Forward Modeling of Natural Fractures within Volcanic Reservoir by Using Continuum Damage Mechanics

By using the continuum damage mechanics’ model and finite element numerical simulation technology, this paper studies natural fractures’ distribution within volcanic reservoir and presents numerical solution of contour of damage variable which represents distribution of natural fractures within target formations. Workflow of numerical calculation was proposed. These techniques were applied to investigate natural fractures distribution generated by tectonic movement within volcanic reservoir of an oilfield in west China. Quasi-brittle plastic damage model is used to simulate fractures generated by technical movement within volcanic formations given in this region. Initial-strain-method is used to simulate loading applied to target formation caused by tectonic movement instead of using boundary displacement loading. Different values of magnitude and orientations of principal strain components have been tested in order to find their proper values. Values of parameters of damage models are calibrated by matching numerical results of natural fractures’ distribution to those obtained by interpretation of seismic data. Principal results are: (1) in the upper Triassic Xiaoquangou formation reservoir and the lower Tiaohu formation reservoir, numerical solution of natural fractures’ distribution is consistent with those obtained from interpretation of seismic data. (2) The maximum value of damage variable of the upper Triassic Xiaoquangou formation is 0.2678, i.e., the degree of crushing is 26.78%. The width of the zone of damage localization is about 10 meters to 300 meter. The maximum value of damage variable in the lower Tiaohu formation is 0.2569, which is slightly less than that in the upper Xiaoquangou formation. (3) Within the target block, zones of natural fractures are mainly distributed in central-east part of the block. Therefore the planned new wells should be located in this part of the block. Zones of natural fractures in the west part of the target block are in the shape of narrow bands which are lack of connections, and thus this west side of the block is not a good place for new wells. This case study provides a best practice for identification of natural fractures within volcanic reservoir.