Abstract Fracture spacing is an important concept for characterizing flow properties of naturally fractured reservoirs, since the main function of fractures that separate matrix blocks is transporting fluids through long distances; however, the estimation of fracture spacing presents some difficulties mainly due to the fact that fractures occur at different scales, going from microfractures in thin sections and minifractures in cores, up to macrofractures in geological outcrops. The scale of interest in this work is that used in reservoir simulation, which is of the order of feet or meters.
This article is based on the ideas developed in a previous paper, where a procedure to locate fractures is presented. That procedure, which makes use of resistivity data obtained through well logging, visualizes the fractures as highly conducting channels within a low conductivity medium (the rock matrix). By using a special way of data processing, it is possible to filter out data that are not associated with fractures, keeping only those data related to fractures. In this way, fracture spacing can easily be estimated. However, that procedure exhibits some uncertainties which must be overcome to make it a more reliable one.
In this work, a study is made to search for an improved procedure to estimate fracture spacing. For this purpose, fractures are considered at two scales: local scale which includes micro- and minifractures present in matrix blocks, and at reservoir scale which refers to fractures separating matrix blocks. These latter fractures, called principal fractures, constitute the main fracture network, and are the subject matter of this work.
Conductivity studies reveal that local scale fractures have a frequency distribution quite different from that of principal fractures. As it will be seen below, this fact facilitates establishing a procedure for estimating fracture spacing without uncertainties.
To make the ideas clear, an application to a carbonate reservoir is presented. The results obtained show that the improved procedure is a simple, reliable, and practical tool for establishing the distribution of fractures along a well, from which fracture spacing can be inferred.
Introduction Non sealed fractures in naturally fractured reservoirs are high conductivity channels; hence, fracture spacing is a factor that controls, to a great extent, the flow properties of such systems. In spite of its importance in areas such as hydrology, geology, geophysics, and petroleum engineering, the problem of estimating fracture spacing has not received the proper attention from researchers, and the specialized literature presents relatively few works treating in depth this theme. Among the currently used techniques for detecting fractures are well testing, core analysis, direct outcrop observation, and well logging.1–4 In this work, an improved way to determine fracture spacing is approached.
In a previous paper,5 a procedure for estimating fracture spacing was developed. That procedure is based on data analysis of formation resistivity factor obtained through well logging. The fundamental consideration of the procedure is that fractures are high conductivity anomalies in a low conductivity medium (the matrix) and, consequently, the basic tool for studying fracture spacing is based on the detection of contrasts in electrical conductivity. To this end, a special analyzing process is used to distinguish between data associated with fractures and non-associated. However, such a procedure does not allow establishing with certainty a discriminating threshold between both types of data.
The fractures referred to in this work are those surrounding matrix blocks. These fractures, called principal fractures, constitute the main fracture network, which has the property of transporting reservoir fluids through long distances, and eventually to the producing wells, in opposition to micro- and minifractures which act at block scale, and whose main function is to convey fluids within the matrix blocks and towards the principal fractures.