Dynamic responses and evolutionary characteristics of waterflood-induced fractures in tight sandstone reservoirs: A case study of oil reservoirs in the 8th member of the Yanchang Formation, well block L, Jiyuan oilfield, Ordos Basin

Prolonged waterflooding leads to the development of waterflood-induced fractures in tight sandstone reservoirs. Clarifying the dynamic responses and evolutionary characteristics of these fractures holds great geological significance for the emplacement of dense well patterns and the tapping of residual oil potential of tight sandstone reservoirs. Integrating data from core observations, logs, oil production, pressure-buildup well tests, and water injection profiles, we explore the dynamic responses and distributions of waterflood-induced fractures across different development stages within the tight sandstone reservoirs in the 8^th member of the Yanchang Formation (also referred to as the Chang 8 Member) in well block L, Jiyuan oilfield, Ordos Basin. The results indicate that waterflood-induced fractures in the tight sandstone reservoirs of the Chang 8 Member within well block L originate from the propagation of natural fractures, and the natural fractures exhibit a preferential opening direction of NEE-SWW and NE-SW, followed by NW-SE. The water injection profiles of injection wells tend to exhibit small water absorption thickness but high water absorption capacity due to the formation of waterflood-induced fractures. Concurrently, the production performance curves of wells display a spurt or stepped upward trend in water cut, while pressure-buildup well tests reveal open double- logarithmic derivative curves that trend upward at a slope of 1/2. In the case where waterflood-induced fractures occur between production and injection wells, the production well test-interpreted formation pressure exceeds that in wells without waterflood-induced fractures and even far surpasses the initial formation pressure. In the initial development stage of tight sandstone reservoirs in the Chang 8 Member within well block L, waterflood-induced fractures in the reservoirs are primarily found in east-central, northeastern, and southeastern parts of the well block, where natural fractures are well developed. Waterflooding causes changes in the reservoir stress and thereby the opening pressure of natural fractures decreases. As a result, in the middle development stage, waterflood-induced fractures striking NW-SW come into being in the southern and north-central parts of well block L, accompanied by the small-scale propagation of pre-existing waterflood-induced fractures. In the late development stage, further waterflooding triggers the opening of natural fractures in different orientations around injection wells, leading to the formation of small-scale waterflood-induced fractures. This further exacerbates the fracture-induced waterlogging of production wells.