A dedicated appraisal campaign and modeling study was carried on a heavy-oil, fractured Shuaiba field in the north of the Sultanate of Oman to assess the feasibility of steam assisted gas-oil gravity drainage (SAGOGD) EOR. In this field, key to a successful SAGOGD is a well-connected fracture network, which was investigated by a dedicated appraisal campaign which included drilling one vertical and 3 near-horizontal wells. BHI, sonic and resistivity logs were run to understand static fracture characteristics; dynamic behavior was assessed with DSTs and PLTs. Fracture models were built with forward modeling algorithms using Shell's fracture modeling software (SVS Fracture-Solution). They are based on fracture characterization that integrates the well data with 3D seismic, field kinematic structural evolution and the regional understanding established by Petroleum Development Oman's (PDO) long-term activities in the area. This integration makes the fracture models more realistic than purely stochastic fracture models that only honor the statistical field data.
Interdisciplinary collaboration during data acquisition and all stages of the modeling assured that only the features relevant to the dynamic behavior of the reservoir were modeled while at the same time the geological details of matrix and fracture models were captured sufficiently in the simulator. Simulations were run in dual porosity dual permeability thermal mode in Shell's dynamic simulator (MoReS). The results are fully integrated model scenarios that can be compared quantitatively with all other PDO fractured Shuaiba fields, facilitating management's technical decision making on the EOR strategies for the fields.
Introduction
Five oil fields located in the Ghaba Salt Basin (GSB) in North Oman and producing from fractured, low permeability limestones of the Cretaceous Shuaiba formation have been studied to assess the feasibility to implement EOR techniques. The subject field (Field B) contains very viscous oil (16 API, 550 cP) in a low permeability (5–20mD) limestone matrix. Based on observations of high fracture density in all Shuaiba fields in the GSB and following the successful SAGOGD pilot project in the nearby Field A (also a Shuaiba reservoir and only 8km away - Ikwumonu et al., 2007), SAGOGD was evaluated as a suitable EOR technique for Field B. For this thermal EOR technique to work successfully in such a setting as Field B, a vertically connected network of natural, open fractures with small spacings is critical (cf. Rawnsley et al., 2005). In a low permeability matrix the fractures are critical for distributing the heat of the injected steam into the reservoir to reduce the viscosity of the oil. The fractures also need to form a dense enough, connected network from the gas cap in the top of the reservoir (formed by the steam injection) to the FWL, to enable gravity-driven oil drainage vertically through the matrix (see Penney et al., 2007 for details). The drained oil then forms a rim in the fractures near the FWL from which it can be produced (see e.g. Wassing et al., 2008 for details).
Obtaining static and dynamic data of the fracture network was hence the main focus of an interdisciplinary study carried out in the PDO Study Centre. With existing and newly acquired seismic, well and interference data and guided by the regional understanding established in PDO, a series of matrix and fracture models were built and cycled with the reservoir simulator. This paper focuses on the fracture characterization and modeling. It exemplifies the value of planned multidisciplinary data acquisition and integration, combined with utilization of regional analogues to predict fracture densities away from wells and to simulate the dynamic behaviour of a fracture network.
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