The presented reservoir in this paper is a tight carbonate reservoir with vertical and horizontal heterogeneous properties. In particular, the low clay content in this reservoir gives low gamma ray counts, making difficult the layer identification. In addition, the high-resistivity response of hydrocarbon-bearing pay-zones and of the tight layers make more difficult the identification of the reservoir sweet spots. Slim-hole magnetic resonance (MR) logging was deployed in wash-down mode for identifying reservoir sweet-spots for providing lithology-independent porosity and formation fluid characterization. The well placement over a deep elongated anticline with steep dips and the lowering of the production liner are challenging. Consequently, the field is being developed using horizontal wells that cut across multiple reservoir layers to maximize reservoir contact and drainage. MR was acquired with dual wait time enabled T2 polarization to differentiate between moveable water and hydrocarbons. After acquisition, the standard deliverables are partial porosities and permeability index. The porosity is divided into clay-bound water (CBW), bulk-volume irreducible (BVI) and bulk-volume moveable (BVM). The effective porosity, the permeability index and the permeability-effective porosity ratio were used to evaluate the rock quality. The classification was based on the gradient of the permeability-effective porosity ratio, where a steeper gradient is interpreted as high flow zone, a gentle gradient as low flow zone and a flat gradient is considered as tight baffle zone. Based on the MR flow units and fluid types (CBW, BVI and BVM), the drilled interval was classified into six compartments of high flow, one compartment of low flow and five tight baffle zones. Accordingly, the perforation plan was optimized to fit the high flow units only. Comparing to the conventional log analysis, MR excluded approximately 700 ft of non-productive reservoirs, leading to more than 250,000 USD cost savings in perforation, drilling time and production optimization. The well was drilled with a distance-to-bed boundary tool; however, enormous potential exists to use this slim MR tool as a non-radioactive sourceless solution for geosteering through reservoir sweet spots while delivering wells that are more productive safely.
The Najmah and Sargelu, two tight carbonate reservoirs of Jurassic age, are oil bearing in the West Kuwait fields Dharif and Abduliyah. Due to their complexity, presence of numerous natural fractures and low matrix porosity and permeability, these reservoirs have been underdeveloped in both fields. A multi-disciplinary integrated study has been performed to thoroughly investigate their potential and design the appropriate future development plans. This paper shows how the use of the 3D seismic data and the rock typing exercise has been maximized to constrain the derived models and reduce the uncertainties. The main reservoir horizons (top Najmah, top Sargelu and top Dharuma) have first been interpreted using the 3D seismic cube and major faults were picked on vertical seismic section and time slices. A seismic characterization work aimed at enhancing seismic data and better describing both matrix and fractures in between wells was also performed. The seismic fracture characterization study led to a set of seismic fracture index maps detailing the fault/fracture corridor network. The matrix characterization study provided an average shale proportion map in Najmah to guide the shaly facies proportions in the geological model. From the available well data, mainly core description and classical logs, a detailed sequence stratigraphy and petrophysical interpretations were performed. Electrofacies were then computed at wells before being propagated in 3D in a detailed geological model combining both fields. Finally analysis of the natural fracture network was made using multiple sources of data: core description for fractures, image logs interpretation, seismic maps, transient well tests, production data … A Discreet Fracture Network (DFN) model was derived and then dynamically calibrated. The equivalent fracture properties (porosity, permeability and fracture spacing) were computed and imported into a dual porosity – single permeability simulation model. Using the limited production and pressure data in both Dharif and Abduliayh fields, the dynamic models were adjusted to fit the production history: uncertain oil-water contacts depth were modified and small modifications of fractures properties in around one of the well was performed. The calibrated dynamic models were then used to test several development scenarios. Dharif potential, due to the presence of a thin oil column, was shown to be limited. Abduliyah potential, on the other hand, is likely to be greater if the fracture network is proved to be as permeable as in Dharif. If this is confirmed, a development with horizontal producers located at the crest of the structure is likely to be the most suited method.
Najmah-Sargelu Formation of Middle Jurassic is tight, fractured Carbonate reservoirs, spread across many fields in Kuwait. These reservoirs are often vertically and laterally heterogeneous because of depositional variability and diagenetic alteration through space and time. Understanding the distribution of hydrocarbons in relation with porosity / permeability heterogeneities is thus of major importance in effective field development and production. A proper sedimentological model is then a mandatory step in matrix characterization to make an accurate numerical reservoir model, predicting fluid flows through time in order to support development scenarios. This paper presents an overview of efforts in building a sedimentological model based on the analyses performed on cores penetrating the Najmah and Sargelu Formations of the Umm Gudair field. The data obtained from the description of cores allowed defining a sedimentary facies scheme that served a basis for building of a 3D sedimentological model. Dominant sedimentary structures and bioturbation traces have been taken into account for the facies classification. The apparent differences in texture between the core-based lithotype scheme and the thin-section based microfacies scheme is due to intense micritization of grains.This model shows a moderate-energy carbonate ramp with a relatively flat morphology, further subdivided into three major depositional environments: The mid ramp, outer ramp, and basin. The basin includes: evaporitic conditions, restricted/anoxic conditions, unrestricted/oxygenated conditions. The basinal deposits are mostly characterized by shaly mudstones that host the deposition of a calciturbidite sheet complex. The mid-and outerramp settings are mostly dominated by relatively fine sedimentary textures where the structures are obliterated by intense bioturbation. TX 75083-3836, U.S.A., fax +1-972-952-9435
The Umm Gudair field is an elongated doubly plunging anticline which bifurcates into two branches NS elongated West UG and the NE-SW elongated East UG structures with a broad saddle in between Fig1A & 1B. West Umm Gudair structure was formed during the Jurassic time and become reactivated in Cretaceous and Tertiary times. East Umm Gudair was formed during Cretaceous as evident from the thicker deposition of overlying Gotnia sediments and its unique fold geometry. A total of 28 wells targeting Najmah-Sargelu (NJ/SR) & Marrat reservoirs of Jurassic have been drilled so far in this field including one deep well in South UG in Divided Zone The Marrat Formation in Umm Gudair field is conventional reservoir with 800ft to 1000ft thick sequence comprising limestone, dolomitic limestone and argillaceous limestone with minor shale and anhydrite. The Jurassic sequence in Kuwait oil fields starts from around 12000ft and is marked by an increase in formation pressure and temperature. Above this lies the Cretaceous sequence where most of the oil field development works have been concentrated in the past. In the last two decades field development has extended to the Jurassic sequence to meet up with increasing production demands. The Jurassic sequence is marked by a series of high pressure and high temperature formations with presence of natural fractures.
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