Lost in the Shadows: Surviving Fracturing Hazards with Fluid Tracking

Abstract

Every unconventional well has a unique set of objectives with the same end goal: effective stimulation. During stimulation, a host of problems can potentially arise. For these problems, a solution is needed, but it is often difficult to visualize. Hydraulic fracturing operations encounter challenges including stress shadowing, thief zones, and fracture-driven interference on a regular basis. A novel application of controlled source electromagnetics (CSEM), called fluid tracking, monitors and images hydraulic fracturing operations. In this paper, we present case studies showing common fracturing hazards and we describe how fluid tracking provides mitigation insight. When fracturing fluid is injected into the reservoir rock, it changes the subsurface electrical impedance. The fluid tracking method involves measuring these changes to image the fluid movement. Operations consist of using a controlled electromagnetic field and a dense network of receivers arranged over the horizontal well trajectory. The data recorded over the course of a stage are then refined into a map view motion picture indicating where fluid is flowing. After visualizing and tracking fluid, engineers either adjust designs or confirm success before completing the next well. During hydraulic fracturing, areas of high signal amplitude indicate regions where the fluid successfully penetrated the rock. Interpretation provides the azimuth and half-length of each stage's induced fracture network. In the first case study, the operator used fluid tracking to investigate the performance of a new completion design. The design increased the total stage count with tighter spacing than the previous design. Results showed stress shadowing effects and inter-stage interference were greater than expected. Unlike the symmetric fracture geometry predicted by models, this completion had less than 50% of its monitored stages with signal on both sides of the well. Thus, the majority of stages were highly asymmetric. In fact, the asymmetric stimulation contributed to fracture-driven interference (i.e., a "frac hit") on an offset well. The operator found the increased stage design did not create more effectively stimulated rock volume. Instead, the engineer decided to lengthen stages, decreasing the number required to stimulate future wells. This resulted in lower completion costs without sacrificing production. The second case study explores effects of varied geology along a lateral. For one stage, although diverter was applied, fracturing fluid intersected a natural fracture network and was carried away from the intended target zone. Fluid tracking identified the results of ineffective diversion. Although the observation does not indicate a definitive conclusion on how to avoid the fracture network, it certainly showed the diversion method was insufficient. Operators choose to monitor treatments with fluid tracking to diagnose fracturing hazards and inform mitigation strategies as they improve completion designs and approach an optimized stimulation. The more understanding the industry gains on inter-well and inter-stage communication along with other unknowns during fracturing operations, the better equipped engineers will be when they determine which design modifications have the largest impacts.

Numerical Investigation of Major Impact Factors Influencing Fracture-Driven Interactions in Tight Oil Reservoirs: A Case Study of Mahu Sug, Xinjiang, China

“…Therefore, a series of surveillance technologies, such as micro-seismic monitoring, fluid tracers, and proppant tracers, have been employed to provide information on fracture and proppant geometries, both of which reflect fracture conductivity and fracture connectivity between wells. Moreover, some new methods, such as controlled source electromagnetics (CSEM) and distributed optical fiber monitoring, have been applied to obtain a better understanding of the connections between wells [9,10]. These field observations substantiate some valuable conclusions and indicate that: (1) horizontal, multi-stage, multi-cluster fracturing results in the development of a highly complex fracture system when applied to formations that are characterized by fracture systems of different scales (NFs, faults, and planes of weakness) and pre-existing heterogeneity; (2) in most cases, FDIs in the rock matrix are insignificant, and frac/frac connections between active and passive wells are often destructive; (3) frac/frac connections through the fracture system pathways can remain conductive and cause damage to passive wells for a long time after the fracturing event; and (4) multiple fractures in a single perforation section often show a non-uniform propagation pattern, making it difficult to accurately control each fracture path during horizontal, multi-stage, multi-cluster fracturing.…”