Some of the Middle East UAE lower Cretaceous Thamama Carbonate reservoirs are characterized by low resistivity pay phenomena, on which log analysis results in high water saturation computation; on the contrary, production tests confirm dry oil. The Sedmintological investigation on thin sections have shown significant percentage of Micro-porous (occluded porosity or non-efficient porosity) network traps irreducible hyper-saline formation water which responds to a low resistivity, even if the oil is occupying the larger size of porosity (efficient porosity). This paper presents a simple and cost-effective approach for better identification and evaluation of low resistivity pay carbonate reservoirs. The approach utilizes the pulsed neutron capture log (PNCL) in real-time logging in proper identification and formation evaluation in these reservoirs through case studies in the Middle East-UAE Carbonate reservoirs. Through number of wells that have recently been drilled using different types of mud and were logged by LWD PNCL; the results have clearly shown the benefit of minimizing evaluation uncertainties compared to those logged without LWD PNCL. For decades, the combination of conventional logs i.e. neutron, density and resistivity are undoubtedly proven technologies to evaluating homogenous reservoirs. Unlikely, accurate determination of Petrophysical parameters and water saturation evaluation would rather be difficult to show especially in so-called low resistivity pay reservoirs. Since the PNCL (Sigma log) provides a resistivity independent water saturation evaluation, the PNCL has become an integral part of the evaluation program in these low resistivity pay reservoirs, particularly using OBM with relatively high rate of penetration, so that well bore surrounding environment would not be greatly altered (minimum invasion). LWD drill and wipe passes evidently demonstrate that the different between logs can be caused only by reservoir fluid change.
The reservoir in discussion is a tight carbonate reservoir with low productivity and relatively under-developed albeit the huge in-place volumes. The expectation is that a detail reservoir characterization will provide insight on factors affecting reservoir productivity, spatial distribution of productive portion of the reservoir and offering solution to overcome reservoir tightness. The case study discusses on how a comprehensive multi-discipline review unravels and presents a robust reservoir heterogeneity framework. A geological review that includes both depositional and diagenetic process is performed to understand distinct components/factors responsible for reservoir heterogeneity. Simultaneously, petrophysical assessment was performed to quantitatively define rock grouping based on porosity-permeability, capillary pressure and pore throat distribution in the log and core domain. The multi-discipline observations were then reconciled to establish relationship between the process origin and the resultant product of specific group/range of reservoir petrophysical properties. The multitude of pore throat characters and its petrophysical properties were linked to the underlying geological processes. The established heterogeneity framework provides clarity on spatial distribution of the reservoir sweet-spot, factors controlling low productivity and the required mitigation. The study provides a complete journey of unlocking tight reservoir potential. It illustrates the geological studies influence toward innovative completion technology selection, design, and execution to overcome reservoir challenge. The study is supported by recent drilling and test results, hence offering insight for adoption and lesson learned.
Carbonate reservoirs introduce challenge in providing accurate water saturation from conventional Archie equation. One of the reasons is due to the variability of the Archie cementation factor "m" because of complex and tortuous nature of these heterogeneous carbonates. The study was performed by integrating core and log data from advance measurements to understand the root cause and range of the variability and an attempt to link sedimentology and diagenetic facies to petrophysical groups. The Study focused on a carbonate reservoir with complex pore network. The formation resistivity factor (FRF) measurements were conducted with high-resolution sampling on a selected well. Each of FRF plug has associated porosity, permeability, thin sections, MICP, NMR and high-resolution dual energy micro CT scan. The m value from FRF is then plotted along the porosity-permeability plot. The capillary pressure parameters (entry pressure, slope, inflexion points) were extracted from MICP and relationship is plotted against m. Diagenetic facies described from the thin sections is compared versus m. Principal component analyses was conducted to identify factors relating to m. The uncertainty on water saturation associated to variable parameter m was assessed using Monte Carlo analysis on multiple wells. An advanced multi-frequency dielectric logging tool was run on couple of wells to provide variable water-phase tortuosity (MN) measurement. Specific analysis was performed to extract the variable m value from the measurement over limited zones, which has been derived from core "m" measurements. Several wells located on the flank of the reservoir below water level were evaluated. Dean stark measurements were performed on a well and used to validate the saturation calculation. It is obvious that the evaluated reservoir has high degree of heterogeneity as indicated by complex pore network with multi modal pore system as shown by the thin sections, MICP and plug CT Scan.
Objectives This study focuses on resolving the water saturation (Sw) uncertainty in a small oil rim that consist of thin high-permeability layers (super K). Resistivity-based water saturation techniques suffer from multiple uncertainties including thin layers with distinct capillarity profile, inaccurate resistivity due to high-angle bed effects and unknown Archie parameters. Accurate saturation using alternate methods than resistivity based is critical for the calibration of saturation height model for reserve estimate and to design the perforation program away from the mobile water zone. Methods To resolve the issue of accurate Sw, an alternate method of computing saturation from sigma measurements while drilling is being investigated. Data from multiple wells has been analyzed- saturation from sigma being computed and compared to resistivity based saturation. The resulting saturation is benchmarked against other known sources of saturation like core data and cased-hole measurements. The input parameters used to compute the sigma-based saturation were validated by well test and PVT lab data. Observations A general assumption for LWD logs is that invasion is so shallow that LWD data acquired during drilling may be sufficient to evaluate uninvaded zone properties However, this premise need to be confirmed by other sources of information, in this case Multi-Depth of Investigation (MDOI) sigma data. It provides valuable insight into the status of formation invasion, at the time of acquiring data and of measurement quality control. In situations where the resistivity data does not suffer from any high-angle bed effects, there is immense value in combining the information from resistivity and sigma. When there are two measurements free from invasion and reading the same volume of rock, this can be used for not only calculating water saturations but also to fine-tune the Archie parameters, which might be of immense value in situations, for use on offset wells with resistivity data only. Novel Information Sigma while drilling is another weapon in the arsenal of the petrophysicist and the multi-depth of investigation sigma measurements in a LWD environment contributes a new opportunity to evaluate fluid saturations in reservoirs with unknown or variable Archie parameters.
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