Petrophysical facies modeling plays a key role in reservoir characterization at all levels. At a well level it helps to delineate the layers on basis of certain similar rock physics characteristics, which further can be used in reservoir engineering computations that include layer wise input of properties. At a field level petrophysical facies helps in mapping of reservoir units in a multi-well scenario.
Pressure transient tests are performed to determine the reservoir properties like horizontal permeability (Kh), vertical permeability (Kv), skin, knowledge of reservoir boundaries and understanding the reservoir structure up to a level etc. All these are used in the field development planning (FDP).
Conventionally, in a pressure transient interpretation a reservoir layer is taken as homogenous, i.e., the reservoir properties are taken uniform across the thickness of sand unit. In highly heterogenous reservoirs, this approach may lead to under-estimation or over-estimation of permeabilities, since a homogenous layer doesn't consider the vertical heterogeneity within the layer. Hence, to address the vertical heterogeneity, multi-layer reservoir model is used in pressure transient interpretations. Each of these layers can be treated as a petrophysical facies.
This paper discusses various ways of petrophysical facies modeling and showcases the usage of these layered reservoir models in pressure transient interpretations. The results from both conventional as well as multi-layered model are compared in different type of reservoir sands.
It is observed that a multi-layer reservoir model gives better results for vertical and horizontal permeabilities in a vertically heterogenous reservoir. The degree of layer division defines the vertical resolution or refinement of permeability values. In a homogenous sand unit, the conventional model can be used up to a certain degree of accuracy.
Vertical Interference tests (VIT) are used to determine the hydraulic connectivity between the formation sand intervals. This paper showcases an innovative workflow of using the petrophysical log attributes to characterize a heterogeneous reservoir sand by making use of ANN (Artificial Neural Net) and SMLP (Stratigraphic Modified Lorentz) based rock typing techniques as well as image based advanced sand layer computation techniques.
Vertical interference test is either performed using a wireline formation testing tool with multiple flow probes deployed in a vertical sequence at desired depth points on the borehole wall or using a drill stem test configuration. Based on the test design, flow rates are changed using downhole pumps, which induces pressure transients in the formation. The measured pressure response is then compared with a numerical model to derive the reservoir parameters such as vertical permeability, hydraulic connectivity etc. The conventional way of model generation is to consider a section of reservoir sand as homogenous, which generally leads to over estimation or underestimation of vertical permeabilities. The technique proposed in this paper utilizes advanced logs such as image logs; magnetic resonance logs, water saturation and other advanced lithology logs to obey heterogeneity in the reservoir model by utilizing ANN/SMLP based rock-typing techniques. These rock types would be helpful in making a multi layer formation model for the VIT modeling and regression approach. The vertical interference test model is then used to determine the vertical permeability values for each of the individual rock types. The paper displays the workflow to utilize the rock type based layered formation model in vertical interference test modeling for a channel sand scenario.
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