Well Logging and Its Applications in Cased Holes

Summary
Workovers and recompletions of old wells, evaluation and monitoring of reservoir behavior during primary, secondary, and ternary recovery, and exploration for bypassed oil in cased wells make the proper selection of cased-hole logs and/or optimal logging suites a necessity. Required information includes the integrity of the cement bond between casing and the formation, lithology identification, porosity, type and distribution of reservoir fluids, formation permeability, and anticipated watercut estimates. These data can be obtained from vitrious log responses, which include natural total and/or spectral gamma ray radioactivity measurements, radiation-type logs (density, neutron), pulsed neutron logs. acoustic measurements, etc. In addition. special time-lapse logging techniques monitor reservoir behavior and state of hydrocarbon depletion, whereas log-inject-log (LIL) techniques enhance the accuracy of residual oil saturation (ROS) estimates.


Introduction
Cased-hole logging capabilities and associated quantitative interpretation techniques have become very important in several applications, including, cased-hole exploration, completion, workovers. and the monitoring of hydrocarbon reservoirs under primary, secondary, or tertiary recovery schemes. Cased-hole well logging concepts are reviewed which are currently used (1) to explore for bypassed hydrocarbons in abandoned or old workover wells; (2) to work in fresh, brackish, or unknown formation water salinities; (3) to evaluate hydrocarbons in new wells in which openhole logs could not be run, (4) to monitor production behavior and depletion in reservoirs under primary driving mechanisms, breakthrough in waterfloods, chemical and micellar projects, C02, and steam- and fireflood projects; (5) for residual oil saturation based on LIL techniques, etc. Several field case examples and references are presented.


Reservoir Characterization
Lithology. Natural radioactivity measurements have phyed an important role in wireline logging operations since about 1935. Most of the natural gamma ray logging techniques applied in formation evaluation measure the total and/or individual contribution of gamma rays from potassium, uranium, and thorium series by means of standard gamma ray and/or spectral gamma ray logging. The fact that natural gamma-ray intensities vary as a function of lithology was recognized as early as 1939. Gamma rays are the radiations originating within an atomic nucleus. A nucleus gives off excessive energy (gamma rays) as the result of radioactive decay or an induced nuclear reaction. Radioactive decay consists of the emission or capture of elementary or composite particles with consequent transformations into daughter nuclei characterized by different atomic numbers and, in some cases, by different mass numbers. As early as the 1950's, field tests in boreholes were carried out to study the feasibility of detecting some of these nuclides by gamma ray spectroscopy techniques that identify characteristic gamma rays. Of particular interest are those of potassium and the uranium and thorium series. Both uranium and thorium are characterized by specific decay series. Potassium consists of three isotopes that exhibit masses of 39, 40, and 41 with abundames of 9318, 0.0119, and 6.9%. The only unstable isotope of potassium is the nuclide potassium-40, the major contributor, which emits a single, easily identifiable gamma ray at 1.46 MeV. Hence, in addition to total gamma ray counts, the Spectralog TM measures and records the gamma rays emitted by potassium-40 (40K) at 1.46 MeV, the uranium series nuclide bismuth-214 (214Bi) emanating gamma rays at 1.764 MeV and the thorium series nuclide thallium-208 ( 208Th) emanating gamma rays at 2.614 MeV. These nuclides are of particular interest to the oil industry since, in various amounts, all are found in subsurface formations as constituents of potential reservoir rocks. Table 1 illustrates the distribution of potassium (K, %), uranium (U, ppm), and thorium (Th, ppm) for several formation constituents detrimental to optimal reservoir rock conditions.
Application of such spectral gamma ray data may be made either qualitatively or quantitatively. As is extensively documented in logging literature, natural spectral gamma ray logging assists greatly in geological studies (lithology identification, recognition of depositional environment, stratigraphic correlation, source rock evaluation, etc.), complex reservoir rock analysis (heavy minerals, mica, feldspar, glauconite, fractures, silt. etc.), shaliness estimates, in-situ clay typing, cation exchange capacity (CEC) estimates, and radioactivity buildup under dynamic fluid conditions (channeling behind pipe, high-permeability streaks, fractures. caverns, watered-out zones, etc.) and/or around perforations.
JPT
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