This paper describes the theory of cased hole formation resistivity measurements and presents examples demonstrating the diversity of their use. Resistivity behind casing has many applications, from evaluating formations in new wells to monitoring water influx and bypassed hydrocarbons in producing wells. These robust deep-resistivity measurements can be combined with data from traditional cased hole and openhole evaluation tools to provide a comprehensive formation evaluation from behind casing. Example A highlights the use of these measurements in the successful completion of a new Cook Inlet offshore well. Traditional openhole logs were not obtained because of hole conditions and the borehole environment, which also limited traditional cased hole evaluation techniques such as sigma and carbon-oxygen logging. The new resistivity measurements were instrumental to the successful completion of this well as an oil producer with low water cut. Examples B and C, from wells in a mature field, demonstrate the use of the measurements to identify water influx and encroachment, which is vital for reservoir monitoring. Both wells produced water from high-permeability thief zones, which prompted profile modification to limit water production and enhance off-take from zones with lower permeability. The new cased hole resistivity measurements provided valuable data, not available from traditional production logs, to assess sweep efficiency, which will be used to guide recompletion or sidetrack decisions. Introduction Traditional openhole formation evaluation techniques use resistivity or nuclear measurements to identify the location of hydrocarbons within the formations penetrated by the drill bit. In the past, once casing had been set in a well, formation evaluation and monitoring have been restricted to nuclear measurements because it was impossible to measure formation resistivity behind metal casing. Recent advances in electronics and electrode design now allow formation resistivity measurements to be made behind metal casing that can be used for both primary evaluation and reservoir monitoring. The ability to measure resistivity behind casing complements traditional cased hole nuclear evaluation techniques by providing valuable data in low-porosity environments and with greater depth of investigation than previously available. Direct resistivity measurements are also valuable for time-lapse monitoring of reservoir depletion. Three examples of the application of formation resistivity behind casing will be presented from wells located in Alaska. Example A demonstrates the application of measurements from the CHFR* Cased Hole Formation Resistivity tool to new well formation evaluation. In this well, openhole resistivity measurements were not available because hole conditions and formation conditions limited the use of traditional nuclear techniques. Examples B and C demonstrate the application of CHFR measurements to reservoir monitoring in a mature field through comparisons to previous resistivity and water saturation (Sw) results. CHFR Measurement Theory The theoretical basis for resistivity measurements behind casing has been known for a long time, having been patented in 19391. Practically, however, this measurement has not been available until recently because of the large resistivity contrast between metal casing and typical formation materials. The resistivity of metal casing is approximately 2×10-7 ohm-m, while typical formation resistivity ranges from 1 ohm-m to 100 ohm-m2. That contrast is approximately seven orders of magnitude. To measure this large contrast, voltage measurements in the nanovolt range are necessary. This capability has only recently been possible with advances in tool electronics and electrode design.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractInelastic capture neutron logging (IC) has been used successfully to identify water influx and by-passed hydrocarbon in the mature oil fields located on the North Slope, Alaska. The North Slope has had commercial production since 1977. Production from these fields are enhanced by secondary recovery techniques including water flooding using Beaufort Sea and produced water, natural gas re-injection and miscible fluid injection techniques. The uses of traditional cased-hole evaluations (sigma and porosity interpretation from pulse neutron and cased hole neutron logs) are limited due to the complexity of the secondary recovery techniques including wide variations in the salinity of injected water. Beaufort Sea water salinity varies widely through out the year due to the influx of fresh water during the spring run off. Inelastic capture neutron logging and interpretation has allowed identification of remaining and by-passed oil and has aided in the planning of sidetrack wells to tap additional reserves. Ambiguities arising out of formation water salinity uncertainties have been clarified through the use of inelastic capture neutron logging techniques.
Alaska state requirements as well as sound engineering practice require that oil and gas operators investigate the zonal isolation in the annulus of water injection wells. Conventional cement evaluation can be a time-consuming and expensive process, especially in high-angle or horizontal wells in which the wireline tools must be conveyed with a tractor or on drillpipe. Cased-hole LWD sonic data was acquired in a North Slope, Alaska, well to evaluate the cement bond zonal isolation in the casing by attenuation and transmission analysis. The LWD data was compared with a traditional wireline cement evaluation logs to validate the application. This paper provides a case study of the techniques used to acquire process and interpret logging-while-drilling (LWD) sonic logs for cement evaluation.In this case study, a horizontal lateral section was drilled after setting and cementing the intermediate casing string. The interval to be evaluated for zonal isolation was behind this intermediate casing. LWD sonic data were acquired for cement evaluation purposes while tripping in the hole to drill out the intermediate casing cement shoe prior to drilling the horizontal section of this well. The ability to acquire cement evaluation data during the normal drilling practice provides significant cost savings over traditional wireline cement evaluation methodologies.This paper discusses the theoretical basis for using LWD sonic data for cement evaluation, planning procedures, data acquisition, and data processing. A comparison of the LWD sonic cement evaluation data with traditional wireline cement evaluation data that was used to validate this technique is provided. Additionally, the advantage of integrating the LWD cement evaluation option into the drilling process will be described.
In this paper, a new series of Logging-while-drilling (LWD) nuclear magnetic resonance (NMR) tools is introduced. The tools incorporate a magnet arrangement which provides a low field gradient minimizing adverse lateral motion effects. T2 distributions are measured while drilling or sliding in a range of hole sizes, including large holes previously inaccessible to LWD NMR measurements. Benefits and trade-offs of key tool design features are presented. Emphasis is placed on measurement quality, operational simplicity and log quality control in real time and memory mode through example logs from operator wells. Real-time T2 distributions provided by the new generation LWD NMR tool enable a full range of NMR answers, including lithology-independent total porosity, bound fluid volume and permeability. Real-time log quality controls monitor tool noise, antenna sensitivity, tuning quality and lateral motion. Acquisition sequences are optimized for porosity precision and accuracy in different environments. Drilling data have been acquired in several operator wells and a test well covering diverse ranges of formations and logging conditions. Environmental factors affecting porosity precision and T2 quality are discussed with reference to log quality indicators and comparison with wireline NMR logs is made where available. In general, the LWD tools deliver NMR answers comparable to the analogous wireline logs in terms of precision, accuracy and vertical resolution. With increased industry focus on LWD services and heightened sensitivity to downhole chemical sources, the need for reliable LWD NMR measurements continues to grow. In addition to a sourceless porosity measurement, NMR provides unique quantitative information on the disposition of producible fluids, which is not available from other logs. Real-time producibility information can be used to optimize formation pressure measurements and sampling as well as for timely completions and well placement decisions. The new generation LWD NMR tools introduced here addresses this need.
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