CONNECTIVITY MAPPING WHILE DRILLING IMPROVES RESERVOIR ANALYSIS
Steve Rutherford, Mark Parchman
Anadarko Petroleum Corp.
Houston
Walter Turpening, Yu-Taik Chon, Tawassul Khan
Reservoir Imaging Inc.,
Stafford, Tex.
Connectivity mapping while drilling uses the bit as an acoustic energy source and receivers in nearby wells to improve reservoir characterization for better infill drilling and development programs.
The drill bit is an ideal source for connectivity map-ping because it is a wide bandwidth, energetic source that can be detected a few miles away. The data are acquired while drilling is in progress; therefore, drilling decisions can be adjusted to achieve the objective of maximizing production.
Infill drilling can increase the vertical and aerial sweep efficiency by contacting isolated and compartmentalized zones. To be economic, infill drilling programs must b conducted in conjunction with detailed engineering and geologic reservoir char acterization. Targeted infill drilling that strategically selects new well location based on detailed geologic and engineering models ca provide better returns o investment.
For better reservoir descriptions between wells, technologies that can map the reservoir with greater resolution are needed. Connectivity mapping (CM) and connectivity mapping while drilling (CWD) can map reservoir continuity within a few feet of resolution.
Sales of producing properties have changed the structure of the U.S. oil and gas industry, and the resulting reanalysis of many fields has highlighted shortcomings in previous reservoir
descriptions. For various reservoir lithologies, up to 80% of the original oil in place is unrecovered.1
For better reservoir descriptions between wells, tools with higher resolution are needed. Anadarko Petroleum Corp. used both CM and CWD technology in the Clearfork sand unit in the Ketchum Mountain field to map geologic complexities and identify discontinuities in the sands.
The project was recorded in two phases:
- Connectivity mapping using the CS-1 system (recorded in 2 days)
- Connectivity mapping while drilling using the drill bit as the source and recording with two CS-1 receiver systems in two nearby wells (recorded continuously over a 23-hr period as the reservoir interval was drilled).
FIELD HISTORY
The Ketchum Mountain field is located in Irion County, Tex., in the southeastern part of the Midland basin and produces from the San Andres, Clearfork, and Sprayberry formations. The field was discovered in 1955 by Sinclair Oil & Gas Co. The unit is currently operated by Anadarko Petroleum Corp., which acquired the property in January 1993 from Arco Oil & Gas Co.
The Ketchum Mountain Clearfork unit includes 3,752 acres and currently has 39 active producers and 21 injectors. The main pay sands in the Clearfork unit produce from an average depth of 4,500 ft.
The Clearfork interval was unitized in 1975, and full scale waterflooding began in 1976. Waterflood response has been erratic and generally poor because of the discontinuity of individual sandstone beds.
The unit is continuing to be developed with infill drilling, well conversions, and workovers and is currently under study to determine additional work necessary to optimize waterflood performance. To date, the unit has produced approximately 4.8 million bbl of oil and 10.2 billion cu ft of gas.
GEOLOGY
The Clearfork producing interval is composed of interbedded marine sandstones and shales deposited in a slope to basin floor environment during middle-to-late Leonardian time. The Clearfork sandstones and shales are overlain by upper Leonard marine shales and are underlain by marine shales and sands of the Sprayberry formation.
The Clearfork sands are locally divided into three laterally discontinuous sand packages (A, B, and C sands). The middle and lower (B and Q sands are hydrocarbon bearing, and the upper (A) sands are water producing.
The quartz sandstones are fine to very fine grained, moderately well sorted, and subrounded. The sandstones contain varying amounts of dolomitic, siliceous, and argillaceous cements. The average porosity is 13% and average permeability is 5-8 md for the hydrocarbon-bearing sands. Individual sands in the B and C packages are discontinuous and usually less than 20-ft thick.
Structurally, the area is characterized by predominantly west dip at a rate o 100 ft/mile. The trapping mechanism for the productive Clearfork sandstones is an updip pinchout along the eastern margin of the field.
CONNECTIVITY MAPPING
Connectivity mapping is cross-well seismic processing technology. 2 The main connectivity mapping concept i that relatively low-velocity layers in reservoirs act a wave guides trapping channel waves in them. Channel waves have high energy an contain considerable information about the layers in which they propogate.
If channel waves with high frequencies (1,000-3,000 hz) are recorded between two wells, the probability is very high that the layer is continuous between the wells.
Connectivity mapping is done in the frequency domain with small apertures. Therefore:
- Exact relative well bore positions are not required (travel times are not picked for velocity calculations).
