A common practice in log interpretation is to cross-plot van"ous porosity log readings in order to determine formation iithoiogy and compute porosi~accurately. LCrossplots of Sonic versus Densi~logs are widely used in the interpretation of shaly sands. For carbonates, Density versus Neutron cross-plots are commonly employed. These plots and the calculations based on them are extremely useful, but, when the lithology is a complex mtiture of ,m~",qg!$ i,nterpretgtie.~OJf the drta OJfte.nMcmnes am-bi~ous. The "Lithe-Porosity" cross-plot is introduced for interpretation in formations of complex lithology. It presents simultaneously the data from all three of the standard porosity tools: the Sidewall Neutron Porosity log, or the GNT; the Formation Density Compensated log; and the Borehole Compensated Sonic log. From the readings of these logs two porosity-independent parameters, "M" and "N'; are derived-M from the Sonic and Density, and N sented by a unique point regardless of porosity. For a formation of complex lithologv, the position of the log data points on the M-N plot relative to the pure mineral points is of great assistance in identifying various minerals in the formation. Litho!ogical information so derived is then used to calculate accurate values of porosity. The computer can be programmed to produce cross-plots of M versus N from logging data recorded on magnetic tape, or on punched cards. The method allows detailed studies of individual formations and compan"sons with other we[[s in a fraction of the time required using manual methods.The Lithe-Porosity technique has many applications in formation evaluation and interpretation. Examples are shown in the paper.
A Devonian shale – specific geophysical log interpretation model was developed to extract as much geological and reservoir data as possible from a suite of air- or gas-filled-hole logs. Development of the methodology and evaluation of the geological and reservoir properties was supplemented with core data, formation tests, mud logging data, temperature logs and borehole TV logs acquired in Devonian shale wells. This new analysis technique improves the identification of hydrocarbon bearing intervals and the types of formations associated with production. As a result, the analysis when combined with other data, allows a more effective selection and treatment of the hydrocarbon productive intervals. Examples are shown which compare the computed lithology and porosity with core measurements. The computed results are also compared to mud logs; noise logs; temperature logs; borehole television data; production logs; and production tests.
Summary. Commonly available tools used by producers for selecting intervals for stimulation provide different, and sometimes conflicting, information on which zones have the best potential to produce gas. Data on the presence of hydrocarbons and permeable features required for migration of hydrocarbons to a well can be acquired from mud logs, noise and temperature logs. borehole television logs, and geophysical well logs. The Gas Research Inst. (GRI) has funded the collection of these data in more than 30 Devonian shale wells as part of its Eastern Devonian Gas Shales project area, permitting an evaluation of the utility of these diagnostic tools for selecting treatment intervals. Special interpretive methods were developed for analyses of data from geophysical logs, borehole television logs, and experimental mud-logging techniques. Data from each technique were analyzed to determine the geologic and reservoir characteristics of the zones with gas production potential. Production logs were collected in the study wells after stimulation and cleanup to determine the relative contribution of each completed zone to the total production of the well. The comparison of actual productivity of each completed zone to the anticipated gas potential as indicated by each diagnostic technique can provide producers with a better understanding of how to interpret and use diagnostic data collected during drilling and completion of Devonian shale wells. Introduction One premise of the GRI's Devonian shale research program is that, to improve productivity of Devonian shale wells, it is necessary to develop a better understanding of the permeable pathways that allow gas to migrate from the matrix to the wellbore. Studies under the U.S. DOE's Eastern Gas Shales Program have shown that gas is contained in the rock matrix throughout the Appalachian basin. Yet productivity of development wells can vary significantly, even when offsetting other development wells. Understanding how to recognize the combination of geologic features necessary for good production can improve the producer's ability to complete wells in the best zones. The objective of the Geologic Production Controls project is to establish a fuller understanding of the relationships between productive gas flows and various geologic features that control the distribution and magnitude of production in Devonian shale gas wells. To accomplish this objective, the GRI is investigating 30 newly drilled wells in West Virginia, Kentucky, and Ohio. The purpose of this study is to determine in individual wells which zones are producing gas and then to characterize the geologic and reservoir properties of both the productive and nonproductive zones. The relative contribution to production of each perforated zone is determined by running production logs in each well after stimulation. The geologic and reservoir properties are then characterized by standard diagnostic methods: mud logs, temperature logs, noise logs, borehole television logs, geophysical logs, sidewall cores, and surface well tests. Through this approach, the GRI plans to determine how to identify productive zones in Devonian shale wells better. One outcome of this study has been an evaluation of the productivity of completion zones selected on the basis of various diagnostic techniques. Through the course of the project, the producers have selected zones for perforation and stimulation. Often, data from several diagnostics supported the selection of a particular zone. Frequently, however, one indicator, such as a temperature anomaly or fractures visible on the borehole television, was used as the criterion for completion. The comparison of the relative contribution to production of the selected zones to the interpretation of the diagnostic methods is discussed below. Production Logs While nearly the last set of data collected in a well, production logs are the most important data in determining where gas is flowing into a well. Production logs were run after stimulation and cleanup of each of the study