A New Surveillance Method for Delineation of Depletion Using Microseismic and Its Application to Development of Unconventional Reservoirs

Abstract

Hess Corporation performed microseismic monitoring of hydraulic fracturing in two infill horizontal wells in the Middle Bakken Play of North Dakota during 2011. Six vertical observation wells were drilled to monitor microseismic events. Each observation well was fitted with a pressure gauge in the Middle Bakken interval, capped by a bridge plug, and instrumented with 40 geophones. The experiment was specifically designed to observe the interaction of the fracs with a pre-existing Middle Bakken horizontal well, which had been on production for about two and a half years. The producing well was shut in for the duration of the experiment and instrumented with a downhole pressure gauge.Experimental results show the hydraulic fracturing did not propagate as anticipated. Instead, the microseismic observations provided a very interesting result early during the stimulation of the first infill well. While fracturing the fourth stage of the first infill well, microseismic events appeared along the length of the nearby production well at distances up to 9,000 ft away from the open frac port. During this stage, and just prior to the appearance of the distant events, a pressure connection was established to the original well as recorded by the pressure gauge in that well. Our interpretation is that these microseismic events were distributed throughout the depleted zone surrounding the original production well. We provide evidence for the development of a cloud of depletion points through the use of pressure measurements, microseismic event speed plots, and Mohr's circle analysis.This experiment provided a unique opportunity to "see" the shape of the depleted zone and to understand its influence in developing Bakken resources. By analyzing the outline of the depleted zone outline, we can now propose a series of reasonable infill well locations at a minimum distance from the original producer. We are leveraging microseismic as a tool to optimize the spacing of development wells. Our objective is to infill drill at the minimum distance required to maximize fracture contact in-zone near the infill wells and yet avoid significant overlap into previously depleted zones. Thus, a new application for microseismic monitoring is envisioned, in which one would pressure-up a producing well in order to measure its actual area of depletion prior to planning subsequent development wells and executing field-wide strategies for hydraulic fracturing.

A Validation Assessment of Microseismic Monitoring

“…In some unconventional reservoirs, microseismicity often shows fairly significant amounts of upward development that often times might not appear reasonable based on volumetrics (e.g., Pope et al 2009;Curry et al 2010;Dohmen et al 2013). It is not clear if the activity is a result of a fracture actually propagating that far vertically, stress effects, pressurization of existing faults/natural fractures (Yang et al 2013), or something else.…”

“…In a similar vein to determining the SRV, a relatively recent application of microseismic monitoring is to use it to determine depleted volumes around wells that have been on production for some time (Dohmen et al 2013;Yang et al 2013). The procedure is to pressurize a producing well to induce microseismicity in the depleted region as a result of the altered in-situ stress field caused by depletion.…”

“…The procedure is to pressurize a producing well to induce microseismicity in the depleted region as a result of the altered in-situ stress field caused by depletion. The geomechanics behind this behavior are well understood (e.g., Teufel and Rhett 1992;Dohmen et al 2013); the difficulties lie in sufficiently characterizing the reservoir stress, mechanical properties, and flow properties to fully interpret the microseismicity field. As with the SRV, it is not clear how the depleted volume could be validated.…”

“…Because a hydraulic fracture has a significant impact on the local stress state in the reservoir, mechanical analyses suggest that fractures could be affected by other nearby fractures, with the case of multistage horizontal wells potentially having moderate to severe effects on successive stages (Roussel and Sharma 2011;Wu et al 2012;Castonguay et al 2013;Dohmen et al 2013;Kan and Olson 2015). Shadowing is expected to have two manifestations-re-orientation and pruning.…”

Stimulated Reservoir Volume: A Misapplied Concept?

“…This example utilizes the Bakken microseismic data presented by Dohmen et al (2013) and builds on the work of Chu et al (2012). To illustrate the impact of fracture geometry and complexity on well performance and RTA, hydraulic fracture and reservoir modeling were performed using reservoir properties, rock mechanical data, and 3D stress regimes derived from a detailed geomechanical study coupled with fracture modeling and reservoir simulation history matching.…”