- Smaller data sets are recorded (25-30% of that used in other cross-well seismic techniques) because sources and receivers need only be in the reservoir interval.
- Recording range between two well bores is greater than that for any other cross-well seismic technology.
- Any broad band seismic energy source in the well bore can be used.
These points are demonstrated in the Ketchum Mountain field project. The project had two phases: first, a connectivity mapping survey recorded using a cylindrical bender source and receiver system in one well pair, and second, two connectivity mapping while drilling (CWD) surveys using the drill bit as the source during the drilling of the reservoir interval.
FIELD PROJECT
Fig. 1 (34456 bytes) is the well layout map in the Ketchum Mountain field, and Fig. 2 (28054 bytes) is the downhole recording configurations for the two phases of the project.
The CS-1 system, which uses cylindrical bender source and receivers, recorded the Phase I connectivity mapping survey between Well C and Well B. A 60 x 60 survey at 2.5-ft intervals was made using a 300-2,100 hz, 700-millisec sweep with a total of 16 sweeps per source level.
In Phase 2, two CS-] receiver systems were used to record the CWD survey in Wells B and C while Well D was drilled from 4,000 to 4,364 ft. The drill bit energy was significantly greater than the CS-1 source so that attenuators were put in the recording system to prevent overloading of the input. Seventy-five receiver levels at 5-ft intervals were recorded 4-6 times per pipe joint (30 ft) drilled down. The drilling rate was 3-5 min/ft.
A four-geophone package, buried in a 200-ft deep hole about 200 ft from the drilling rig, monitored drilling conditions. During processing, these data were used to balance the cross-well data for changes in weight on bit, revolutions per minute, etc. No recording occurred when a joint of pipe was added. The mud logging crew coordinated bit depth and time of day.
CM INTERPRETATION
Anadarko's Ketchum Mountain field is a Clearfork formation marine turbidite sandstone reservoir. In this field, shale intervals have lower horizontal seismic velocities than the reservoir sands and therefore behave as wave guides. The results of the connectivity mapping project confirmed that certain low-velocity shale markers correspond to time stratigraphic sequences. These sequences, identified from the CM and CWD results, were used to reinterpret the distribution of sand reservoir discontinuities and heterogeneities.
In a given reservoir, connectivity mapping (CM) identifies low velocity units and characterizes their wave guide properties. Such properties are not the usual velocity and density of conventional seismic data but the energy and frequency spectra of channel waves propagating in the units. These properties are then translated into an image to correlate with the well information.
The Clearfork interval is broken into three units: the A sand sequence (uppermost), the B sand sequence, and the C sand sequence (lowermost). Historically, the individual sands were numbered top-down within each sequence.
The interconnection of the shales is addressed with an alphabetic nomenclature: a denotes the uppermost identifiable shale in the sequence, and b the next, and c the next with increasing depth. This improved identification is based on the connectivity maps. Figs. 3-5 are the connectivity maps with the shale connections shown.
WELL C TO B
Shales are the low velocity zones; therefore, the strong connections colored yellow to red on the CM displays are the continuous shales (Fig. 3)(56748 bytes). The depositional environment is interpreted to be turbidities. Fig. 3 (56748 bytes) is the connectivity map between Well C and Well B (1,320 ft separation).
Most striking is the shale continuity between 4,3604,370 It in Well C (Cd shale) and the shale at 4,410-4,415 It (Cd) in Well B. This shale cuts across the log correlation of the sand (C2 sand) at the bottom of Well C and the sand at 4,395-4,410 ft (C2) in Well B (Fig. 6)(63614 bytes). Note the connection between the Cd shale and the Cb shale in Well B, as well as the Cb shale connection to the Cb shale in Well C. This complex interconnection of shales isolates the sands as direct flow units between Wells C and B.
In the B interval there is a distinctive change in the sequence, denoted by the Bb shale being low energy and the gamma ray signature in Well B less well developed. Conventional correlation of the logs would have placed the sand correlation at 4,355 ft in Well B. This correlation of the B sequence implies that there is an expansion of the section in the upper C sequence or lower B sequence west and south of Wells B and C.
WELL D TO C
Fig. 4 (58745 bytes) is the connectivity map between Wells D and C. The seismic source in Well D was the drill bit as the well was drilled through the lower Clearfork interval (4,000-4,364 ft). The vertical resolution there was about 5 ft, which is slightly less than that for the connectivity map (without drilling).
The C sequence has striking correlations. The Cd shale in Well C is connected to the Ca and Cd shales in Well D (Fig. 4)(58745 bytes). This correlation means that the three C sands encountered in Well D are isolated from the C sands in Well C.
The B sequence shows connectivity with uniform energy levels in both wells.
