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Oilfield Anisotropy: Its Origins and Electrical Characteristics ANISOTROPY ANISOTROPY ANISOTROPY Oilfield Anisotropy: Its Origins and Electrical Characteristics Barbara Anderson Klaus Helbig Ian Bryant Consultant Martin Lüling Hannover, Germany Brian Spies Ridgefield, Connecticut, USA Getting a grip on anisotropy of the earth can mean the difference between success and failure in reservoir evaluation and development. Accounting for the affects of anisotropy in measurements of the earth begins with understanding the geologic foundations of anisotropy—how sediments are laid down, converted to rock and deformed. Here is a summary of some geologic mechanisms for anisotropy, and some recent progress on anisotropy of electrical properties of rock formations. Across the many disciplines of the oil field, a power now allow anisotropy to be factored stock exchange. A smart one will figure out nearly universal phenomenon is anisotropy into field development decisions. A full how to survive, but not without a struggle. —the variation of a property with the direction mathematical description of anisotropy Physicists first met this challenge early in in which it is measured. Where anisotropy remains difficult—for example, representing this century when they left the laboratory to arises, convenient assumptions fall. Seismic flow in three dimensions requires six vari- make measurements of the earth’s subsur- reflectors appear at the wrong depth. Seismic ables.1 But accounting for the simplest face. They brought with them the mathe- lines don’t tie. Waterflood programs fail. effects of anisotropy is coming within reach. matics of materials assumed to be Induction logs are misinterpreted and mis- What is meant by anisotropy depends on isotropic—having properties with the same take water for pay. who is talking. Geophysicists generally focus value in all directions—and homogeneous. As producers seek a finer comb to draw on variation of seismic wavefront velocity or For the most part, the convenient assump- through measurements of the earth, they are on the polarization of shear waves. Petro- tions held remarkably well. Where the changing the status of anisotropy from physicists may measure resistivity anisotropy. assumptions began to fail, there arose the unwelcome guest to hard-working collabo- Drillers and geologists may think first of second difficulty with anisotropy—if you rator. Advances in theory and computing anisotropy in rock strength or stiffness pro- don’t have the tools to deal with anisotropy, duced by earth stresses. Stratigraphers may the temptation is to ignore it or sweep it concentrate on anisotropy of magnetic prop- under the rug. For help in preparation of this article, thanks to David Allen, Schlumberger Wireline & Testing, Sugar Land, erties. And reservoir engineers need go to Assumptions about isotropy began to Texas, USA; Bülent Baygün, Bob Burridge, Darwin Ellis, great lengths to characterize permeability crack as early as the 1930s, when measure- Charlie Flaum, David Johnson, Bob Kleinberg, Stefan anisotropy to plan an optimal production ments made with electrodes laid in different Luthi, Bill Murphy, TS Ramakrishnan and Lisa Stewart, Schlumberger-Doll Research, Ridgefield, Connecticut, strategy (see “Case Study: Anisotropy for directions on the earth’s surface were seen to USA; Stuart Crampin, University of Edinburgh, Edinburgh, Steering Horizontal Wells,” page 50). give different results when strata were dip- Scotland; Gregor Eberli, University of Miami, Miami, In all geoscience disciplines, however, ping than when strata were flat.2 In geo- Florida, USA; Stan Gianzero, Consultant, Austin, Texas; Roy Nurmi, Schlumberger Wireline & Testing, Dubai, there are two difficulties in dealing with physics, the introduction of shear wave UAE; Jon Roestenburg, Schlumberger Wireline & Testing, anisotropy. One is that the conceptual sources in mid-1970s showed that shear Jakarta, Indonesia; Mike Schoenberg, Schlumberger Cambridge Research, Cambridge, England; Frank Shray, underpinnings of anisotropy originate from wave anisotropy was often significant and Schlumberger Wireline & Testing, Stavanger, Norway; the laboratory study of crystals—pure, could be analyzed quantitatively (next Julian Singer, ONGC-Schlumberger Wireline Research homogeneous materials under pristine, con- page).3 A leap, however, took place in the Centre, New Delhi, India; Sven Treitel, Amoco Production Company, Tulsa, Oklahoma, USA; Don Winterstein, trolled conditions. Today’s physical models mid-1980s, when sensors again reclined Chevron Petroleum Technology Company, La Habra, of the earth draw much from this work, with the expansion of horizontal drilling. In California, USA; Mark Zoback, Stanford University, even though the earth is a composite, het- vertical wells, electrical anisotropy was often Stanford, California. erogeneous material sampled under any- In this article, CDR (Compensated Dual Resistivity) and thing but pristine, controlled conditions. RST (Reservoir Satuation Tool) are marks of Schlumberger. Applying the physics of pure materials to the earth is like putting an economics pro- fessor to work on the