Fracture Geometry Characterizations through Physical Modeling

Abstract

Abstract Fractures have been known to exist in reservoirs for the last half century, yet the practice of characterizing fractured rock reservoir system has been extremely slow. Why is this so? The greatest contributor to this point of view is that fractured reservoirs are extremely complex. The complexity is attributed to vast number of both dependant and independent geometrical variables that dictates final reservoir response. Fracture characterization is essential step to understand the overall reservoir performance, and to accomplish this, it is imperative to integrate all facets of information to achieve optimization of permeability response. A basic physics-understanding is absent of the fracture morphology that commonly occur in naturally fractured-reservoirs (NFR). This knowledge will help understand flow and recovery patterns in fractured reservoirs. For example, fracture-population and fracture-spacing, fracture area (length & width), fracture opening (fracture-porosity), and fracture-orientation. The model will simulate several scenarios of hypothetical fracture geometries in pursue of overall reservoir permeability. Anyone who has dealt with natural fractured reservoirs will realize that these variables are few of other numerous variables that when combined properly would have a better prediction for the reservoir overall performance. Systematic fractured reservoir physical-models are proposed for this study. It is known that almost all fractured reservoirs respond in a unique way according to the class of their fracture. Therefore, the principal objective of the study is to construct a physical model of an artificial permeable reservoir that will host many sets of controlled fracture geometry and morphology. These fractures will be designed and tested. Further these fractures will be studied and modeled in order to seek fundamental knowledge of their impact on total reservoir performance. Objectives The overall objective of establishing the fractured reservoir physical model research is to study the impact of fractures geometry and their distribution on reservoir performance. The study proposes detailed and integrated characterizations of naturally fracrured reservoirs - NFR - reservoirs using six different characterizations of fracture parameters: length, aperture (width), orientation, density, spacing, and porosity. Introduction Fractured rocks that comprise the reservoirs are formed in variety of geometric shapes due to dynamic diagenesis. Some of the dimensions of these fracture masses are very large; it is impossible to observe from a single vantage point on the ground the overall pattern of their distribution and relationships to other rock masses with which they are in contact. Therefore, scientists and engineers have developed techniques that enable them to understand the interrelationship of rock masses and their fractures and thereby make some interpretations regarding the extent to which they have been deformed since the time they originated (1). Fractures are present in all rock subsurface formations. Their physical character of these fractures is dictated by their mode of origin, the mechanical properties of the host rock, and subsurface diagenesis. These factors combine to develop a feature that can either increase or decrease reservoir porosity and permeability. Fractures when they occur in sufficient spacing or length that their effect on fluid flow becomes important. To accurately assess this effect (either negative or positive) it is important to know the fluid flow properties of individual fractures and how many of these fractures of a given orientation exist in a given reservoir volume (1). The origin of fracture system is postulated from data (if available) on fracture dip, morphology, relative abundance. Often times, these data obtained from full-diameter oriented core, borehole imaging tools, and applied empirical models of fractures generation (1). Available fracture models range from tectonic to of primarily digenetic origin. The interpretation of fracture system origin involves a combined geological/ rock mechanics approach to the problem. It is assumed that natural fracture patterns depict the local state of stress at the time of fracturing, and that subsurface rocks fracture in a manner qualitatively similar to equivalent rocks in laboratory tests performed at analogues environmental conditions. This study is different than other fracture studies in terms of studying the origin of fractures. This study is concerned with studying the effect of the petrophysical determinations of the rock matrix in which the fracture system resides. It is also to determine the reservoir properties as that is detrimental to the fluid flow.