During waterflooding, formation heterogeneity controls water sweep efficiency. The location of water-injection wells and operational parameters should be designed based on the heterogeneity of the formation in order to maximize oil production and to avoid early water breakthrough. However, initial geologic and reservoir characterization studies may not provide an accurate view of the location of high-perm zones, channels, and fractures. In this study, we present the application of multi-well pressure pulse testing in the dynamic monitoring of waterflooding sweep efficiency and in the investigation of interwell connectivity. In a pressure pulse test, a periodic injection-rate signal comprising a series of active and shut-in cycles is superimposed on the regular schedule of a candidate injection well, and the corresponding pressure response is recorded at several surrounding wells. Because of the periodic characteristic of the pressure pulse test, pressure response at observation wells can be extracted from background reservoir pressure data; therefore, no well shut-in is required during the test. Pressure data are then transformed into the frequency domain using fast Fourier transform (FFT). Finally, using a modified analytical solution in the frequency domain, the hydraulic diffusivity coefficient is obtained. The modified analytical solution only takes pressure data at the pulsing well and the observation well; no injection-rate data is required in the analysis.
We verified the accuracy of pressure pulse testing using a waterflooding case in an inverted five-spot well pattern. The measured hydraulic diffusivity from pulse testing was in agreement with the hydraulic diffusivity coefficient calculated from formation properties. In the next step, we applied pressure pulse testing in a five-spot well pattern with a channel between one of the well pairs. Again, time-lapse analysis of the hydraulic diffusivity coefficient successfully identified the direction of the high-permeability channel, which helped prevent an early water breakthrough. In the last simulation case study, we investigated pressure pulse testing in a synthetic field-scale waterflooding project. We considered a lognormal permeability distribution to mimic formation heterogeneity. Using the information derived in this step, the zones with higher water invasion were determined, which led to modification of well operational parameters such as injection rate and bottom-hole pressures. Optimization of waterflooding design significantly improved sweep efficiency and increased the recovery factor of the process. Pressure pulse testing reasonably predicted the heterogeneity of the formation through interwell hydraulic diffusivity coefficients. Such information substantially assists in future planning and development of the formation.
