Summary Between 2014 and 2016, ConocoPhillips drilled five deviated wells adjacent to a multistage, stimulated horizontal producer to sample the physical characteristics of the reservoir stimulation caused by hydraulic fracturing in the Eagle Ford Formation in DeWitt County, Texas. The design, execution, and results of the pilot are described. This pilot establishes the paucity of pre-existing natural fractures in this locale and enables the determination of the spatial characteristics of the stimulation using information derived from the core, cuttings samples, borehole-image logs, tracer logs, microseismic, distributed temperature sensing (DTS)/distributed acoustic sensing (DAS), and pressure data. Results show that stimulation effectively breaks the reservoir rock and makes a complex array of hydraulic fractures that are more closely spaced near the producer. Some fractures, however, extend interwell distances of more than 1,000 ft. The pilot data indicate that abundant proppant transport appears to be limited to distances less than 75 ft from the producer, which suggests that the stimulated rock volume (SRV) might be greater than the volume of rock that can be effectively drained.
In chalk reservoirs such as the Ekofisk Field, fluid flow and geomechanical effects combine to change both the location and properties of the reservoir and overburden. Pore pressure and fluid saturation changes cause reservoir compaction and perturb the elastic properties of the reservoir rocks. The overburden responds to the compaction with piston-like seafloor subsidence and length changes (strains). These overburden strains change the seismic velocity. The resulting velocity changes are observed on time-lapse seismic data as time-shifts that accumulate though the overburden. These processes are being monitored by GPS surveys of the production facilities, repeat logging of radioactive marker bullets, repeat bathymetry surveys, core sample analysis, and time lapse seismic data. The goal of this paper is to combine these measurements to better understand the relationship between overburden strains, changes in overburden velocity, and resulting time-lapse time-shifts.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractIn chalk reservoirs such as the Ekofisk Field, fluid flow and geomechanical effects combine to change both the location and properties of the reservoir and overburden. Pore pressure and fluid saturation changes cause reservoir compaction and perturb the elastic properties of the reservoir rocks. The overburden responds to the compaction with piston-like seafloor subsidence and length changes (strains). These overburden strains change the seismic velocity. The resulting velocity changes are observed on time-lapse seismic data as time-shifts that accumulate though the overburden.These processes are being monitored by GPS surveys of the production facilities, repeat logging of radioactive marker bullets, repeat bathymetry surveys, core sample analysis, and time lapse seismic data. The goal of this paper is to combine these measurements to better understand the relationship between overburden strains, changes in overburden velocity, and resulting time-lapse time-shifts.
Results from time-lapse seismic data on Ekofisk Field have been successfully used in monitoring reservoir compaction and well planning for more than five years. In the past, the time between repeat surveys was up to 10 years , and these surveys observed large time-lapse signals dominated by time shifts attributed to reservoir compaction and overburden subsidence. In addition to continued use of time shifts, joint interpretation of three streamer surveys (1989, 1999, 2003) has demonstrated a useful, though quite noisy, time-lapse amplitude signal at the reservoir level.As the reservoir development matures, we expect the time-lapse signals to become smaller and remaining infill well targets more difficult to locate. Reservoir management needs also demand current information on dynamic reservoir behavior. To meet this need, more frequent surveys are required. As the survey interval is decreased, reservoir changes will become smaller and more difficult to detect. To continue to impact drilling decisions, a future seismic monitoring system must be able to detect these smaller signals and relate them to dynamic processes occurring within the reservoir.To help design a seismic monitoring system to meet these future needs, we have constructed a model of 4D signal detectability for a particular repeat interval and timelapse noise level ( Figure 1). The synthetic time-lapse signal is generated by rock physics and seismic modeling of changes in reservoir porosity, fluid saturation, and pore-pressure output from the reservoir simulator. By simply adding appropriate amounts of band-limited random noise, we were able to match the general signal and noise levels of the field data. Once this calibration step was completed, further modeling was done to estimate various time-lapse noise levels that would be expected from different reservoir monitoring concepts that are being evaluated for Ekofisk (streamer and permanent OBC).The project was designed to evaluate the detectability of a range of survey intervals between six months and four years beginning in June 1999 and ending in January 2005. To evaluate the impact of different seismic acquisition systems on 4D detectability, we estimated the 4D noise level expected for several seismic acquisition systems. These expectations are based on past experience and industry reports. The analyzed noise levels were:• 30% nrms: This noise level is expected from streamer programs where the data have been processed with 4D techniques but there are significant positioning differences between the surveys. This expectation is based on experience with past streamer surveys at Ekofisk. • 20% nrms: This noise level is expected from streamer programs where there has been a determined effort to repeat source output, source positioning, and streamer positioning combined with industry standard 4D processing. This expectation is based on experience with past streamer surveys in the North Sea. • 15% nrms: This noise level represents the best possible streamer survey repeatability and also served as the w...
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