach is to integrate the direct measurements on cuttings with the Logging While Drilling (LWD) practices and tools, to support drilling, geosteering and formation evaluation. In this framework, it is key to accurately characterize drilling cuttings from a mineralogical and lithological point of view. This characterization can be achieved performing an elemental analysis, by means of Energy-Dispersive X-ray Fluorescence (ED-XRF), in real time, with a portable instrument capable for use at the rig-site, integrated inside the mud logging operations. The portable X-ray Fluorescence (XRF), coupled with a methodology that enables to convert the elementary analysis into a mineralogical composition, produces a characterization comparable to what can be obtained by full scale X-ray Diffraction (XRD) laboratory equipment. An experimental set up was deployed, to assess the ability to model the mineralogy from geochemically analyses of a set of rock samples. After portable ED-XRF geochemical analyses, further lab-based analyses were carried out on the same samples: laboratory Wavelength-Dispersive XRF (WD-XRF) and ED-XRF in state of the art facilities. The geochemical data from the portable ED-XRF were compared to the lab-based XRF results (both WD-XRF and ED-XRF) and showed good agreement, particularly between the two ED-XRF instruments. As a final result, the modeled mineralogy from geochemical whole-rock data showed good agreement with the mineralogy determined from XRD analyses. Finally, a first field geochemical monitoring, by portable ED-XRF while drilling, was then successfully executed in Saudi Arabia.
Glacial continental sandstones are much less frequently encountered as hydrocarbon reservoirs than their fluvial and marine sandstone counterparts. Sands transported by water are subject to grain size fractionation during the transport process while glacial sandstones are not. Glacial sandstones are commonly very poorly sorted, ranging from silts to boulders, and neither size sorted nor density sorted. Hence glacial sands may also contain dispersed heavy minerals that are not removed by aqueous transport processes or by long lateral fluvial transport. The provenance of glacial sediments reflects components sourced from their high altitude origins which are mountainous terrains with igneous and/or metamorphic rocks. For similar reasons, glacial sands are commonly bereft of primary shale and clay. The formation evaluation starting point for glacial and reworked glacial sandstone analysis is therefore quite different from the more commonly encountered fluvial and marine sands. Instead of starting with a sand – clay or sand - shale analysis, we start with a sand – heavy mineral analysis. Lower to middle Unayzah Group siliciclastic sediments in central and eastern Saudi Arabia represent a major glaciation and deglaciation of Gondwana. The glacial sediment origins and proximity to igneous and metamorphic outcrops ensures enrichment, preservation and recycling of heavy minerals such as zircon, monazite, apatite, rutiles and sphenes as observed by Knox and coworkers. Such minerals are strongly associated with the presence of rare earth elements (REE). REE's can be strong neutron absorbers and their variable presence in glacial and glacial reworked sandstones complicates the sandstone formation evaluation. These variations are anticipated to drive an important and variable sand matrix correction to the thermal neutron well log porosity for Unayzah Group sandstones. Gadolinium (Gd), is the strongest contributor to the neutron absorption cross section of the REE group. Conventional thermal neutron well log interpretation of sandstone assumes that the dominant neutron processes are related to the presence of chlorine and hydrogen in the pore space fluids and in formation clays. These are not the only contributors in this case. If undetected, the REE presence can affect the porosity and matrix determination by neutron-density crossplot and thus affect the effort to use the apparent hydrogen index for distinguishing gas from oil. Geochemical and petrographic studies of the Lower to middle Unayzah Group sediments demonstrate the significant presence of REE within the glacial/deglaciation complex where fine silts and boulders are equally transported within and by glaciers and deposited in distal parts of the basin. The use of neutron and density log data to estimate porosity is valid in most fluvial-deltaic sandstone reservoirs but this does not hold true for the heavy mineral bearing sands of the Unayzah Group. In this work, the evidence for REE presence in Unayzah sandstones is reviewed using an extensive (tens of thousands) cores and cuttings samples database analyzed for elemental composition by inductively coupled plasma-mass spectrometry (ICP-MS) and optical-emission spectrometry (ICP-OEs) for 50 elements. Analysis of these data demonstrate that total REE's in Unayzah sediments are directly proportional to the lab-measured and wireline measureable Gadolinium (Gd) concentrations. These insights combine to indicate a significant relationship between the neutron capture cross section of the formation matrix and the geochemical well log detection of Gd, and thereby, insights into an improved method for the use of the neutron well log in the formation evaluation of the Unayzah Group.
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