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PROCEEDINGS, INDONESIAN PETROPLEUM ASSOCIATION Thirty-Fourth Annual Convention & Exhibition, May 2010

INTEGRATED PETROPHYSICS IN KUJUNG CARBONATE RESERVOIRS,

EAST JAVA, INDONESIA

Sigit Sutiyono* ABSTRACT Carbonate reservoirs are often considered as less significant than clastic reservoirs, however large accumulations of hydrocarbons can be found in carbonate rocks. The challenges faced in exploitation of carbonate reservoirs are far more complex than those in clastics. The Early Miocene carbonate reservoirs in the Ujung Pangkah Field offshore East Java Indonesia possess very complex reservoir properties. Petrophysical challenges the subsurface team has to deal with include how to establish reliable porosity, water saturation and permeability models used in generating representative geological and reservoir modeling. Further complexity is presented by how to develop a thin oil-rim with a long transition zone in fractured and vuggy reservoirs. The data collected from appraisal wells provides only partial information of the field that has led to planning and development decisions in some cases found unrepresentative of the actual subsurface conditions encountered during development; fractures were not commonly seen in the appraisal wells: most of these were vertical wells, seismic resolution was not at level to define fractures, and only limited cores were taken. More information about the field was obtained during development; fractures were intersected by horizontal wells, and tendency of oil-wet reservoirs was revealed from new cores. Integrated Petrophysics is one of the keys to the future Pangkah development strategy. The integration includes how to incorporate more recent data including sedimentological study, various special core analyses, such as capillary pressure data, relative permeability analysis, wettability and Core NMR, advanced wireline logs, image logs and NMR logs. In addition how to deal with the high degree of heterogeneity demonstrated by very large variations in both log response and core measurements such as saturation exponents that * HESS (Indonesia Pangkah) Ltd.

cause problematic water saturation estimation. Hydraulic Flow Unit generation to characterize porosity, water saturation and permeability, fracture characterization from image log and transition zone thickness identification also are parts of this rigorous integration effort to not only provide inputs for a new 3D geological model, reservoir simulation and volumetric estimate, but also to give feedback for mitigation strategies such as well placement and completion type. INTRODUCTION Ujung Pangkah Field, an Early Miocene platform margin carbonate reef build-up, is located offshore East Java right in front of the present day Solo river delta, northwest of Madura Island. The field was discovered with the drilling of Ujung Pangkah-1 in 1998 by Premier Oil. Amerada Hess has been operating the Pangkah PSC since 2001. Ujung Pangkah POD was approved in July 2004 for gas development, and in July 2006 it was revised to include oil development. Currently KUFPEC owns 25% share in the Pangkah PSC. A high degree of petrophysical uncertainty in the main producing Kujung reservoirs, part of the Kujung-I formation, was recognized shortly after the Phase-I Field Development started. The wells that produced water early in the production life provided an indication that the reservoirs are more complex than had been anticipated. Petrophysical challenges including how to establish reliable porosity, water saturation and permeability models to be used in generating representative geological and reservoir modeling, and how to provide inputs for mitigation efforts in thin oil-rim development with a long transition zone and in fractured and vuggy reservoirs with the main objective to minimize water production. METHODS AND DISCUSSION OF RESULTS Porosity estimation from the logs shows limited success when compared to core porosity and image

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log porosity. Vuggy and fracture porosity are not well defined from conventional porosity logs - density, neutron and sonic porosity (Sutiyono, 2009). A factor to correct this conventional log porosity is required to appropriately represent true porosity. Figure 1 shows the comparison between core and log porosity. Early permeability modeling efforts suggested a generally good reservoir unit based permeability prediction. However, each reservoir unit has more than just one rock type or hydraulic flow unit (HFU); the initiative was then taken to extend the prediction based on multiple rock types within each reservoir unit. This approach has assisted tremendously in water saturation modeling efforts. Core calibrated HFU prediction for the entire field is performed utilizing Facimage software in Paradigms Geolog package. Figure 2 is a core porosity-permeability plot that demonstrates the clustering of each individual hydraulic flow unit used as the basis for FHU prediction approach using Facimage. Various capillary pressure data from porous plate, mercury injection and centrifuge methods were analyzed to help rock type and water saturation assessments. The evaluation indicates that water saturation is the biggest petrophysical uncertainty in the Pangkah field. Special core analysis (SCAL) capillary pressure data suggests the following; there is a poor relationship between porosity and irreducible water saturation (SWirr), no clear trend can be generated for pressure entry with porosity, Brook-Corey model seems not to be particularly useful, and J-function approach is considered to be poorly applicable for Pangkah field. These findings make the water saturation modeling efforts difficult. This is not uncommon for carbonate reservoirs. Water saturation estimation utilizing capillary pressure method works generally well in a system at equilibrium. It is very likely that Pangkah Field at present is not at an equilibrium condition. However, what does help in addressing the water saturation modeling issues in Pangkah field is Nuclear Magnetic Resonance (NMR) data both from logs and core sample tests. It is found that the NMR log requires slight adjustment for higher T2 cutoff than currently used to match the core NMR measurements, lower SWirr from NMR log was discovered using 120s T2 cutoff. The NMR data is tremendously useful especially for calibrating log derived water saturation calculation. There are some issues associated with water saturation calculation from the logs including large range of Archies exponent measurements revealed from SCAL data over short intervals, resistivity with various measurement issues and difficulty in

