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Formation Testing - Background, Perspectives and New Industry Requirements

Oil and gas industry petrophysicists are well versed in well logging methodologies. And over the past several decades, a large suite of formation evaluation instruments have seen commercial introduction, tools measuring, for example, resistivity, sonic speed, gamma ray, nuclear magnetic resonance properties, and so on. We know what they measure, or at least, think we do. Take resistivity analysis. We intuitively conjure up images related to DC resistance, a property often highlighted by the differences in intensity associated with dim versus bright light bulbs. But the interpretation of measured quantities in reality depends on subtleties beyond the experience of even experienced analysts. For instance, one might ask, "How does invasion affect measurements and how are these corrected?" "Were predictions derived from phase or amplitude data?" Or, "How extensive was the simulation domain used to derive interpretation charts and what is its influence?"

1.1 Formation Testing - A Brief Introduction.

The point is, for just about all logging tools, analogous subtleties arise that are not easily addressed - measured data in all cases are indirect and subject to cross-checks between different tools, plus (hopeful) familiarity with the geological asset. When one of the authors asked Rob Kirby, an experienced directional driller in Houston, "Which logging measurement would you prefer, given all that are commercially available?" His response was, "I'd take formation testing any day." And the reason? It was obvious. The formation tester provides direct (and not inferred) observations of fluid properties: sample chambers transport actual liquid or gas samples to the surface for analysis, while properly interpreted pressure transient measurements yield mobility, the ratio of permeability to viscosity, and for multiprobe tools, the anisotropy ratio kh/kv - and, of course, there's compressibility and pore pressure.

Despite the importance of formation testers, noting that oil service companies derive significant percentages of their revenue from sampling and real-time pressure transient analysis operations, such tools command relatively low visibility at annual conferences and trade shows - the bulk of existing math modeling papers and patents, in fact, were published in prior decades and assumed some expertise on the part of the reader. Those new to the industry are taught logging basics. For instance, they readily identify resistivity tools as instruments with coils wrapped around drill collars; or, sonic instruments as tools with perforated slots in the metal body; or, gamma ray logging tools, as devices with trademark photo-multipliers and detectors.

But what does a "formation tester" look like? Are there multiple engineering concepts? What are their relative advantages and limitations? How do recent designs differ from earlier ones and how can these be improved? And what exactly underlies advanced tool design, that is, what analysis models are used to develop integrated mechanical and interpretation systems that ensure accurate measurements and predictions in the field? This book addresses these questions, and for completeness, starts with an initial review of prior industry work and also methodologies developed by the authors. The narrative supporting the initial portion of the text is mainly visual, an approach that conveys the greatest volume of information in the least amount of space, while for the latter, beginning with Chapter 7, we focus on three-dimensional quantitative models, computational details and very rigorous validations.

What started as a standalone review on "ideal source models," our scope of work during the early 2000s, has evolved into a companion volume due to Qin et al. (2021) which complements the multiprobe methods of the present book. Like the 3D methods above, source approaches are now routinely used in day-to-day analysis. Importantly, our expositions are deliberately readable - our analysis methods are explained in "plain English" terms with sufficient references to basic math and related prior art - with more than enough detail that motivated students should be able to produce equivalent software prototypes with minor effort. The authors hope that our two latest volumes will provide comprehensive, state-of-the-art reviews for students as well as professionals - and, in fact, a needed resource offering the latest analysis and interpretation techniques, so that formation testing methods become a ubiquitous part of petroleum engineering activities typical of well logging, drilling or reservoir simulation.

Figure 1.1. Formation testing research monographs, Chin and COSL authors, Scrivener Publishing, 2014, 2015 and 2019.

Figure 1.2. Drawdown-buildup pressure responses with dynamic pumping action and flowline.

Remarks. The formation tester is a well logging instrument with nozzles, which when pressed against the borehole sandface, extracts insitu reservoir fluids for surface analysis. By-products of this process are pressure transients that can be "interrogated" using advanced math models for properties like mobility, anisotropy, compressibility and pore pressure. Testers may be "single" or "multiple probe," positioned axially and/or azimuthally along the tool body. Active probes may withdraw or inject fluids, while passive "observation probes" measure pressures. An example showing two drawdown-buildup curves is given above. Despite the simplicity, sophisticated math models are required for interpretation.

Figure 1.3. Downhole, surface and logging truck operations.

Remarks. Transient pressure responses are collected along the length of the well and predictions for pore pressure and mobility are presented to the petrophysicist as functions of depth. Depending on the data processing requirements of the host math model, real-time calculations can be performed within the tool, at the surface in the logging truck, or when these options are not possible, at the home office where more computing resources are available (data are transmitted for off-site calculation and received as well logs). A logging truck such as that shown will support a wide variety of formation testing software apps, for example, as described in Qin et al. (2021) for "source models" and the present book for triple probe forward and inverse applications.

1.2 Conventional Formation Testing Concepts.

Formation testing design concepts are rich and varied. A pumping probe, operating as a "sink" or a "source," also tracks pressure transient responses. Other pressure probes may reside along the tool body, displaced axially, azimuthally or both, which may actively pump or function as passive observers. While the primary formation tester function is fluid sampling, in which in-situ reservoir fluids are collected and transported to the surface for analysis, pressure measurements represent critical byproducts that are important to formation evaluation. Examples of testers offered by different manufacturers for wireline and MWD applications are given in Figures 1.4 - 1.7.

Figure 1.4. Conventional formation tester tool strings.

Figure 1.5. Formation testers, additional developments.

Figure 1.6. Conventional dual and triple probe testers.

Figure 1.7. Dual probe tester with dual packer.

1.3 A New Triple Probe Tool - Design Concepts and Well Logging Advantages.

A well known industry "triple probe" tool is sketched in Figure 1.8. A "sink probe," shown at the top right, withdraws fluid from the formation and initiates a pressure drawdown. The pressure drop is communicated to a "horizontal probe" that is displaced 180° circumferentially about the tool - that is, both sink and horizontal probe lie in the same plane normal to the tool axis. A third "vertical probe" is often found on the tool body at the same azimuth and displaced axially, say about thirty inches away. This conventional triple probe tool is different from the COSL triple probe tool shown in Figure 1.9. The COSL tool consists of three probes set in the same normal plane, each displaced 120° from the others - additionally, one or more axial probes may be installed along the tool string. These are discussed later in this volume, drawing on our 2014, 2015 and 2019 book publications, which have already addressed forward and inverse modeling algorithms for axial probes. The new triple tool hosts several advantages. First, the design offers redundancy and provides an important margin of safety in the event of downhole mechanical failure. Second, if all three probes are identical, the ability to see at different angles allows the tester to detect heterogeneities that may possibly remain hidden with single probe tools. Third, it is possible to measure dip angle, if distinct thin layers exist. Fourth, equipping the tester with different sized probes, for example, with small, medium and long slotted nozzles, enhances the tool's ability to log formations where little is known about the formation. This is a crucial advantage...