New Method for Identifying Fluid Type of Reservoirs in Tough Conditions
Authors: Liu Cheng Bing 
Published: October 2008

This paper presents a new fluid type identification method called Correlation Between LogS method (CBLS). The CBLS method combines porosity and pore size information (total porosity [?t] and bound fluid porosity [?b]), measured by a shallow-investigating, nuclear magnetic resonance (NMR) tool, with the deep-resistivity information (deep reistivity [Rd]), measured by a deep-investigating resistivity tool to differentiate hydrocarbon and water in the "virgin" zone of the reservoirs. The CBLS method compares the value variations with depth among Rd, ?t and ?b curves rather than the actualvalues. The CBLS method can be used in both simple and tough conditions with a high success rate.

Here, we briefly review a few traditional methods for identifying reservoir fluid type such as resistivity absolute value method (RAV), multiresistivity invasion profile method (MRIP) and NMR method (NMR). We also discuss, in detail, the theory of the CBLS method with application examples from low-and no-resistivity-contrast sandstone reservoirs, and low-porosity, tight-fractured and/or vuggy, carbonate, or igneous reservoirs. We also note possible limitations of the method.

Introduction

Qualitatively identifying reservoir fluid type based on log curves before further data processing and interpretation is often the key to successful formation evaluation. A petrophysicist will often wish to qualitatively identify the fluid type (hydrocarbon or water) of the reservoirs first, and then quantitatively compute various physical properties (mineralogy, porosity, saturation, intrinsic and phase permeabilities and production rate etc) of the reservoirs using this knowledge of fluid type to guide the interpretation 3.

In tough conditions such as low-resistivity, low-and noresistivity-contrast and low-porosity reservoirs, most of the existing fluid type identification methods do not work. There is a strong imperative from the oil industry to develop a new, reliable and low-cost technique to identify reservoir fluid type in both simple and tough conditions.

Review of Current Fluid Typing Methods.

Currently there are three main methods for qualitatively identifying reservoir fluid type: RAV, MRIP and NMR. These three methods each have advantages, disadvantages, limited application ranges, and are only used in certain conditions.

RAV Method.

The RAV method identifies reservoir fluid type using the deep-investigating resistivity log readings in different reservoirs or different parts of one reservoir (Fig. 1). The logs required for the RAV method are Rd, ?t and clay volume (Vcl). This method requires an assumption that the resistivity values of formation water (Rw) in the different reservoirs or the different parts of one reservoir associated with the analysis are equal. The working conditions required for this method are that the ?t and Vcl in the different reservoirs or the different parts of a reservoir associated with the analysis are equal and there is at least one reservoir or one part of a reservoir that is water bearing and at least one reservoir or one part of a reservoir that is hydrocarbon bearing. The advantage of this method is that it only requires Rd, ?t and Vcl measurements for identifying the fluid type.