Chinmaya Pattnaik

Structural Evolution Model for the North Kuwait Carbonate Fields and its Implication for Fracture Characterisation and Modelling

This paper presents a new structural model for the North Kuwait Carbonate fields as well as its implications in term of fracture modelling and field development. It also describes a workflow which can be used as foundation for further fracture modelling study at production and exploration scales alike. This workflow consists of a four step approach: 1) elaboration of a regional structural model, 2) creation of 3D conceptual fracture diagrams, 3) elaboration of constraints capturing the key elements of the conceptual diagrams and 4) creation of fracture model properties for further dynamic simulation. The application of this workflow resulted in the creation of a series of fracture models for the North Kuwait Carbonates fields. During the first step of the study, a new structural model has been elaborated based on key kinematic observations from well and seismic data, as well as experimental and field analogues which have been linked to the known regional phases of deformation. These main phases of deformation are 1) post Triassic rifting, 2) Alpine 1 - late Cretaceous transtension and 3) Alpine 2 - Mid Tertiary compression related to the Zagros formation, which has the greatest impact on the formation of the pre-Gotnia structures and fracture development. The major difference between the new model and previous structural thinking is that the formation of the compressional folds in the Carbonate fields (an event that shaped the current outline of the fields) has happened during the Tertiary time instead of Jurassic time. The proposed structural evolution has been used to define characteristic structural domains. These structural domains have defined a foundation to elaborate conceptual fracture diagram to support fracture modelling study work. The fracture conceptual models have potential implications on fracture development and preferred direction of horizontal and deviated wells. Greater fracture connectivity is expected in compressional ridges developed in Tertiary time, while in the area between the compressional ridges, less dense fractures and probably more cemented fractures (likely to have developed before hydrocarbon emplacement ) are expected. The new view on the timing of the structural development (i.e., late uplift of compressional ridges regionally) also has possible implications on maturation/charge history as well as reservoir properties development. The new proposed model for structural evolution is now being used as a foundation for appraisal and fracture modelling activities of the pre-Gotnia carbonate reservoirs. A fracture characterisation study integrating all available static and dynamic data is ongoing.

Use of High Resolution Sequence Stratigraphy in Building 3D Reservoir Geological Model - A Case Study from Kuwait

A number of structures are producing hydrocarbons in commercial quantity from the carbonate reservoirs of Middle Marrat Formation in the Northern part of the State of Kuwait. Seismic resolution of these deep carbonate reservoirs of Toarcian age is strongly affected by multiples from overlying Gotnia-Hith salt-anhydrite layers impacting true understanding of the internal depositional architecture from seismic data making it difficult to make realistic geological models. An extensive coring campaign adopted over a period of time for improving the understanding of depositional settings and diagenetic processes in these reservoirs and subsequent studies helped in developing a correlation based on sequence stratigraphic principles. In the Middle Marrat Formation seven cycles with sequence boundaries and maximum flooding surfaces were identified. The best reservoir facies are located in High Stand Tract, corresponding to the progradation of the carbonate platform in the form of clinoforms sloping towards the basin. 3D geological model was built with a framework base on this correlation capturing the higher order sequences with the prograding and aggrading carbonate shelf. Sedimentary facies were identified from electrologs with complete calibration from core description and ECS calibrated ELAN processing results. Gross depositional environment maps were prepared to guide the distribution of primary sedimentary facies within the model for different depositional environments like Slope/Basin, Outer Shelf, Inner Shelf, Shoreface, Barrier/ Shoal, Backbarrier/ Backshore, Lagoon, Tidal Flat, and Sabkha using templates developed for different sequence boundaries. These carbonates are characterized by seepage reflux dolomitization process and burrows giving rise to enhanced porosity and higher productive layers. This diagenetic imprint was modeled on the primary facies in shoreface and inner shelf areas. This facies model was used to distribute other reservoir properties like porosity, permeability and water saturation using different techniques. The developed model was further calibrated with isotope geochemistry studies and was also validated from comparison with exposed and well studied geological analogue examples. This model showed better predictive capability for wells drilled subsequently. In this carbonate reservoir, a sequence stratigraphic framework holds the key for distributing the reservoir facies within a 3D geological model in a realistic manner giving better predictability of reservoir properties and facies thereby reducing the field development uncertainty.

Field Development and Well Planning in Tight Carbonate Reservoir Using Fracture Characterization and In-Situ Stress Mapping From Core Reorientation Studies: Kuwait Case Study

Fracture characterization is vital for efficient field development of naturally fractured Carbonate reservoirs. Successful development of fracture reservoir in the study area was possible due to early recognition of fracture play right from the exploration phase and through careful execution of relevant data acquisition campaign in the initial stages of field development. Comprehensive and integrated studies have been carried out over the past few years to arrive at an understanding of the conceptual model in deciphering structural evolution of North Kuwait Jurassic. Extensive core and image log data was acquired in the initial stages of field appraisal, which helped in comprehensive forward planning in design of deviated and horizontal wells. The data analysis steps included accurate and reliable reorientation of the cores. These data were calibrated with the image logs, along with available seismic attributes, which resulted in better understanding of structural evolution and sweet spotting of horizontal wells. This reorientation of the core data also helped in establishing a number of quantitative fractures attributes such as frequency, spacing, dip-azimuth and aperture along with mapping of in-situ stress directions. The detailed integration of these data also helped in accurately mapping the local and regional present day stress and its variations spatially across the fields. Stress direction across the field was helpful for deciding the azimuth of wells during well planning along with selection of completion strategy for current set of horizontal drilling Campaign. Drilling and testing results have been encouraging through enhanced reservoir performance in these tight carbonate reservoirs, based on these integrated studies.