Lost in the Shadows: Surviving Fracturing Hazards with Fluid Tracking

摘要

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 stressshadowing,thief zones,and fracture-driven interference on a regular basis.A novel application ofcontrolled source electromagnetics(CSEM),called fluid tracking,monitors and images hydraulic fracturingoperations.In this paper,we present case studies showing common fracturing hazards and we describe howfluid 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.Operationsconsist of using a controlled electromagnetic field and a dense network of receivers arranged over thehorizontal well trajectory.The data recorded over the course of a stage are then refined into a map viewmotion picture indicating where fluid is flowing.After visualizing and tracking fluid,engineers either adjustdesigns or confirm success before completing the next well.During hydraulic fracturing,areas of high signal amplitude indicate regions where the fluid successfullypenetrated the rock.Interpretation provides the azimuth and half-length of each stage's induced fracturenetwork.In the first case study,the operator used fluid tracking to investigate the performance of a newcompletion 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.Unlikethe symmetric fracture geometry predicted by models,this completion had less than 50% of its monitoredstages with signal on both sides of the well.Thus,the majority of stages were highly asymmetric.In fact,theasymmetric stimulation contributed to fracture-driven interference(i.e.,a”frac hit”)on an offset well.Theoperator 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.Thisresulted in lower completion costs without sacrificing production.The second case study explores effectsof varied geology along a lateral.For one stage,although diverter was applied,fracturing fluid intersecteda 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 howto 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 informmitigation strategies as they improve completion designs and approach an optimized stimulation.The moreunderstanding the industry gains on inter-well and inter-stage communication along with other unknownsduring fracturing operations,the better equipped engineers will be when they determine which designmodifications have the largest impacts.