致密砂岩储层注水诱导裂缝动态识别及演化特征

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

吕文雅 安小平 刘艳祥 李德生 曾联波 皇甫展鸿 唐英航 张克宁 张玉银

Wenya LYU Xiaoping AN Yanxiang LIU Desheng LI Lianbo ZENG Zhanhong HUANGFU Yinghang TANG Kening ZHANG Yuyin ZHANG

致密砂岩储层长期注水开发导致储层诱导裂缝发育，研究注水诱导裂缝动态响应及演化特征对致密砂岩油藏井网加密部署及剩余油挖潜具有重要地质意义。综合利用岩心、测井、生产数据、压力试井和吸水剖面等资料，以鄂尔多斯盆地姬塬油田L井区三叠系延长组8段（长8段）致密砂岩储层为例，研究了L井区长8段致密砂岩储层不同开发时间段注水诱导裂缝的动态响应特征及展布特征。研究结果表明：①L井区长8段致密砂岩储层注水诱导裂缝为天然裂缝扩展延伸导致，天然裂缝优先开启方位为NEE-SWW向和NE-SW向，其次为NW-SE向。②注水诱导裂缝的形成会导致注水井吸水剖面呈现出吸水厚度小、吸水量大的特征。采油井生产动态曲线含水率陡增或阶梯状上升，压力试井双对数导数曲线开口并呈现“1/2”斜率上升。采油井与注水井间存在注水诱导裂缝，因而采油井试井解释地层压力高于未发育注水诱导裂缝井甚至超过原始地层压力。③L井区长8段致密砂岩储层开发初期注水诱导裂缝主要分布在井区中部偏东、东北部以及东南部的天然裂缝发育带。注水导致储层地应力变化，天然裂缝开启压力降低，开发中期在井区南部以及中北部进一步形成NW-SW向注水诱导裂缝，先存注水诱导裂缝存在小范围扩展延伸。开发后期受注水开发的不断深入，导致注水井周围不同方向开启天然裂缝，形成小规模注水诱导裂缝，进一步加剧了采油井裂缝性水淹。

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 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.

10.11743/ogg20240516