An Integrated Approach To Obtain Reliable Permeability Profiles From Logs in a Carbonate Reservoir
- Piero Balossino (Eni E&P) | Ferdinanda Pampuri (Eni E&P) | Corrado Bruni (KPO) | Karazhan Ebzhasarova (KPO)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2008
- Document Type
- Journal Paper
- 726 - 734
- 2008. Society of Petroleum Engineers
- 1.6 Drilling Operations, 4.3.4 Scale, 5.6.4 Drillstem/Well Testing, 1.2.3 Rock properties, 5.6.1 Open hole/cased hole log analysis, 1.6.9 Coring, Fishing, 5.5.2 Core Analysis, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.1 Reservoir Characterisation, 3.3.1 Production Logging, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.8.7 Carbonate Reservoir
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Permeability is one of the key petrophysical parameters in reservoir evaluation. Information about permeability is commonly derived from cores and test data, which generally cover only part of the reservoir section, but can also be derived from logs and then extrapolated to uncored intervals and wells. Two logs provide such information: acoustic and nuclear magnetic resonance (NMR). In the Karachaganak field, an approach based on the acoustic tool was preferred because of the textural characteristics of the vuggy carbonate reservoir.
The approach relies also on the use of image logs to obtain a detailed description of the reservoir-rocks texture and to discriminate between rocks with primary interparticle porosity or very small vugs and lithotypes with multimodal distribution of pores, enlarged vugs, and touching vugs.
More than 900 m of core have been used to validate the permeability log derived from the analysis of Stoneley waves in 25 wells from this field. A correlation between the validated log-derived permeability and the textural facies from image logs has allowed the relationship between permeability variations and the geological framework to be established.
The results have been compared with dynamic data from production logging through the definition of flow units from Stoneley-wave-derived permeability and porosity-log data and the use of a stratigraphic-modified Lorenz plot (SMLP) to identify possible fluid-entry points.
Three main permeability trends have been identified:
- For undolomitized or patchily dolomitized biohermal deposits
- For pervasively dolomitized lithologies characterized by lower mean permeability values
- For facies characterized by well-developed vuggy porosity with enhanced dissolution phenomena (i.e., touching vugs and microfracturing)
Also, well-test results have highlighted the occurrence, in some wells, of very high k values in the biohermal deposits, which are clearly unmatchable by log-derived matrix permeability. Open fractures have been observed in these wells on the image logs, thus suggesting that the enhanced permeability and the improved well performances have to be related to their presence
The comparison of log-derived permeability and well-test results has given a new perspective in the interpretation of well-test results with respect to the geological framework and a well-defined sequence-stratigraphic model. The extension of this methodology to new wells could improve knowledge of the petrophysical characteristics of the reservoir and allow better prediction of reservoir productivity.
This paper presents the interpretative methodology--integrating wireline and image logs, cores, and test data—that has been followed to describe vertical trends of the main petrophysical parameters and correlate them throughout a carbonate reservoir. The study has been focused mostly on understanding the complex variations in the porosity/permeability relationship and has made use of the flow-unit concept and, partially, of the graphical method described by Gunter et al. (1997).
Porosity and permeability are key petrophysical parameters for a detailed reservoir description. Porosity can be obtained confidently from cores and logs. However, especially in complex reservoirs, image logs can add valuable information about texture and porosity-type distribution. Permeability usually is derived from core measurements and from the interpretation of production tests. As is well known, there are no wireline logs that measure permeability directly. However, continuous permeability profiles can be extracted from the analysis of the Stoneley waves acquired by an acoustic tool or from NMR T2 distribution and from the Coates equation. In the latter case, a good correlation must exist between pore size and pore connectivity.
In the study field, Karachaganak (western Kazakhstan), the main reservoir consists of platform carbonates and biohermal deposits in which multiple diagenetic events have created and destroyed porosity and have generated complex porosity/permeability relationships (Hendry et al. 2002). Because of the widespread presence of vuggy and moldic porosity and the overall textural characteristics of the reservoir, the approach based on Stoneley-wave attenuation has been considered as more suitable to derive reliable permeability profiles from logs. Moreover, acoustic-waveform data were available together with image logs in at least 25 wells and, thus, provide good coverage in all the zones of the field.
More than 900 m of core from eight different wells was also available and has been used to ascertain the validity of the computed permeability. Further discrimination of unreliable intervals has then been performed by integrating textural-facies data.
Once the permeability and porosity data have been validated, flow units have been defined by analyzing the vertical variations in storage capacity and flow capacity on an SMLP and have been correlated throughout the field with help from the results of production tests.
A good correlation has been established between flow units and the textural facies from image logs that show vuggy-porosity distribution. The best flow units are associated with the most-heterogeneous facies, characterized by touching vugs and cavernous pores. On the contrary, a unique correlation between flow units and lithofacies is often not verified.
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