WELL D TO B
With the understanding gained from the two 0.25 mile connectivity maps between Wells D and C and between Wells C and B, one can begin to understand the long spacing connectivity map between Wells D and B (Fig. 5)(49779 bytes).
Without the information from Well C, there is clear indication that in Well D the three C sands between the Ca and Cd (CO shales are isolated from the main C2 sand in Well B (Fig. 5)(49779 bytes). There is also an indication that the C2 sand in Well B truncates in the region nearer to Well B (approximately in 800-900 It of the well). The Cd shale is a clear, continuous shale from Well D to Well B. In this current nomenclature, Ca is connected to Cb.
The B sequence is changing character between the two wells, and the expansion of the lower B/upper C section is evident. The A sequence shows continuity, but the shale interconnection between Wells C and B is blurred.
RESULTS
With clear indications that the C sequence shales are isolating the sands between Wells B, C, and D, it is highly unlikely that the water injection in the C2 sand in Well C is contributing to oil production to the west, north, or east.
Because hydrofracing has been extensively used in this entire area of the Midland basin, some early attempts may have opened fractures to thief zones, and water is being lost. Alternatively, other reservoir units may be receiving the injected water, and prospective units have been created behind pipe.
The structural cross section before the cross-well surveys shows original sand correlations for the B and C
hydrocarbon-bearing sandstones (Fig. 6)(63614 bytes). The base of the A sandstone shown on the cross sections represents the base of the overlying water-bearing sandstones in each well bore and does not represent a correlation marker.
The structural cross section after the cross-well surveys is a new interpretation which takes into account some of the information provided by the connectivity maps and the shale correlations identified on the images (Fig. 7)(63517 bytes).
The B sandstones are interpreted to have a more discontinuous character on the after cross-well survey section, most notably between producer Well B and injector Well C. The uppermost B sandstone shales out to the west, and the lower B sandstone in injector Well C correlates to the solitary sandstone in producer Well B. This interpretation is more consistent with what is shown in Fig. 3 where a connection occurs at Bb.
The major change in the interpretation of the C sandstone correlation on the later cross section occurs in producer Well B. In Well B, the thickest sandstone is interpreted to be discontinuous and is not present in injector Well C to the east nor is it present in injector Well A to the north. This discontinuity implies that this sandstone, at least locally, has a channel character and trends northeast/southwest. This interpretation is consistent with the shale correlations Cd from the connectivity maps.
The lowest sandstone in injector Well C is shown to thin to the west and is less than 4 ft thick in producer Welt B. It is possible that these individual sandstones are not connected and is so indicated on the cross section by the dashed lines between the two wells. This interpretation would support the shale correlations Cd from the connectivity map. The same situation exists to the east of injector Well C for the lowest C sandstone. This interpretation is supported by the Cd shale correlation from the connectivity maps.
The results of connectivity mapping (CM) and connectivity mapping while drilling (CWD) showed that reservoirs in the Ketchum Mountain field are more complex and heterogeneous than initially believed. This information upgraded the previous geologic model in which the producing sands were mapped as continuous across the field. The latest subsurvace geologic interpretation supports the CM and CWD results.
Anaarko is planning an infill drilling program in Ketchum Mountain field during 1995-96 that will test these interpretations.
CWD data were recorded at 1/4 mile and 1/2 mile ranges. The signal strength was strong enough to enable successful cross-well mapping over a distance of 1 mile.
ACKNOWLEDGMENT
The authors thank Anadarko Petroleum Corp. for permission to publish this article and Mike Cochran and Lee Petersen and Anadarko for reviewing this material. The authors also thank Brad Milliken, field foreman of the Ketchum Mountain field, for his support and help during data acquisition, and Dan Kernaghan and Kevin Renfro in Anadarko's Midland office for their support with the project.
This project has been partially funded by Gas Research Institute (GRI). The authors thank Anthony W. Gorody and the GRI staff for their support and assistance. The generous support provided by Cloy N. Causey, director of the Energy Research Clearing House, has been the primary vehicle for making this project possible.
REFERENCES
1. Tinker, G.E., Turpening, W.R., and Chon, Y-T., "Connectivity mapping improves understanding of reservoir continuity," OGJ, Dec. 20,1993, pp. 87-91.
2. "Crosswell Bed Connectivity Analysis," abstracts from the 1993 Society of Petroleum Geologists International Exposition and 63rd Annual Meeting, Washington, D.C., Sept. 26-30, 1993.
3. Fisher, W.L., "Factors in Realizing Future Supply Potential of Domestic Oil and Natural Gas," July 1991, Aspen Institute Energy Policy Forum.