floor of the Tokyo 48 Oilfield Review W E ° E Fractures, N 45 Slow shear, S 45 ° E ° E Fast shear, N 45 observed to be negligible and could be nHow fractures split shear waves. the vertical direction. This is called trans- ignored. With horizontal boreholes, acoustic A notable feature of acoustic anisotropy is verse isotropy in the vertical direction and and electrical anisotropy became obvious shear wave splitting, or polarization, typi- derives from the early days of logging, when cally caused by fractures. If a shear wave 4 and demanded consideration. For many, of a polarization is not parallel to the strike anisotropy was observed in vertical wells at the simple, isotropic days were ending and a of a fracture set, the wave will be split into 90° (transverse) to uniform (isotropic) flat- new way of thinking was required. two components as it passes through the lying beds.5 Resistivity, for example, would This article gives an overview of the geo- fractures. The first, faster component will appear to be the same for any wellbore logic basis of anisotropy as it is understood have particle motion aligned parallel to azimuth, but be different from the value in the fracture strike. A second, slower com- today in the oil field. It begins with a review ponent will have a wavefront aligned per- the vertical direction. of basic concepts and geologic mechanisms pendicular to the fracture strike. A growing usage today, especially among that produce various types of anisotropy and [From Lynn HB: “Seismic Detection of Oriented geophysicists, is to qualify isotropy with then focuses on recent advances in the mea- Fractures,” Oil & Gas Journal 84, no. 31 (August 4, respect to an axis of symmetry. Transverse 1986): 54-55. See also Crampin S: “Evaluation of surement and interpretation of electrical Anisotropy by Shear-Wave Splitting,” Geophysics isotropy in a vertical well that crosses hori- anisotropies. Two crucial and well-charac- 50 (January 1985): 142-152; Crampin S: zontal beds would be transverse isotropy terized anisotropies—acoustic/seismic and “Anisotropy and Transverse Isotropy,” Geophysi- with a vertical axis of symmetry, abbreviated permeability—are detailed in other articles cal Prospecting 34 (February 1986): 94-99.] TIV. Properties measured in a horizontal in this issue (see pages 24 and 36). well that crosses a series of vertical fractures What is Anisotropy? 1. Each variable here is a scalar, a quantity that has mag- Crampin S, Bush I, Naville C and Taylor DB: “Estimat- A material is anisotropic if the value of a nitude but no direction. For an introduction to the ing the Internal Structure of Reservoirs with Shear- vector measurement of a rock property analysis of anisotropy: Lake LW: “The Origins of Wave VSPs,” Geophysics: The Leading Edge 5, no. 11 Anisotropy,” paper SPE 17652, Journal of Petroleum (1986): 35-39. varies with direction. Anisotropy is typically Technology 40 (April 1988): 395-396. Alford RM: “Shear Data in the Presence of Azimuthal used to describe physical properties, which, 2. Maillet R and Doll HG: “Sur un Théorème Relatif aux Anisotropy,” Expanded Abstracts, 56th SEG Annual for the purposes of geoscience, can be Milieux Electriquement Anisotropes et ses Applica- International Meeting and Exposition, Dallas, Texas, tions à la Prospection Electrique en Courant Continu,” USA (November 2-6, 1986): 476-479. thought of as parameters intrinsic to the Ergänzungshefte für angewandte Geophysik 3 (1932): 4. For acoustic anisotropy due to layering in deviated body of the rock at a given state. The 109-124. and horizontal wells: White J: “Recent North Sea notable exception is that anisotropy is often For a review of resistivity anisotropy: “Formation Experience in Formation Evaluation of Horizontal Anisotropy: Reckoning With its Effects,” Oilfield Wells,” paper SPE 23114, presented at the SPE Off- used to describe a state of stress, which is Review 2, no. 1 (January 1990): 16-23. shore Europe Conference, Aberdeen, Scotland, not a property but a condition that results in 3. Crampin S, Evans R, Üçer B, Doyle M, Davis JP, September 3-6, 1991. anisotropy of intrinsic physical properties. Yegorkina GV and Miller A: “Observations of Dila- For a discussion of electrical anisotropy, see refer- In the simplest form of earth anisotropy, a tancy-induced Polarization Anomalies and Earthquake ences 27-36. Prediction,” Nature 286 (August 1980): 874-877. 5. Maillet and Doll, reference 2. vector measurement has constant magni- A shear wave is like the wave that travels down a tude in any horizontal direction that is dif- jump rope that is attached to a tree at one end and at ferent from the magnitude of the vector in the loose end, given a shake either side to side or up and down. Particle motion in a shear wave is perpen- dicular to the direction of wave propagation. October 1994 49 Homogeneous Heterogeneous a a a a might have transverse isotropy with a hori- a a a a a a a a a a zontal axis of symmetry, called TIH (see a a a a aaa a a aa a a a page 37). The nomenclature clings to isotropy as the frame of reference, since it aa a aa a aaa a a aaa a a remains far easier to model than anisotropy.
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