William Connelly
Pangea International Inc.
Golden, Colo.
Jack Krug
Questa Engineering Corp.
Golden, Colo.
Wells drilled in Russia are broadly classified as "research" wells and "production" wells.
Research wells are drilled by the local geologic institutes, known as geologia, and include exploration and delineation wells. Production wells are drilled by the local production institutes, called neft, for the purpose of developing and producing delineated fields.
Oil and gas prospects usually are identified through seismic prospecting and hydrocarbons are confirmed by drilling and testing exploratory research wells by the geologic institutes.
After hydrocarbons are discovered, fields are delineated with additional research wells. These wells are extensively evaluated and provide data for preparing the Russian geologic TEO not to be confused with the joint venture TEO which includes:
- Estimating original hydrocarbons in place and reserves.
- Designing a development plan.
- Estimating development economics.
Reserve estimates are submitted to the Central Geologic Group in Moscow (presently being shifted to the Area Geologic Committees) for certification.
Fields are offered for competitive bid only after their reserves are certified. Once certified, the responsibility for a field historically has been transferred to the local production company (neft), which drills production wells, constructs intrastructure, installs production facilities, and produces the field.
Depending upon the level of development, fields may be partly or fully delineated or may be at varying stages of development and/or production when a new joint venture is formed. The data available for evaluating a property are different for each of these situations.
Most research wells are evaluated with open hole logs, cores, and flow tests. The typical suite of logs for wells drilled since about 1985 includes the spontaneous potential (SP), lateral, conductivity, microlog, caliper, and acoustic logs.
Some logging suites also include a gamma ray and neutron log. In addition to the open hole logs, prospective formations may be conventionally cored, open hole drillstem tested, and/or flow tested through casing.
In contrast to the relatively complete data package gathered from research wells, production wells generally are evaluated with an SP in combination with a lateral or conductivity log and are not cored or flow tested. The most useful logs for formation evaluation are the SP, conductivity, microlog, and acoustic logs.
Russian logs typically are recorded with the mechanical stylus equipment and the multiple log traces and are not confined to specific tracts or standard scales common to western logs. Depending on the zone being logged, the curves may overlay each other and make them difficult to read.
Fortunately, many log traces are color coded, which makes them more legible. However, even color coded, their presentation makes them difficult to quickly interpret and inherent to scale errors. The following three sections log data, core data, and saturation calculations, review open hole logs and core data as they pertain to estimating net pay thickness, porosity, and hydrocarbon saturations.
LOG DATA
LATERAL/CONDUCTIVITY LOGS
The available suite of Russian lateral logs is summarized along with common logging units and abbreviations (see table).
Western oilmen use lateral logs primarily for correlation and for comparison with older logs. Laterals were the primary resisitivity logs run in Siberia before the early 1980s but are being superseded by induction conductivity logs, which are more accurate and easier to use.
Spacings on lateral logs range from .25 to 9.25 m with the 2.25 m spacing (A2M0.5N tool) being the most common.
Fig. 1 is a sample of the 2.25 m lateral log in combination with the SP across a Jurassic formation in Tomsk oblast; the lateral is recorded in ohm meters and the SP in millivolts.
This SP lateral log combination is referred to as the electric log and is an excellent correlation log. Lateral logs use linear scales with multiple 5X backup and resemble those run in America in the 1950s and early 1960s.
Laterals are plagued by thinbed effects, lack of calibration, and deep invasion of relatively fresh mud filtrate. A full suite of lateral logs is run in many research wells, and the resulting resistivities are manually plotted on a complex series of cyclone charts to correct for invasion and estimate the true formation resistivity.
This procedure is time intensive and renders questionable results. Many Russian petrophysicists prefer using the newer induction conductivity devices to estimate deep resistivity. Conductivity derived resistivity varies significantly from lateral log resistivity.
The "601" induction conductivity log is the most popular of several conductivity tools and is equivalent to the 6FF40 resistivity tool. Conductivity measurements are converted to skin effect corrected resistivity using the nomogram shown in Fig. 2. Although not borehole corrected, these resistivity measurements are the most reliable and consistent for making saturation calculations.
SP LOGS
Westerners use the SP log for correlations, estimating Rw, and sometimes for estimating sand and porosity thicknesses.
In addition to these applications, Russian petrophysicists often use SP to estimate porosity used in saturation calculations and pore volume estimates.
Core data are preferred over the SP for porosity estimates, but core analyses commonly are not available for three years after drilling of a well. Since reserve estimates are required much sooner, the research facilities have derived a series of empirical linear equations to calculate porosity based on the relative deflection of SP.
These equations are updated periodically based on new core data and are provided to district offices by field, by formation.
Where micrologs are not available, sand and porosity thicknesses are estimated using the SP in conjunction with conductivity. With beds exceeding 2 m, this approach is acceptable. However, for beds less than about 2 m, the SP estimated thicknesses are anomalously large.
MICROLOGS
The microlog is a very important log in the Russian suite.
It provides the best indication of permeable and porous formations and is used to construct porosity isopach and net pay maps. There are two common micrologs in use, the A0.025M0 025N and the A0.05M.
These tools correspond to approximately 1 in. and 2 in. electrode spacing. Microlog separation correlates with caliper log mud cake buildup (Fig. 3) and indicates permeable zones. There is good agreement between microlog permeable zones and flow test results.
