David H. Johnston

Physical properties of shale at temperature and pressure

Deformational properties, P‐wave and S‐wave velocities, and electrical resistivity were measured for three North Sea Malm shales in the laboratory under pressures to 800 bars and temperatures to 100 °C. These data were used to evaluate how factors such as mineralogy, microstructure, compaction, and pore‐fluid conductivity affect a shale’s seismic and electrical responses. Deformation in the shales is dominated by inelastic processes which cause time‐dependent changes in velocity, resistivity, and pore pressure. Overall, shales are less sensitive to pressure changes as compared to sandstones of similar porosity. However, changes in temperature result in large changes in physical properties as compared to sandstones or shaly sands. P‐wave and S‐wave velocities may decrease by as much as 10 percent over the temperature range studied, and calculated activation energies for surface conduction are nearly twice those observed in shaly sands. These comparisons emphasize fundamental differences in fabric among the...

Time-lapse seismic analysis of Fulmar Field

This report discusses the analysis of time‐lapse seismic data from Fulmar Field in the central North Sea. This case study evaluates the potential of time‐lapse seismic technology for monitoring fluid movement and pressure changes in a reservoir using legacy (preexisting) data.

On the inversion of time‐lapse seismic data

As the number of time-lapse (4D) seismic applications increases; so do the prospects for quantitative estimates of dynamic reservoir properties, such as fluid saturation and pressure, based on time-lapse seismic data. These estimates can be obtained by the application of appropriate rock physics models that map seismic-derived estimates of timelapse Pand S-impedance changes to changes in the dynamic properties. Therefore, it is important that reliable 4D impedance models are available for this process. There are, however, a number of ways to estimate impedance change from 4D seismic data. The purpose of our study was to assess the merits of different inversion workflows by using a synthetic 4D data set and comparing models resulting from the inverse calculations with actual impedance models. This study demonstrates the importance of coupling the inversion of base and monitor surveys to mitigate the impact of inherent non-uniqueness in seismic data inversion. The results from uncoupled inversions can mislead time-lapse interpretation and compromise reservoir management decisions.

VSP Detection of Fracture-Induced Velocity Anisotropy

A multiple S-wave polarization VSP survey using gyroscopically oriented three-component downhole recording documented azimuthal anisotropy associated with the preferential orientation of fractures within the Austin Chalk. The anisotropy is manifest in a significant velocity contrast between S-waves polarized parallel to and perpendicular to the dominant fracture orientation as determined by bore-hole televiewer data. In addition, as discussed by Becker and Perelberg in another paper, for S-waves polarized 45/sup 0/ to the principal axes of symmetry, particle motions recorded within the Austin exhibit birefringence or splitting.

Using legacy seismic data in an integrated time-lapse study: Lena Field, Gulf of Mexico

Seismic monitoring (time-lapse or 4-D seismic) has the potential to significantly increase recovery in existing and new fields. While future field developments should benefit from seismic acquisition designed for time-lapse monitoring, the portfolio of current seismic monitoring opportunities for most companies consists of existing fields for which one or more 3-D surveys have already been acquired. These legacy seismic data sets were not acquired for the purposes of seismic monitoring and are often very different in terms of acquisition and processing parameters. In addition, the seismic acquisition is rarely timed to optimally map reservoir changes or impact development decisions. Repeatability of the seismic data in the nonreservoir portion of the data volume and the robustness and credibility of the seismic difference anomaly within the reservoir are key issues for the application of legacy data in time-lapse analysis. Another issue is whether 4-D seismic differences can be interpreted in terms of reservoir production changes. If these issues can be effectively addressed, then legacy 4-D seismic may be used as a tool for reservoir surveillance or reservoir management. The objective of this paper is to understand the magnitude of the processing effort required to obtain reliable time-lapse differences and to interpret the seismic difference observed in the B80 reservoir of the Lena Field through the use of geologic modeling, flow simulation, and seismic modeling. The Lena Field (Mississippi Canyon Block 281) is south of the modern Mississippi delta in 1000 ft of water. The field is on the western flank of a salt dome within a fault-bounded intraslope basin. Hydro-carbon production is from six Pliocene-age sands. As shown in the seismic section (Figure 1), the B80 reservoir is about 10 500 ft below sea level at approximately 3 s TWT. The interval is interpreted as a low-stand fan systems tract representing deposition …

