Abstract
To maximise drainage, the full field development phase of the Wandoo oil field required three new horizontal wells to make a total of five crossings with two existing horizontal wells. The low relief structure and potential coning from nearby fluid contacts required three of these crossings to be within a vertical separation of 2 m. Because this clearance is much smaller than the closest safe approach possible using geometric collision avoidance, geosteering was used as the collision avoidance technique. This required a detailed resistivity and gamma ray model of the stratigraphic sequence, which was used to establish the relative position of the existing and new wellbores. The well-crossings were made by geosteering in the formation layer either over or under that of the existing well. Three crossings requiring the closest approach had separations computed to be 2.6, 2.4 and 1.7 m. These separations are much closer than the minimum allowable 5.2 to 7.1 m for geometric collision avoidance, but are far enough apart to avoid fracturing between the wells.
As the horizontal drainholes dropped down the updipping sequence they were geosteered within two reservoir units. One unit has a 0.6-m sweet spot, and each unit has its own upper and lower coning boundaries. This reservoir geosteering also required a resistivity and gamma ray model, in order to build a real-time geological cross-section. This cross-section was used to intersect and follow the sweet spot, and design sidetracks if faults or rapid dip changes caused the well to enter a reservoir unit outside its coning boundary.
The well crossings allowed approximately 7 million barrels of reserves to be successfully exploited without putting the existing wells at risk. Production from the field was above budget estimates, which was due primarily to the high efficiency in placing the well bore in good quality rock and the contribution from the crossed wells.
