Characterizing Permeability for the Fractured Basement Reservoirs

Abstract

Permeability is one of the major controls for production in fractured basement. It is critical to be able to identify and characterize permeable zones in the basement reservoirs not only for evaluating well producing potential but also for designing perforation, well completion and injection. Even though geoscientists and reservoir engineers have made considerable effort over the years to better characterize the permeability profile in fractured basement, it is still very challenging to achieve this objective. This paper presents a newly developed approach aimed at better understanding the permeability profile. The technique was developed based on case studies from wells in two different basement fields located offshore in the south of Vietnam. Traditionally, fractures have been characterized using core and borehole image data. In this study, borehole image data were integrated with other open-hole logs to derive a permeability curve. The result was calibrated with core data and then validated with dynamic data i.e. production log data, well testing data, mud losses, gas shows. As demonstrated in the case studies, it is believed that the permeability in the basement could be reasonably evaluated using this method. However, in a zone where fractures are cemented or partially cemented with conductive minerals, the log-derived permeability needs to be calibrated with the degree of cementation. Introduction Permeability is one of the most difficult parameters to assess in a reservoir and it is even more challenging when the reservoir is fractured. There is no proved mean for directly measuring the permeability in fractured basement except for well testing and coring, which imply high cost. In addition, well testing only provides permeability height product KH based on many "best guess" assumptions and can lead to unclear range of contacted hydrocarbon volumes. Core analysis usually focuses on the worse portion of the reservoir due to the fact that core recovery has rarely been good in a highly fractured zone. Therefore the permeability measured from the core sample is often not representative. Reservoir modeling could provide critical insights to the permeability system in fractured basement. However, the model is stochastic or based on fault line objects, and is grossly constrained by dynamic pressure data but rarely by well data. This paper focuses on new techniques that allow multiple sources of information to be incorporated into the estimation of permeability along the path of a wellbore in the reservoir. These permeability properties can then be incorporated and upscaled into reservoir models to provide more accurate representations of the fractured reservoir. Fracture Basement Reservoir of the CuuLong Basin The fractured basement reservoirs of CuuLong Basin are prolific hydrocarbon bearing and naturally fractured. Individual wells in these fractured basement reservoirs are capable of sustained production in excess of 20,000 bbls/d. The fracture systems are present in 4-way dip closed structures and have been reviewed by Long(1), Guttormsen(2), Schmidt(3), Cuong(4), and Olson(5). The productive CuuLong basement structures are oriented NE-SW with range of structural styles from large flat horst blocks to half horst structures (Figure 1, map of CuuLong Basin). These structures are typically bounded by strike slip faults that exhibit positive and negative elements along their strike. The largest structural complex in the block is the Bach Ho (White Tiger) structural complex and particularly its prolific central dome (Figure 2, Map of major producers in CuuLong Basin). The individual structures exhibit a variety of internal structural complexities as they accommodated stress during the complex structural history of the basin. The horst reservoir is a granitiod body with a history of poly-phased intrusives. Post emplacement tectonic events related to the intrusives can be summarized into seven episodes(3). These are from oldest to youngest:Pre-Rift Magmatic upwelling and Paleozoic metasediments deformationPre-Rift active poly-phased intrusive and cooling phasePre-Rift Pre-basin compressional phase (deformation cycle 1 - D1, D2)

Basement exploration, West of Shetlands: progress in opening a new play on the UKCS

“…Naturally fractured crystalline basement reservoirs (basement reservoirs) are a global phenomenon, as depicted in Figure 3 (see also Aguillera 1995;Nelson 2001;Petford & McCaffrey 2003;Gutmanis et al 2012). Despite the proven commercial success of basement reservoirs it has long been recognized that basement reservoirs are a globally underexplored play (Eggleston 1948;Hubbert & Willis 1955;Landnes 1959;Landnes et al 1960;P'an 1982).…”

“…The Lancaster reservoir is the Lewisian Basement and is considered a Type 1 fractured reservoir (Nelson 2001). Type 1 fractured reservoirs owe their productivity and storativity to a hydrodynamic fracture network which is a subset of the natural fracture network, consisting of fractures that are spatially connected and collectively capable of transmitting fluid.…”

“…The identification of the prospective fracture types present in the Lewisian basement is a key step in developing a conceptual exploration model; however, it is also important to evaluate the potential for epithermal and hydrothermal processes to have contributed to the properties of the hydrodynamic fracture network, as the analysis of global analogues indicate that such processes are important geological components in producing basement fields (Dimitriyevskiy et al 1993;Petford & McCaffey 2003;Li et al 2004;C&C Reservoirs 2005;Geoscience 2007). The Lewisian Basement has a long geological history of subaerial exposure and orogenic activity; there is therefore significant potential to enhance fracture network drainage through epithermal and hydrothermal processes.…”

An Integrated Workflow for Granitic-Basement-Reservoir Evaluation

“…Fracture-aperture and related fracture-porosity estimation, as well as fracture classification using electrical image logs, has been well documented (Luthi 1990;Li 2004;. This technique can estimate fracture aperture on the basis of the increased conductivity in front of a fracture.…”

Parallel, Multigrid Finite Element Simulator for Fractured/Faulted and Other Complex Reservoirs based on Common Component Architecture (CCA)

“…Sibbit (1995) quantified porosity from well logs in fractured basement reservoir and estimated permeability from that field. Li et al (2004) describes that permeability is one of the most difficult parameters to assess in a reservoir and it is even more challenging when it is the basement reservoir. They also report on a method to quantitatively evaluate basement reservoir's 5-5 permeability based on image logs with the integration of other open-hole logs, mud gas data, drilling data, dynamic well testing and production logging data.…”