wells immediately before drawdown and buildup well tests were conducted. These logs were interpreted to determine the relative contribution to total production of each perforation in the well. A production profile was constructed for each well that showed an estimate of the actual productivity of each zone. This production profile was used to compare actual productivity of each zone to the potential productivity indicated by each diagnostic method. While the contribution of each perforation can be determined by careful interpretation of production logs, it is not possible to determine from the available data whether the gas is produced directly from the perforated zone or whether the gas has migrated vertically through an induced fracture to that perforation. Recording useful poststimulation production logs in typical low-gas-flow-rate, low-pressure Devonian shale wells required well-maintained, high-quality equipment and special logging procedures and interpretation techniques. The production log suite run in Jackson Well 10 in Jackson County, WV, included the full-bore flow-meter (spinner), gradiomanometer (fluid density), manometer (pressure), and thermometer (temperature survey). This logging suite permits the detection of gas entries into the wellbore through the perforations, but not necessarily where gas was stored in the formation, especially when the well has been hydraulically stimulated. In addition, the gas flow rate at standard conditions can be determined at any depth in the well. This permits evaluation of the contribution of any perforation to the total flow rate from the well. For example, the perforation in Jackson Well 10 at 4,276 ft [ 1303 m] (Fig. 1) contributes 24.6 Mcf/D [697 meters cubed per d] to the total production. Mud Logging Natural gas in the drilling fluid is detected at the flowline at the surface and recorded on the mud log. A hypothetical mud-log total-gas curve recorded in a well where air was used as the drilling fluid is shown in Fig. 2. (Interpretation of this figure assumes constant drilling fluid flow rate and constant drilling rate of penetration of the formation.) JPT P. 187^
Summary The newly developed borehole acoustic flowmeter provides data to locatedepth precisely and to quantify gas production from individual gas entries, even when they are spaced closely together, for low-flow-rate gas-producingzones in a typical Devonian shale well in the Appalachian basin. Data from afield test conducted in Aug. 1987 are presented, along with the measurementtheory involved. A comparison to a conventional spinner flowmeter on the samewell is also presented. Introduction One premise of the Gas Research Inst.'s (GRI's) Eastern Devonian shalesprogram is that to improve productivity of Devonian shale gas wells, it isnecessary to develop an understanding of the permeable pathways that allow gasto migrate from the matrix to the wellbore and of the effects that variousstimulation methods have on enhancing natural producibility. Production logs, nearly the last set of data collected in a well, are among the most importantdata available in determining where gas is flowing into the wellbore. Production logs run before and after stimulation are interpreted to determineeach perforation's relative contribution to total production. Production profile logs permit evaluation of not only the diagnostic methods used toselect perforations for completion, but also the effectiveness of the procedureused to stimulate the well. Recording useful conventional production logs intypical low-flow-rate, low-pressure Devonian shale gas wells requireswell-maintained, high-quality equipment and special logging procedures andinterpretation techniques. It became clear early in the research program thatconventional production logging tools were approaching the lower limits of accuracy and that a flowmeter designed specifically for these conditions wouldbe desirable. One outcome of this research was the development of anexperimental flowmeter tool based on the time of flight of an acoustic wavepropagated between two ultrasonic transducers mounted in a probe lowered intothe wellbore. Flowmeter Tool Description The borehole acoustic flowmeter measures the time of flight of acousticwaves propagated between two ultrasonic transducers located in the downholetool. The transducers are mounted axially, 7–7/8 in. apart, in a 1–11/16 in.-ODflow tube (Fig. 1). Gas enters the open-ended flow tube at the bottom of thetool (inlet) below the lower transducer and exits through exhaust ports abovethe upper transducer. A pressure sensor is mounted on a bulkhead immediatelyabove the exhaust ports. A temperature sensor attached to the same bulkheadprojects down to the exhaust ports. A gamma ray sensor located above the flowsection provides depth control, and a caliper measures hole diameter. In-linecentralizers near the top and bottom of the tool string maintain the sensorarray in the center of the casing. The acoustic flowmeter operatingspecifications are shown in the Appendix. Acoustic Flowmeter Field Data Log A field test of the experimental acoustic flowmeter tool was made in the Columbia Gas Transmission Corp., Pocahontas Land Co. Well 20460G in Martin County, KY, on Aug. 28 and 31, 1987. The well was producing natural gas atrates between 62 and 77 Mcf/D from perforations in the Sunbury shale, the Bereasandstone, and the Devonian shale. The data in Fig. 2 were obtained with thetool descending over the Sunbury and Berea formations only (File A). (Note thatthe measurements should be made with the gas entering the tool at the inlet andmoving upward through the flow section to exit at the exhaust ports.) In Track1 (left track), the gamma ray log, recorded for depth-control purposes, and thecable velocity (CVEL), used to correct the computation of gas velocity for toolmovement, are shown. The upward and downward time-of-flight (UTOF and DTOF, respectively) measurements, along with the apparent speed of sound in the gas(CGAZ), are displayed in Track 2 (center track). Displayed in Track 3 (righttrack) are the downhole temperature (TEMP) and pressure (PWF) and the apparentgas velocity (VGAS) computed from the times of flight. The apparent gasvelocity is not zero below the lowest perforation because additional gas isbeing produced from the Devonian shales (not shown) below the Berea formation. The steps on the temperature curve (1F), times of flight, and speed of soundresult from the conversion from analog to digital. The field-recorded pressurewas replaced by a calculated value with the measured wellhead pressure and gasspecific gravity on this log display.
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