An Integrated Geology and Reservoir Engineering Approach for Modelling and History Matching of a Vietnamese Fractured Granite Basement Reservoir
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Advances in Granitic Basement Reservoir Evaluation
Granitic basement reservoirs have been the focus of increasing attention in the Asia Pacific region in recent years, following several new oil and gas discoveries in this complex reservoir type. Accurate formation evaluation in fractured, crystalline, granitic reservoirs is notoriously difficult. Furthermore relatively little research has gone into understanding logging tool response or pressure transient behavior in these reservoir types, and developing suitable workflows for formation evaluation. In this paper the authors propose an improved methodology for integrating various open-hole logs, production logs and well test data to better evaluate the reservoir potential of a fractured granitic formation. Since the wells are either horizontal or highly deviated, the framework also serves as one of the primary methods to asses the lateral extent of the reservoir. A case study from the region is used to illustrate the workflow. Image log interpretation, advanced acoustic measurements, nuclear logs and production logs with distributed local sensors are combined with well test data to derive the best possible evaluation of the fracture network around the borehole and the degree of connectivity with the reservoir at large. The advantages and limitations of the proposed workflow are also discussed and the stage set for further work in this complex environment. Background Fractured granitic reservoirs differ from other types of fractured reservoirs in that they are generally considered to have no primary porosity. All the pore space in the rock is formed through fracturing and diagenetic processes. The resulting pore structure heterogeneity makes formation evaluation extremely challenging. Another complicating factor is the lithology, where the granite composition can vary and sub-vertical extrusive dykes cause abrupt lateral variations in matrix properties. This paper deals specifically with granitic oil bearing formations from southern offshore Vietnam. The typical basement reservoir consists of a faulted basement high. The majority of the porosity is thought to be associated with fractures and fault zones. High fracture density enhanced by hydrothermal alteration forms the majority of the effective pore space. Unlike basement plays in other regions the weathered zone at the top of the structure and the dykes are usually not productive, though the interface between dykes and host rock can be (Le Ngoc 2007, Nguyen 2003, Tandom 1999). In this environment the majority of new wells drilled are highly deviated or horizontal, and drilled to intersect sub-vertical fault zones. Most of the fractures are small and form the storage capacity of the reservoir. To be productive, a well should also encounter enough larger, permeable, fractures which are sufficiently connected to the storage capacity of the reservoir. The objective of the following interpretation methodology is partly to quantify as much as possible the total fracture porosity, but mainly to identify the large permeable fractures which are necessary for the well to be productive.
Advances in Granitic Basement Reservoir Evaluation
Granitic basement reservoirs have been the focus of increasing attention in the Asia Pacific region in recent years, following several new oil and gas discoveries in this complex reservoir type. Accurate formation evaluation in fractured, crystalline, granitic reservoirs is notoriously difficult. Furthermore relatively little research has gone into understanding logging tool response or pressure transient behavior in these reservoir types, and developing suitable workflows for formation evaluation. In this paper the authors propose an improved methodology for integrating various open-hole logs, production logs and well test data to better evaluate the reservoir potential of a fractured granitic formation. Since the wells are either horizontal or highly deviated, the framework also serves as one of the primary methods to asses the lateral extent of the reservoir. A case study from the region is used to illustrate the workflow. Image log interpretation, advanced acoustic measurements, nuclear logs and production logs with distributed local sensors are combined with well test data to derive the best possible evaluation of the fracture network around the borehole and the degree of connectivity with the reservoir at large. The advantages and limitations of the proposed workflow are also discussed and the stage set for further work in this complex environment. Background Fractured granitic reservoirs differ from other types of fractured reservoirs in that they are generally considered to have no primary porosity. All the pore space in the rock is formed through fracturing and diagenetic processes. The resulting pore structure heterogeneity makes formation evaluation extremely challenging. Another complicating factor is the lithology, where the granite composition can vary and sub-vertical extrusive dykes cause abrupt lateral variations in matrix properties. This paper deals specifically with granitic oil bearing formations from southern offshore Vietnam. The typical basement reservoir consists of a faulted basement high. The majority of the porosity is thought to be associated with fractures and fault zones. High fracture density enhanced by hydrothermal alteration forms the majority of the effective pore space. Unlike basement plays in other regions the weathered zone at the top of the structure and the dykes are usually not productive, though the interface between dykes and host rock can be (Le Ngoc 2007, Nguyen 2003, Tandom 1999). In this environment the majority of new wells drilled are highly deviated or horizontal, and drilled to intersect sub-vertical fault zones. Most of the fractures are small and form the storage capacity of the reservoir. To be productive, a well should also encounter enough larger, permeable, fractures which are sufficiently connected to the storage capacity of the reservoir. The objective of the following interpretation methodology is partly to quantify as much as possible the total fracture porosity, but mainly to identify the large permeable fractures which are necessary for the well to be productive.
The development of basement reservoirs can add significant upside to regional reserves but because of perceived drilling complexity are often overlooked or underexplored. Modern drilling technologies, such as directional drilling and formation evaluation while drilling, have been shown to improve commercial viability of appraisal and development projects in frontier areas. A granitic Type-1 fractured basement reservoir offshore West of Shetland was horizontally drilled and evaluated by means of directional drilling (DD) and logging-while-drilling (LWD) technologies. This was the first time that drilling and completion of a 1-km horizontal basement well had been attempted in the UK, and the project goals were successfully achieved. The project as planned fell into the extended-reach drilling (ERD) envelope, defined as a well in which the ratio of measured depth (MD) is greater than two times the vertical depth. The ERD ratio [MD to true vertical depth (TVD)] was 2.21. This success resulted from a combination of drill bit selection, bottomhole assembly (BHA) stabilisation optimisation, rotary steerable drilling system usage, and close cooperation between the operating company and service providers. The logging data acquired were of sufficient quality to support detailed petrophysical study. The basement reservoir section was shown to have significant storage capacity within a network of fault zones, joints and microfractures, as identified using a LWD resistivity microimaging tool. The LWD imaging data were compared to offset well wireline imaging data and used to evaluate the microfracture and joint network across the horizontal section. This evaluation confirmed the storage capacity of the reservoir and was later supported by successful commercial extended well testing. In addition to fracture characterisation, the LWD data were successfully used to evaluate the fracture porosity. Fracture porosity and fracture characteristics in the horizontal well were both consistent with those derived from previously drilled inclined wells, providing the operator with confidence in its reservoir model. This paper documents the drilling and petrophysical evaluation of the first 1-km horizontal well in a fractured basement reservoir on the UK continental shelf. Further development of similar hydrocarbon-bearing basement fields would greatly increase regional recoverable reserves.
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