Jotun 4D: Characterization of fluid contact movement from time-lapse seismic and production logging tool data

Time-lapse (or 4D) seismic is a reservoir monitoring technology that has matured to the point where it can be considered a reliable reservoir surveillance tool. By analyzing differences of multiple seismic surveys acquired over a producing reservoir and by integration with conventional reservoir monitoring tools, 4D seismic data can provide valuable insight on dynamic reservoir properties such as fluid saturation, pressure and temperature. Many published case studies demonstrate that 4D seismic technology can optimize reservoir management and improve production efficiency. In this paper we describe the application of time-lapse seismic methods to the Norwegian North Sea Jotun Field to determine the extent of the oil-water contact (OWC) movement during the first three years of production.

Seismic attribute calibration using neural networks

Neural networks are used to predict sand percen t f rom se i smic a t t r ibu tes fo r severa l in t e rva l s in a Cre taceous bas in . Neura l ne tworks a re h igh ly s impl i f i ed computer models of biological neural systems and have found applications in a number o f a reas inc lud ing pa t t e rn recogni t ion , c l a s s i f i c a t i o n , a n d s i g n a l processing. These networks are not programmed but rather are trained by repea ted p resen ta t ion o f inpu t da ta ( s e i s m i c a t t r i b u t e s e x t r a c t e d a t w e l l loca t ions ) and the cor respond ing des i red output (sand percent measured in the w e l l s ) . I n t h i s c o n t e x t o f s e i s m i c a t t r i b u t e a n a l y s i s , t r a i n i n g a n e t w o r k i s e q u i v a l e n t t o a c a l i b r a t i o n .

4D in the deepwater Gulf of Mexico: Hoover, Madison, and Marshall fields

The use of time-lapse or 4D seismic technology is well established in the North Sea and West Africa where the application of 4D for reservoir production monitoring and surveillance has resulted in documented gains in recovery efficiency, increased reserves, and more efficient field operations. However, for several reasons, including challenging acquisition conditions, 4D technology has not been widely applied in the deepwater Gulf of Mexico. In late 2005 and early 2006, ExxonMobil acquired a deepwater Gulf of Mexico 4D survey covering the Hoover, Madison and Marshall fields in the Alaminos Canyon/East Breaks area (for simplicity, we will refer to this as the Hoover survey).

Geophysical and geochemical logs from a copper oxide deposit, Santa Cruz project, Casa Grande, Arizona

In support of an in‐situ leaching experiment, five holes drilled into a copper oxide deposit have been logged with geophysical and geochemical tools developed for use in the petroleum industry. When combined with geological description, chemical analyses, and mineralogical data from core and cuttings, the logs provide information regarding the alteration, fracturing, copper distribution, porosity, and permeable zones. Correlations among sonic velocity, rock strength from mechanical tests on core, and alteration indicators from neutron and potassium logs demonstrate a close link between the state of alteration and the mechanical state of the rock. Neutron activation, natural gamma‐ray, and density logs, in combination, correlate so well with copper assays that log‐based prediction of copper content is possible; in addition, an estimate of whole‐rock mineralogy is presented in log format. Based on comparisons of flow logs and acoustic logs obtained in the same holes, reductions in acoustic velocity appear t...

Interpretation redefined for the year 2000, Part 1

At a 1993 postconvention workshop in Washington DC, the changing role of geophysical interpreters was discussed. Part 1 of the edited transcipts, published last month, dealt with the driving forces for change in our profession: Technology, Global economics and geopolitics, and Business. In Part 2, we discuss the responses to those forces in the areas of Corporate culture and Professionalism. We then attempt a summary “definition of interpretation.”