A simple petrophysical model proposed by Waxman and Smits (WS)1 in 1968 and Waxman and Thomas (WT)2 in 1972 accounts for the results of an extensive experimental study on the effects of clays on the resistivity of shaly sands. This model has been well accepted by the industry despite a few inconsistencies with experimental results. It is proposed that these inconsistencies resulted from the unaccounted presence of salt-free water at the clay/water interface. Electrochemistry indicates that this water should exist, but is there enough to influence the results? Both a theoretical study and reinterpretation of Waxman-Smits-Thomas data show that there is. The corresponding new model starts from the Waxman and Smits concept of supplementing the water conductivity with a conductivity from the clay counterions. The crucial step, however, is equating each of these conductivity terms to a particular type of water, each occupying a representative volume of the total porosity. This approach has been named the "dual-water" (DW) model because of these two water types - the conductivity and volume fraction of each being predicted by the model. The DW model has been tested on most of the core data reported in Refs. 1 and 2. The DW concept is also supported by log data3 and has been successfully applied to the interpretation of thousands of wells. However, the scope of this paper remains limited to the theoretical and experimental bases of the DW model. The Petrophysical DW Model The purpose of this model is to account for the resistivity behavior of clayey sands. For petrophysical considerations, a clayey formation is characterized by its total porosity, ft; its formation factor, F0; its water saturation, SwT; its bulk conductivity, Ct; and its concentration per unit PV of clay counterions, Qv. The formation behaves like a clean formation with identical parameters ft, F0, and Swt but containing a water whose conductivity, Cwe, differs from the bulk formation water. Neither the type of clays nor their distribution influences the results. Since the formation obeys Archie's laws,Equation 1 The clayey sand equivalent water conductivity, Cwe, can be considered a mixture of two waters. 1. A clay water surrounds the clay particles but has a conductivity independent of the type and amount of clay. Its conductivity, Ccw, comes exclusively from the clay counterions. The volume fraction of clay water, Vcw, is directly proportional to the counterion concentration, QvEquation 2 where vQ is the amount of clay water associated with 1 unit (meq) of clay counterions. 2. The water further away from the clay is called far water. Its conductivity, Cw, and ionic concentration correspond to the salinity of bulk-formation water. The volume fraction of this water, Vfw, is the balance between the total water content and the clay water.Equation 3 The implicit assumption is that the far water is displaced preferentially by hydrocarbons.

Cwvwht 1=, Socwty of Pe4mJeum EngIneem, Inc lhs paper W8 prepared b pmwntaWn d the 1993 SPE Annual Tac+md Conlerenca and Exthb! held m Owwor, cdorado US A,&il Od&.erl W6 mm papr was selected hn p+ew!tabon by an SPE Program Grnmdtm tdbnung m%v C4 mlon'natmn contained m an abstmd subrmtted by the author(s) Gntentx ot h paper, at pfesentd, ham not been renwmd by the Soc!ety 01 P@K4wm Engmoem and we sub~to conectwm by the auttw($) 7M material, as presented does M n-nly retkt any powtmn d thaScaety d P6@iwm Engmaem, is c#hc8m, of !wmM8 Papm pmsentad at SPE meetmgs am subjed to publtdmn rewew by Edttonal Commitees d ttw Somty d P&oleum Engineers Pernvssu.n tn COPY m restmtad to an abstiact d MI mom Lhan X0 words Illustrations may ml be cqued Ttw ak4?wt should contain ccmspuuou$ acknowt. edgment d Acre acd by wimm the papef was presented Write Lbrmmn, sF'E, P O Sox 833S33, Richardson, Tx 750tW3&5, U S A, fwO1-214-8S2-M3S AbstractPuked nuclear magnetic resonance W) logging has until now been limited to measurements of capillary bound water and of free fluids, the sum of which is considered the 'MTcctive porosity" of rock, Clay-bound water and fluids trapped in micropores generally exhibit NMR relaxation times too fast to be detected, given the echo sampling rates and sensitivity limitations of current state~f-the-arl NMR logging tools.Core studies performed on representative clay samples confirm a linear relationship between the transverse relaxation time TZ and the water content At 1 MHz, clays with the largest specific surface areas (smectiles) exhibit T2's in the sub-millisecond range; illites have characteristic T2's of one millisecond, and kaolinite, having the smallest spceifrc surface areas, relax with T2's in the rarrge of ten milliseconds.A new MRIL application was implemented based on the industry-standard MRIL logging tool. By incorporating twice the standard sampling rate and an acquisition scheme designed to boost the signal-to-noise ratio of very fast decay modes, the tool is sensitive to transverse decay components as short as 0.5 ms. During a field test campaign, the tool demonstrated the feasibility of simultmcous acquisition of ctTcctivc porosity and total porosity. Neither porosity measurement requires prior knowledge of rock ma(rix pro~rties.In shaly sands, the difference between MRIL total porosity and effective porosity can be interpreted as the clay-bound water volume, relevant as the clay correction term for resistivity analysis.

No abstract

Core samples are examined using a 10 MHz NMR spectrometer and results are compared with those from the Magnetic Resonance Imaging Log*(MRIL) (operating frequency 1 MHz) in the same rock formations. The frequency dependence of this technology and its link to petrophysics is investigated, thus offering the first link between core and log NMR data for this new tool.

Carbonate reservoirs pose many challenges to geologists and engineers. Lithofacies variations are often the key to the determination of commercial value and efficient reservoir exploitation. Thus, techniques which permit the identification of the involved lithofacies would be useful. This paper illustrates a successful application of multidimension techniques in two-dimensional space, by using cross plots of a variety of well logs that have a principle component sensitivity to the lithofacies. This paper also examines the appropriateness of techniques which relate log data to core-derived permeability. These approaches have previously been associated with the intergranular shaly sand rocks and not carbonates. The technique shown approaches this from a Kozeny-core correlation which is used to predict the surface area parameter. This is then followed by development of a link to log-derived bulk water values. The log derived variables can be made to work providing irreducible conditions are present or can be established.