Permanent Microseismic Monitoring System in a Long Horizontal Well

摘要

A permanent microseismic monitoring array has been installedin a horizontal well with the objective of imaging propagationof acid fractures and water injection fractures in offset wells.The array consists of 16 geophones placed at 500 ft intervalsalong the horizontal section of a producing well, deployedinside the pre-perforated liner and coupled to the formation bygravity. Optimal monitoring conditions are achieved whenproduction from the well is closed in, however, many eventshave been detected whilst the well is producing by applying aproduction noise filter. Microseismic events are detected, oneper week on average, when the monitoring well is producingand 1-2 events per day when the well is shut in.The microseismic monitoring well is located in theHalfdan field in the Danish North Sea. This chalk reservoirhas been developed with a line drive waterflood configuration,alternating oil production and water injection wells. Parallelhorizontal producers and injectors are drilled in the directionof maximum horizontal stress, 600 ft apart with reservoircompletions in the range 10,000-15,000 ft. The relatively closewell spacing is necessary to maximise recovery from the tightchalk reservoir. Multiple acid fractures are placed inproducing wells to maximize productivity while fractures areinduced by water injection in injection wells in order toachieve voidage replacement. Orientation of induced fracturesalong the direction of the horizontal wellbore is critical toavoid early water breakthrough and inefficient sweep patterns.Approximately 450 microseismic events have been locatedfrom a total of 13 acid fracture stimulation treatments in twooffset producers, both 1,200 ft from the monitoring well.Definitive fracture orientations have been determined for 8acid fracture stimulation treatments. Contrary to expectations,the fractures have propagated orthogonally to the wellbore.This has resulted in a modification in the completion methodology for subsequent Halfdan producer wells. Waterinjection fractures have not been imaged with themicroseismic network.