With current market conditions, operators are increasingly looking to maximize investment. It has been estimated that the typical hydraulic fracture completion is only able to recover a small percentage of the resources in the reservoir and re-stimulations, also known as ‘refracs,’ have shown to be able to increase estimated ultimate recovery (EUR) by 53% in the Eagleford and by 69% in the Bakken. Here, we compare and contrast original completion and refracs where microseismic monitoring was used to image both types of completion. When interpreting both programs, we consider the number of events in each dataset, the volumetric extent and complexity or conductivity of the fracture network, and seismic source attributes of each event, such as the stress, energy released and the deformation that occurred. Using these attributes in addition to the spatial-temporal extent of the events, the mechanics of the fracturing process can be understood, which can be further used in the evaluation and optimization of refrac designs.
The aim of hydraulic fracturing is to enhance the pathways for oil and gas extraction using pathways that create and re-activate pre-existing fractures through the injection of high pressure fluids and proppant. When refrac completions are monitored, generally a larger number of microseismic events are observed which suggests that it is often easier to initiate fracturing on a previously completed reservoir, than ‘virgin’ rock. This difference may be indicative of previously open drainage pathways being closed during the refrac process. Additional goals of refrac may include improving the original fracture geometry, fracturing by-passed pay zones in the reservoir, and fracture re-orientation due to changes in the local stress regime (Vincent, 2010).
In our study, we show that in addition to simply re-opening fractures from depleted zones, the amount of energy required for deformation is higher in the refrac compared to the initial completion. In other words, the reservoir has memory and only when original frac'ing conditions are exceeded is it possible to generate or open new fractures as potential fluid pathways. By comparing the stress release for each completion, which quantifies the amount of energy dedicated to deformation compared with the amount of energy dedicated to radiation, more energy is devoted towards radiation outwards and less into the deformation of the reservoir for refracs. This suggests that for refracs it is easier to initiate fracturing and deform the reservoir compared with the initial completion. This paper suggests that event attributes during refracs can be used to better understand and evaluate the effectiveness of these completions.