deriving true formation resistivity (Sutiyono, 2009). Comparing the NMR derived SWirr with other SWirr estimates from different methods discovered that NMR SWirr is the most optimistic water saturation. The combination of NMR SW with capillary pressure based Brook-Corey SW and Height function SW provides a measure of quantitative SW uncertainty assessment for the interval above the transition zone. Figure 3 illustrates various water saturation curves, core NMR, NMR log, height function and conventional logs. A different approach is now being pursued to characterize rock type from trim ends mercury injection, this method is relatively fast by relating pore size distribution to rock type to further help water saturation modeling. The pre-ageing and post-ageing resistivity index measurements result in high resistivity index values from post-ageing compared to those from pre-ageing tests, suggesting an oil-wet system. Unsteady state relative permeability test results also confirm an oil-wet reservoir below the gas-oil contact, however, the values indicate a rather mild oil-wet condition compared to those from post-ageing result. Analysis of Corey Exponents to determine wettability characteristics from a number of samples taken from different zones suggests the samples are in an intermediate or mixed wettability to oil-wet range. United States Bureau of Mines USBM wettability tests performed on selected core plug samples from gas, oil and water zones indicate the reservoir has a property of intermediate oil-wet. Wettability determination is very important aspect in developing the thin-oil rim found Pangkah field, especially if it is associated with a long transition zone, as the water in the transition zone for an oil-wet reservoir can be very mobile. Therefore, determination of well placement vertically in the oil column is critical to minimize possible large water production. An accurate estimation of transition zone thickness from SWirr model and SW calculation provides help in optimizing well placement above the transition zone. Pangkah area has experienced extensional, compressive and strike-slip tectonic activities that have led to current complex structure and faulting system. Image logs run in the producing zone in Pangkah are required for fracture and rock fabric characterization. It is very important to know about the type and density of fractures penetrated by the lateral section for completion strategy. Fractures have been blamed for early water production in the field as this very permeable feature provides a conduit connecting the productive interval to a

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mobile water zone. Open fractures can be mitigated by having a form of isolation in the completion string, and when high density fractures are intersected that any form isolation can not handle, a decision to side track the well become an option. Figure 4 shows an example of a well with image log and conventional log responses penetrating a number of conductive fractures. CONCLUSIONS A combination of well data acquisition in both vertical and high angle wells should have taken place during the appraisal period not only to get information about reservoir heterogeneity but also to provide enough time for appropriate evaluation; some SCAL tests take very long time to do and to utilize them in petrophysical modeling. The integration has helped in building a reliable new 3D model to account for quantitative petrophysical uncertainties, more accurate reserve estimates and providing inputs for the Phase-II Pangkah development strategy. It is clear that the Kujung reservoirs in Pangkah field are very complex

reservoirs. Even with the data collected so far, Pangkah field still has high degree of reservoir properties uncertainty, additional data is required to confirm this. It is very unfortunate that in dealing with carbonate reservoirs we need to be ready with mitigation strategies to overcome the unexpected. ACKNOWLEDGEMENTS High appreciation and many thanks to the Direktorat Jendral MIGAS, BP MIGAS, KUFPEC and HESS Managements for their permissions to publish this paper in the IPA Convention & Exhibition 2010, as well as to all colleagues in Hess Indonesia Subsurface team for their supports and constructive inputs and fruitful discussions. REFERENCE Sutiyono, Sigit, 2009, Reservoir Water Saturation and Permeability Modeling in the Pangkah Field. The 33rd Indonesian Petroleum Association Convention, IPA09-G-022, May 5th 7th, Jakarta, p. 37.

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Figure 1 Comparison between core porosity and log porosity suggesting the high core porosity that could

not be matched by log porosity are vuggy porosity, the difference can be up to 4%, the low core porosity is most likely caused by a combination of log resolution issue and drilling mud fines influenced core plug samples, which is not experienced by full diameter samples.

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Figure 2 Core porosity and permeability modeled based on Hydraulic Flow Units (HFU) with distinct

clusters in different colors with the porosity-permeability transform for each unit.

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Figure 3 Water saturation curves; core NMR SWirr in red mark, remodeled NMR SWirr curve in green,

SW as height function and core porosity in blue mark, and calculated SW in light blue curve. Large offset between SWirr and calculated SW or SW as height function represents movable water in the transition zone

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Figure 4 Image log shows very conductive fractures with various overture size, large overture fractures in

red arrows in this well are well recognized also in the conventional logs, GR, Resistivity and Density-Neutron logs characterized by highly conductive features. The smaller overture fractures are hardly identifiable in the conventional logs in blues although they have similar conductivity but very thin, these conductive fractures are most likely open fracture that may become good conduit connecting production zone to mobile water zone.