Structurally-Enhanced Carbonate Matrix Reservoirs: Utilising Tailored Conceptual Models and Relative Deformed Reservoir Rock Volumes to Understand Flow Behaviour, Fracture Modelling Requirement, and Potential Development Pitfalls.
Authors: Edwards Ewart , Shekhar Ravi 
Published: November 2019

Structural characterization of faults and fractures is a key component of reservoir description, however, data consolidation often does not allow decisions on modelling or explanation of fluid flow anomalies. For the task of assessing fracture modelling need, data has often not been gathered systematically. Most carbonate fields’ models in the area require permeability modification/enhancement but where do these sit with regard to K multiplier modifications versus DFN models, for example? A key conundrum i s that most conventional UAE hydrocarbon fields would, according to the Nelson (2001) classification, be matrix-dominated reservoirs with fracture enhancement within comparatively open, low-strain, 4-way dip-closed structures, making them theoretically marginal to fracture involvement. However, within stacked sequences of contiguous reservoirs, or due to fault damage zone associated fracturing effects, the specific classification of individual zones within a field may vary between entirely matrix-dominated to highly fracture-dominated reservoirs. It then becomes difficult to determine whether fracture modelling or proxy methods of enhancing permeability is the appropriate practical way forward. This paper addresses a means to assess this need, based on conceptual models and relative deformed rock volume. A means to pre-assess the need for fracture modelling and to understand the role and need for fracture assessment is a key first step founded on basic structural principles and data. A set of Early Cretaceous reservoirs from a supergiant offshore Abu Dhabi oilfield, as well as single reservoir examples country-wide, are utilised to illustrate this concept.

Building conceptual models involves collaborative structural geology to develop a concept, including static and dynamic data relevant to the structural components present at all scales (faults, fracture corridors, layer-related mechanical fractures). The concept may include fault character and temporal evolution, along with dynamic data for understanding connected flow paths as much as possible. Once built, variants can be generated around the base case, reflecting uncertainty of structural aspects or specific sub-zone nuances. A series of conceptual models considers the variation between different stacked reservoirs and end-member variants for a given single reservoir within different structural domains or different traps. Comparing the conceptual models with standard ‘fractured field’ classifications allows determination of the type of reservoir (Nelson, 2001, scheme) and hence likelihood to requiring modelling by fracture enhanced K-multipliers, DFNs, or dual-media models.

Assessing the level of deformation, as a bulk strain within a carbonate reservoir, provides a ‘relative deformation volume’. Proportionally, this is the volume of rock within the reservoir that contains visible or mapped deformation, as per conceptual model features, where at least a component of fluid flow would be controlled by such discontinuities. Assessment can be for an entire field, for specific reservoir(s), or for a specific geologically defined flow domain, and offers unhindered comparisons. Analysis of a series of different stacked reservoirs, with varying matrix properties, in the same field, are ranked according to deformed volume, which shows relationships with (a) dominance of fractured layers and (b) water cut evolution and hence future field development options. Visual assessment of the conceptual model allows differences between reservoirs and variants within a reservoir that reflect different effects of structure (e.g. trap shape, fault-proximal effects) to be compared. Using conceptual models with relative deformation levels, the appropriate need and means of fracture modelling can be quickly established.