Geological Modeling Of Reservoir Systems – An Adaptive Concept

Summary The construction of realistic geological models of petroleum reservoir systems is an essential prerequisite to the design and implementation of their development scenarios. Unlike most of the conventional approaches of geological modeling, an adaptive concept takes into account the uncertainty and incompleteness of initial data. First, the adaptive models have the number of layers no more than can be distinguished by means of detailed correlation. Typically, the adaptive models consist of 8 to 16 layers with an average layer’s thickness of 5 to 10 m. “Mechanical cutting” of the layer’s thickness down to the traditional 0.4 m is added nothing to the information content of the model, instead such tiny layers make a false representation. When a certain layer is selected, it is implied that it is exactly correlated over the entire region of the model, but this is completely wrong. If it cannot be correlated using logs, then such layer has no justification and only creates a precedent, asserting that there is not in reality. A 95% of any geological model quality depends on the correctness of detailed correlation. According to the seventh Shanonn’s theorem, no one mathematical transformation can increase the amount of information. Only time and energy spending might increase it. This is the time and effort of a specialist who makes detailed correlation and creates new information, which initially was not in logs. This information is the input data of the model and thereby increases its quality. If there is not time and energy spending in detailed correlation, then the model is reduced to a pure formality. The layer selected according to detailed correlation is a separate geological subsystem with its own historical evolution, which is reflected in the distribution of gross and net pay thicknesses and other petrophysical parameters. Therefore, in the adaptive concept, a multilayer geological model is constructed using the superposition principle. At first, the geological models of separate layers are built entirely independently, and then they are summed up into a multilayer model. Historically, the sedimentary cover is formed successively layer by layer in its own paleogeographic environment. Because of this, it is fundamentally incorrect to apply any 3D interpolation. When the multilayer model is constructed in a layer by layer mode, the uniqueness of each layer can be distinguished avoiding their averaging. In in the adaptive concept, traditional methods of interpolation are not used. The basis of the model’s construction is the seismic data – the structural surfaces of reflecting horizons. There should be at least three such surfaces. They have everything that is needed. The distances between them show the rates of sedimentation, the absolute marks - the structure of the section, and the degree of curvature - the tectonic stresses that affects the properties of permeable formations. The essence is that there is a vector of seismic data both at wells and in the inter-well space, so a multi - parametric fuzzy logic function can be created by means of which any geological parameter can be obtained from the vector. At the same time, such function cannot be one for the entire region of the model, that is why, a so-called fuzzy grid is constructed that is a grid with a step of 200 – 300 m which nodes contain local functions connecting the geological parameter with the seismic data.