Reservoir sampling and pressure (RS&P) testing acquisitions are fundamental in Exploration settings to evaluate reserves and/or well productivity. Wireline Formation Testing (WFT) is well known in our industry for conventional reservoirs. However, there has not been any available WFT technology to perform these acquisitions in Naturally Fractured Carbonate (NFR) formations under HPHT conditions. A new platform developed for all terrains of formations and environments was used for the first-time in in Mexico, allowing to conduct a full RS&P program to delineate reserves in a large gas condensate reservoir. The new platform consists of a new focussed radial probe inlet, featuring a dual flowline with dual downhole pumps to allow focused sampling mode. Given the larger flow area of the new probe system, flow tests can be conducted in sub mD permeability zones, typical in NFR carbonates, and fluid identification can be performed in-situ while flowing thanks to a group of new sensors. Sampling for PVT analysis is achievable, and sample quality is ensured via the focused sampling system. Post sampling buildups provide pressure transient test data which allow characterizing the layer productivity. The new technology was employed in two appraisal wells belonging to the Veracruz basin in Mexico. The reservoir facies are constituted by packestone and wackestone carbonates belonging to the mid Cretaceous, over very large thickness. Reservoir quality lies in vugular porosity and micro-fractures; however, the matrix porosity is low. The petroleum system originates from the Upper Jurassic source rocks which led to gas condensate hydrocarbons at the reservoir level today. Preliminary simulations indicated that despite the low matrix quality, with the new platform, it would be possible to test and recover quality samples within 4-8 hours depending on the invasion depth scenario and with a drawdown below 300 psi. Four conclusive tests were made in the first well which helped the operator delineate reserves and two conclusive tests were made in the other well fulfilling objectives and following the designed test durations and outcomes, in temperature conditions up to 182 °C and pressure of 20kpsi. The lowest mobility flow test and fluid identification was conducted in 0.06 mD/cP. The new platform test results compared very well with actual test results that were conducted. These two cases highlight the possibility to test and sample fluids in HPHT conditions, NFR reservoirs with tight matrix properties with the new formation testing platform, in environments where previously formation testing was not possible. A significant amount of Mexican carbonate reservoirs faces this extreme environment.
Low resistivity low contrast (LRLC) reservoirs have been successfully produced for many years; however detection and detailed description of their properties and potential would remain a challenge in absence of an exhaustive formation evaluation program. Proper understanding of the geological evolution of such reservoirs to explain their distribution and variations in petrophysical properties is also vital. Low resistivity pay reservoirs encountered in West Africa are often characterized by variation in resistivity values in vertical and horizontal directions due to fine grains and conductive layers within the coarse grained sands and clearly marked sand-shale laminations. This is accurately solved by tri-axial induction resistivity measurement in combination with high resolution measurements able to define any contributing layer level-by-level through robust anisotropic interpretation methods. However, heterogeneity, mixed clays effect, and complexity in rock texture require new technology and innovative interpretation models in multi-domain approach. Advances in logging technologies, interpretation software, and analytical methodologies enable better and more refined reservoir models to be fashioned and tweaked as needed on a case-by-case basis. The case study analyzes log responses, implication of heterogeneity and mixed clays content on the generation of LRLC pay reservoirs in deltaic environment offshore Nigeria. Precise application of advanced log measurements and integration of core data in a common workflow, built around the concepts of evolution of LRLC reservoirs lead to accurate pay quantification. Borehole image interpretation suggests that the low resistivity contrast is attributed to dispersed clays coating around the sand grains in the toe part of a delta front in major coarsening up and feeble fining up sequences. This is also confirmed by variations of elastic properties of the matrix. Petrophysical logs recorded at high resolution correlate inferring the main causes of LRLC pay are clay content and distribution, and small grain sizes intermingled to the reservoir rock, hence resulting in low resistivity values in all directions and drastically increased irreducible water. The logs based model is confirmed by calibration to core analysis results. The confident results of the study confirm the power of collaboration between petrophysics, rock mechanics and geology in innovative interpretation workflows for enhanced reserves estimate and Producibility prediction in heterogeneous media.
Recent advancement in logging technology and data analytics enables measuring a comprehensive set of formation petrophysical properties and rock composition in cased-hole environments. Using state-of-art pulsed neutron logging technology and processing algorithms enables recording capture and inelastic elemental spectroscopy for rock elemental concentrations, including total organic carbon, detailed mineralogy and matrix properties, simultaneously to sigma and other neutron-based outputs. The integration of the interpreted lithology from cased-hole elemental spectroscopy data with electrofacies from high-resolution imaging tools recorded in the open-hole provides the characterization of heterogeneity challenges by building a synthetic core in old wells with limited data gathering from open-hole logging or absence of conventional coring. An effective way to incorporate those measurements has been developed and adapted to the use of cased-hole spectroscopy logs. The dry weight elemental fractions measured by the advanced pulse neutron technology are corrected for wellbore contribution and converted into dry weight mineralogical outputs. Using an automated processing workflow converts the capture and inelastic gamma-ray yields from the energy spectrum measured behind casing into the dry weight of elements and mineral fractions in the formation. The computed mineralogical outputs are then defined based on a standardized ternary diagram approach to developing dry-weight mineralogy-based lithofacies. This classification is then combined with the calibrated micro-resistivity image data collected during the open-hole logs evaluation to present a high-resolution rock typing (after Kumar & Kear). The resulting log is dry weight mineralogy-based high-resolution lithofacies that contain vital information to support geological and petrophysical reserves modeling adjustments during development and production. The paper demonstrates the applicability of the method to cased-hole environments in fields with mixed lithology and complex geological background. Once a robust lithofacies classification is achieved, this is applied for detailed stratigraphic analysis, well-to-well correlation, or refined static reservoir modeling. A standardized mineral-based facies scheme guides the selection of higher completion-quality intervals, otherwise difficult to define in old wells with limited original evaluation. Besides, thin beds that were previously bypassed can be detected and characterized for high-resolution net pay calculation leveraging the high-resolution lithofacies output from this approach. The lithofacies classification (synthetic core) provides important input to the study of reservoir connectivity in the development phase and production optimization. Moreover, a synthetic core description would be critical when reassessing mature fields and defining completion and production strategies where core data is not available. The approach and workflow can be implemented in various cases as a cost-effective solution in multiple scenarios and different formation types.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.