May-June 2000
Volume 1 No. 3

RESEARCH NOTE
Comparison of Modeling Codes for Resistivity and MWD Instruments: Part 1  1-D Radial Invasion

Liang C. Shen

 
ABSTRACT
The objective of this survey is to compare different computer modeling codes that have been developed by various academic and industrial groups. Only the numerical results are compared. The computation speed, memory requirements, and other factors are not covered in this project. In Part I of this series of reports, we examine the codes that are used to compute the effects of the borehole and radial invasion on the readings of induction and MWD instruments. Five groups participated in this exercise for the MWD instruments and four groups for induction instruments. Agreements between results obtained from those codes are generally good, except at a few isolated points. The maximum difference in computed apparent resistivity is less than 1.5 ohm-m or 7.2 percent in MWD codes and 0.08 ohm-m or 0.2 percent in induction codes.

Dual Laterolog Response in 3-D Environments

Hanming Wang, Liang C. Shen and Geng Ji Zhang
 

ABSTRACT
Computer codes based on the finite element method (FEM) have been developed to model dual laterolog responses in 3-D environments. Validation of the 3-D FEM codes is carried out by showing that results of the codes agree with those obtained by an analytic solution, and by 2-D FEM codes. Using the 3-D FEM codes, dual laterolog responses in highly deviated wells, horizontal wells, dipping anisotropic formations, and fractured carbonate formations are studied .
For a laterolog instrument in a formation consisting of a resistive bed between two conductive shoulder beds, one tends to believe that the greater the dip angle is, the lower the reading of the apparent resistivity at the center of the bed will be. However, such a behavior is not always true. The resistivity reading also depends on the bed thickness and on the resistivity contrast between the resistive bed and the shoulder beds. In some cases, it is seen that when the dip angle varies from 0 to 75 degrees, readings at the center of the resistive bed actually increase with the dip angle. Therefore in thin dipping beds, the response of both the deep and the shallow laterolog instrument is difficult to predict without carrying out computer modeling.
The study also illustrates the dual laterolog response in a horizontal well in a formation containing a resistive bed. The readings of both the shallow and the deep arrays are strongly affected by the shoulder beds when the arrays are in the resistive bed. Even when the bed thickness is as thick as three meters, shoulder beds still have a strong influence on the deep laterolog.
The effect of anisotropy on the dual laterolog is small even at large dip angles. The apparent resistivity responses of the dual laterolog are determined mainly by the horizontal resistivity of the anisotropic formation.
As expected, in a fractured formation the apparent resistivity response decreases as the fracture opening width is increased, or as the resistivity of the fluid in the fracture is decreased. It is shown that in some cases the fracture with  a five-meter extent has the same effect as the one with an infinite extent; and a 100 micrometer fracture opening has an apparent thickness of one meter.  When there are five or more fractures per meter along the borehole axis, then the fractured formation can be modeled as an anisotropic formation.

Interpretation of Wettability in Sandstones With NMR Analysis

Gigi Qian Zhang, Chien-Chung Huang and George J. Hirasaki
 

ABSTRACT
Using nuclear magnetic resonance (NMR) analysis, a systematic interpretation of wettability alteration was carried out using three types of sandstones with varying clay content. Two fluid systems were investigated. Soltrol 130 was used as the refined oil. A deep-water Gulf of Mexico crude oil was used as the crude oil that is known to alter the wettability in restored-state core analysis.
Fluids that are in molecular contact with mineral surfaces have a relaxation time that is less than the bulk fluid relaxation time due to surface relaxation phenomena. Mixed wettability can result in lengthened NMR relaxation of brine compared to 100% brine saturation and shortened NMR relaxation of oil compared to bulk oil when oil comes in contact with part of the rock surfaces. Thus, NMR proton spin-lattice relaxation time (T1) distribution provides a qualitative description of wettability alteration.
Bentheim and Berea were water-wet with refined oil. When saturated with crude oil and brine at Swir they became mixed-wet after aging. After forced imbibition, residual oil had a shorter relaxation time compared to the bulk oil, indicating that patches of crude oil remained on the rock surfaces.
North Burbank sandstone was mixed-wet when either saturated with refined oil at Swir or crude oil after aging. Micropores formed by chlorite flakes were filled with brine. Oil in the macropores was in contact with the tips of chlorite flakes. After forced imbibition, brine in the macropores was partially shielded from the pore walls by a film of oil spanning the tips of chlorite clays. The evidence for this was that brine in the macropores relaxed slower than 100% brine saturation but faster than bulk brine, and oil relaxed faster than bulk oil. Thus North Burbank appeared mixed-wet with either refined oil or crude oil. However, the extent of wetting change was greater with crude oil. This special wetting behavior of North Burbank came from its microporous structure formed by pore-lining chlorite flakes.