ACOUSTIC LOGS
Second to core data, the acoustic log provides the best estimates of porosity. However, the tool is an uncompensated, single receiver, dual transmitter device that has inherent problems with hole rugosity and alignment.
There are different sonde spacings available, so one needs to be careful when calculating delta T from T1 and T2; data are recorded in microseconds per meter.
If used carefully, the sonic porosity can be used for qualitative water saturation calculations. However it is first necessary to determine the matrix velocities by crossplotting delta T with core porosities from several key wells.
Special core analysis may be available that calibrate the sonic travel time with core porosity at various laboratory and reservoir conditions.
GR, NEUTRON LOGS
The gamma ray log is used for correlations and estimating shale content in sandstones. It is run in combination with a single detector neutron log in many research wells.
Gamma ray logs are reported in impulses per minute (conventional units) using linear scales; they are not calibrated to a standard API count basis. Westerners use the gamma ray with limited success to determine shale volumes in water saturation calculations; the erratic nature of the curve sometimes renders it unusable.
The scale on some gamma ray logs needs to be estimated based on lithologic "standards" in the wellbore because their printed scales indicate responses significantly different from offset wells.
The virtue of the single detector neutron log is not apparent to the authors. Russian petrophysicists indicated they use it only to identify gas caps.
A neutron log is run prior to running casing. Normally at this time the mud filtrate invasion is deeper than the neutron log radius of investigation, so the log does not record beyond the wet invaded zone.
A second neutron log is run through casing three months later. If there is a significant shift in the cased hole curve compared with the open hole curve, it is interpreted to indicate gas moving back into the zone of investigation, hence the zone is gas bearing.
DIGITIZING LOG DATA
Before starting log analysis, the authors recommend digitizing the required logs across zones of interest.
Scales on logs are linear, and it often is difficult to determine which curves are backup and which are not. There may be as many as four backup curves over some zones. Our best results occur when a log analyst color codes each curve and corresponding scale before digitizing.
Russian log curves are not confined to specific tracts; therefore several curves can migrate over each other and make it difficult to follow traces unless they are color coded. Fig. 4 illustrates the resulting three tract log of the example logs.
In addition to the digitizing difficulties, the authors have encountered significant depth shifts on some logs. Depth correcting log traces is tedious but critical to obtaining usable log calculations. Depth shifts in excess of 10 m are not uncommon. The authors recommend inputting core data with the digitized logs and insuring it, too, is depth corrected.
CORE DATA
Core data are by far the best measure of porosity.
It cannot be overemphasized how critical it is to gather and analyze core data during feasibility reviews of projects. These data are used for log calculations, calibration of sonic data, and most importantly for pore volume estimates of original hydrocarbons in place.
In addition to reviewing the core analyses, the authors recommend an inspection of cores to determine the extent of fracturing, the depositional environment, and the core sampling method and frequency. If time allows (and it usually does not), it is preferable to study the cores in detail.
Russian petrophysicists generally report core analyses in a fashion similar to western core reports. Sampling of cores for analyses tends to be greater in porous sands than in tight sands. Because the porosities used in volumetric calculations will be weighted average core porosities, it is important to understand the sampling technique.
In addition to conventional core analyses, special core analyses sometimes are available. Special core analyses provide mineralogical components, saturation indices, formation factors, grain densities, and acoustic travel times. From these studies the tortuosity constant (a), cementation exponent (m), saturation index (n), and matrix travel time are obtained and provide input data for the log calculations.
Lengthy petrographic descriptions are available on many cores. The authors recommend translating these core descriptions from Russian to English because they contain a wealth of information about reservoir quality, depositional environment, and explain log responses in some pay zones.
SATURATION CALCULATIONS
Since Russian logs lack modern sophistication, an analyst quickly reaches a point of diminishing returns during analysis of the data.
Problems with the data include lack of calibration, no borehole corrections, no depth corrections, no deep invasion correction, lack of Rmf control, and the fact there is only one useful porosity tool.
In the western Siberia basin, the problems are compounded by multi mineralogic sands that include conductive minerals such as glauconite, micas, and pyrite, in addition to the presence of thin laminated beds and shaly sand sequences.
Nevertheless, some log calculations are reliable if the analyst is careful. The following methodology has proven efficient in western Siberia:
- Digitize the SP, conductivity, gamma ray, and sonic curves over zones of interest.
- Depth correct all curves.
- Convert conductivity to resistivity with appropriate tool correction chart(s).
- Determine Rw from fluid analyses, SP, and Hingle and/or Pickett plots.
- Correlate core data to acoustic travel time and destermine matrix velocity.
- Calculate sonic porosity for the zones of interest.
- Create a pseudo calibration for the gamma ray log using wellbore lithologic control points to estimate shale volume. Use the SP log as an alternative shale indicator.
- Assume "a," "m," and "n" are 1, 2, and 2, respectively, for a first pass calculation. If special core analyses are available, consider recalculating the logs with these data.
- Compute water saturations using Archie, modified Archie, and Simandeaux equations.
- Compare saturation calculations with flow test results to determine the reliability of the calculations. Calculated water saturations do not always agree with reported test results.
Fluid contacts often are recognizable based on conductivity and lateral logs. However, test data are the most reliable data for determining fluid levels.
The second part of this series deals with fluid levels and other subjects pertaining to reservoir description and volumetric estimates of original hydrocarbons in place.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
