Doğal çatlaklı rezervuarların kuyu loglarından saptanması ve saha uygulamaları

Abstract

Bu çalışmada doğal çatlaklı reservuarların kuyu loğlarının yardımıyla değerlendirilmesi ele alınmıştır. Porosite loğları, resistivite loğları ve diğer bası loğlarla yapılan değerlendirmeler olumlu sonuçlar vermiştir. Çeşitli loğlardan elde edilen sonuçlar yardımıyla göseneklilik ve mineral dağılımını gösteren bir log oluşturulmuştur. Ayrıca dipmetre ve kuyu dibi kamerası, (BHTV^ gibi aletler de değerlendirmelerde kullanılma olanağı olmamakla birlikte son derece olumlu sonuçlar sağladıkları dikkate alınarak ayrıca anlatılmıştır.

In this study, methods used for detecting and evaluating naturally fractured reservoirs by means of well logs are evaluated. The naturally fractures may be a research subject for geologists, petrologists as well as drilling, production and reservoirs engineers. From the view point of a log analyst the fractures can simply be defined as seconary porosity. Except Borehole Televiewer there are no direct means for detecting and evaluating fractures in the well. Fractures, consequently, can be evaluated by means of indirect methods. These are well logs and well tests. Even each of these methods have particular- advantages and drawbacks, well logging is generally more preferred than well tests for evaluating fractures. Well logs used for identifying the fractures can be classified in two parts: qualitative identifier and quantitative identifier. Most of logs can just be used for qualitative analysis. The combination of various logs, however, might help for quantitave analysis of fractures. If it is not supported with some other sources, such as core analysis, the results will be ambigious. This study discusses the well logs in three chapters : 1. The first chapter deals with the identification of fractures by using porosity logs. The porosity logs are mainly the sonic, neutron and density logs. In homogenous formations these logs are used for identifying either lithology or matrix porosity. Except the sonic log, the other two logs assume the secondary porosity as a part of total matrix porosity. These logs are used in pair to produce crossplots each of which provides insight into lithology and permits the determination of matrix porosity. Chart selection depends on the anticipated mineralogy. Neutron- density crossplot can be used to differentiate between the common reservoir rocks ( sandstone, limestone, dolomite ), shale and some evaporites. The sonic-neutron crossplot can be used to differentiate between the common reservoir rocks when clay content, is negligible. The sonic-density crossplot can be used to differentiate between a single known reservoir rock and shale and to idetify evaporites. In order to use any of these charts, The required neutron, sonic or density values are entered to "the absisca or ordinate. The intersection point defines the lithology (minerology) and the porosity. The litho-porosity crossplot is introduced for interpretation of complex lithology. It presents simultaneously the data from all three of the standart porosity tools: the sidewall neutron porosity log or GNT, the formation density compensated sonic log and the borehole compensated sonic log. FFrom the reading of these logs two porosity-independent parameters; "M" and "N" are derived. In the plot of M versus N, each pure rock mineral is represented by a unique point regardless of porosity. For a formation with complex lithology, the position of the log data points on the M-N plot relative to the pure mineral points is of great assistance in identifying various minerals in the formation. Lithological information so derived is then used to calculate accurate value of porosity. If there is secondary porosity in the medium, M value will be higher, since the M value implies the sonic log readings. Unlike the density and neutron logs the sonic log reading is affected by the secondary porosity. This effect shows a shift upward in the M-N plot. Dual mineral method is a method for evaluating lithology and porosity. The main drawback of this method is that the formation should contain just a mineral pair of common reservoir rocks: either sandstone- limestone, dolomite-limestone or dolomite- sandstone. After having figured out density-neutron graphic for clean water saturated sones, 0ao and fmv values are obtained. Then a mineral pair is chosen. From the sonic graphic 0s is found The comparision of 0s and 0hd gives the idea of whether the mineral pair is correct or not. If the choice is wrong, another mineral pair will be chosen. Cases where sonic derived porosity is less than those obtained from neutron-density plot are interpreted as to have secondary porosity. The secondary porosity index is defined as the difference between crossplot porosity and sonic porosity. Sonic porosity greater than crossplot porosity in clean zones is used as an indicator that incorrect mineral pair was used in crossplot. VI When porosity logs are used for lithology and porosity evaluation, the results sometimes are far away from each other. There should be such a porosity value and mineral combination that all porosity logs agree on them. This forces one to attempt for solving the problem. Three unknown minerals and an unknown porosity with three equations derived from the porosity logs should be supported by one more equation: material balance equation. The sum of mineral constituents and porosity will be equal unity. The equations can be solved by means of a computer. The positive results are only taken into account. In that case, choosing the appropriate mineral combination is of main concern. Appropriate mineral combination is obtained by means of M-N plot and other crossplots. These crossplots help eliminating uncorrect mineral combinations. In this study the investigations are concetrated on three-porosity methos and two applications are discussed. The results of three-porosity method have been supported by those of M-M plot and dual-mineral method. 2. In the second part, fracture detection from resistivity logs is investigated. Measured resistivity may be either the sum of the resistivities positioned in series around wellbore or the sum of resistivities positioned in parallel around wellbore. Fracture detection could be done by combining these two types of resistivity logs. If induction and later o log (LL8) tools are run together, the fracture may be detected by evaluating the laterolog response with a medium-deep investigated induction tool. If L.L8 response is lower than that of induction, this phenomena can be interpreted as existence of vertical fractures. The success of the method depends on various factors such as mud resistivity, borehole rugoisity. A dual-laterolog tool can be affected greatly by a fracture system filled with conductive fluid. A device with shallow depth of investigation is more affected than one that reads more deeply. The fracture detection can be readily done if matrix porosity is low. The conductivity network of a fractured sone is largely unidirectional. This tends to significantly increase the apparent resistivity as compared to the value expected in an isotropically conductive medium with the same average resistivity; this increase is particularly important for deep-reading focused Vll devices. This increase can be approximately predicted for the deep and shallow later o logs: LLd and LLs. LLd could read up to six times too high and LLs about three to four times; thus, the ratio LLd / LLs shoud be too high by a rather constant factor expected to range The combined effect of the DLL and Ri0 curves is very indicative if the fractures are saturated with hydrocarbon far from the wellbore. The presence of fractures is already indicated by the difference between LLd and LLs which, in addition, is accentuated by the R=r0 response. As discussed earlier the DLL may help in the evaluation of the fracture porosity expressed as a fraction of non-porous bulk. Another approximation relays on the fact that the apparent value of porosity exponent, m, in a fractured sone should be much less than that of non-fractured zones. For non-fractured zones, it is around 2 but for fractured sones it is between 1. 2 and 1. 4 For a fracture system such that matrix and fractures are parallel, the following equation is derived.