In an industry environment where new opportunities for exploration and development are dwindling, every bypassed pocket of oil and every unswept zone becomes a crucial consideration for operators. There is a new imperative facing operators. They must image the subsurface to obtain the most detailed picture of reservoir fluid dynamics possible, and at a reservoir scale. The limitations and pitfalls of well-to-well correlation in divining what fluids or reservoir volumes lie between wells are compelling. Too many assumptions based upon correlations have been proved to be totally wrong.

Even advanced wellbore logging and seismic combinations present imperfect solutions. Logs offer sharp detail in the near-wellbore area, but cannot see more than a few inches or feet into the formation. Accordingly, the presence of commercial hydrocarbon volumes or dynamic reservoir fluid behavior cannot be characterized between wells from wellbore data. Seismic can provide reservoir-scale images, but at a resolution so coarse as to inhibit conclusive determination of the reservoir’s fluid properties. Because interval velocities must be assumed, quantification of bed thickness from seismic is all but indeterminate in any useful detail.

Bridging the interwell gap

The DeepLook-EM electromagnetic crosswell reservoir imaging and monitoring system from Schlumberger is the first tool to successfully bridge the interwell information gap. At the reservoir scale, the system is capable of measuring interwell resistivity when the wellbores are hundreds, or even thousands, of feet apart. In fields under waterflood, the tool tracks the distribution of injected fluid volumes and the resulting swept zone by measuring resistivity sensitivity to changes in fluid saturation and temperature. These measurements make it possible to infer structure, temperature distribution, and residual saturation of affected reservoir volumes. Operators can use the system to image and monitor the effects of steam or water saturation changes, thus helping to guide field development and enhance reserves estimates.

Well A is the observation well, well B is the abandoned producing well, and well C is the steam injector. The crosswell survey mapped the steam-affected zone in the interwell space. Cooler colors (blues) indicate the lower resistivity of steam-swept zones. Warmer hues represent the high resistivity unswept zones. (Image courtesy of Schlumberger)

DeepLook-EM transmitters are deployed in one well, and receiver arrays are deployed in an adjacent well (or wells). Wells may be open or cased, and interwell gaps as large as 3,280 ft (1,000 m) can be imaged. The magnetic moment produced by the transmitter is 100,000 times greater than a conventional induction tool. The resulting interwell image compares to a CAT scan.

The optimum use of DeepWell-EM is in time-lapse mode to track fluid movement. For example, in a water-alternating-gas injection scheme, operators can assess, over time, water migration through a reservoir; determine the ideal injection profile for increasing oil recovery; and avoid water override. Sweep effectiveness can be assessed in real time and bypassed pay avoided.

One successful crosswell survey in California involved a project in an aging heavy oil field under cyclic “huff ‘n’ puff” steam injection. The operator sought to ascertain the steam-saturated volume around the injector wells in relation to the reservoir geology. However, the reservoir features were complex, steeply dipping beds, making it difficult to determine the affected volume around each injector, both vertically and radially.

DeepLook-EM technology was deployed in an abandoned cyclic steam producer well and a more recently drilled observation well less than 50 ft (15.3 m) away from the injector well. The crosswell survey mapped the steam-affected volume in the interwell space. A hidden zone of high permeability was discovered that had been siphoning off the steam flood to a shallower level, impairing its effectiveness.

The new DeepLook-EM technology has been shown to be highly promising for monitoring sweep efficiency, detecting bypassed pay, and optimizing simulations for improved reservoir management. Most importantly, it can telescope the timeline for tracking and monitoring reservoir fluid movements to a matter of months rather than years.