Abstract A permanent microseismic monitoring array has been installed in a horizontal well with the objective of imaging propagation of acid fractures and water injection fractures in offset wells. The array consists of 16 geophones placed at 500 ft intervals along the horizontal section of a producing well, deployed inside the pre-perforated liner and coupled to the formation by gravity. Optimal monitoring conditions are achieved when production from the well is closed in, however, many events have been detected whilst the well is producing by applying a production noise filter. Microseismic events are detected, one per week on average, when the monitoring well is producing and 1-2 events per day when the well is shut in. The microseismic monitoring well is located in the Halfdan field in the Danish North Sea. This chalk reservoir has been developed with a line drive waterflood configuration, alternating oil production and water injection wells. Parallel horizontal producers and injectors are drilled in the direction of maximum horizontal stress, 600 ft apart with reservoir completions in the range 10,000–15,000 ft. The relatively close well spacing is necessary to maximise recovery from the tight chalk reservoir. Multiple acid fractures are placed in producing wells to maximize productivity while fractures are induced by water injection in injection wells in order to achieve voidage replacement. Orientation of induced fractures along the direction of the horizontal wellbore is critical to avoid early water breakthrough and inefficient sweep patterns. Approximately 450 microseismic events have been located from a total of 13 acid fracture stimulation treatments in two offset producers, both 1,200 ft from the monitoring well. Definitive fracture orientations have been determined for 8 acid 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. Water injection fractures have not been imaged with the microseismic network.