裂缝型碳酸盐岩储层酸压数值模拟

酸压是碳酸盐岩储层重要的大型增产改造措施,酸压中,准确预测酸液作用距离对酸压设计至关重要,其决定酸压改造范围、影响酸压目标缝长设定及酸压效果预测。裂缝型碳酸盐岩储层非均质性极强,基质较致密,天然裂缝所在区域渗透率很高,酸液滤失严重。目前各种酸压模型假设裂缝面上渗透率均匀分布,也不能考虑天然裂缝对滤失的影响,这些模型预测的酸液作用距离较长,酸压设计缺乏可靠的模拟模型。针对该问题,本文进行了裂缝型碳酸盐岩储层酸压数值模拟研究,首先基于地质统计规律建立人工裂缝面上渗透率分布模型、天然裂缝分布模型,并基于地质统计参数生成裂缝面上渗透率及天然裂缝分布；然后建立酸液流动、酸液滤失、酸岩反应数学、粗糙酸蚀裂缝表面形成过程模拟模型；再将以上模型耦合求解,形成酸压模拟模型。基于该模型,进行了广泛的数值模拟研究,分析了天然裂缝参数对酸液作用距离、酸蚀裂缝表面的影响。研究发现,天然裂缝对活酸作用距离和酸蚀裂缝表面影响显著,考虑天然裂缝时,其作用距离显著低于常规模型预测的距离,酸蚀裂缝表面更粗糙；裂缝型储层酸压设计中,天然裂缝是不可忽略的因素,新模型预测的酸液作用更合理。该研究为裂缝型碳酸盐岩储层酸压设计提供了更可靠模拟工具。

Acid fracturing modeling of naturally fractured carbonates

acid fracturing is one of large scale stimulation technologies for carbonates. In acid fracturing, accurate prediction of live acid penetration is vital for an acid fracturing design because it determines the area of stimulation and affects setting the target length of effective acid fracture length as well as predicting acid fracturing performance. Naturally fractured carbonates have strong heterogeneities with tight matrix and high perm natural fractures. Acid leakoff is serious in acid fracturing. Currant acid fracturing models assume uniform permeability distribution on the fracture surfaces and are unable to account for the effect of natural fractures. These models predict much longer live acid penetration distance than what it is in real situations. Acid fracturing designing lacks of reliable models. Aiming at this problem, we build a new acid fracturing model in this paper. Firstly, we build a model of permeability and natural fracture distribution on the fracture surfaces in terms of geostatistics, based on which to generate permeability and natural fracture distributions. Then we build a model for acid flow, acid leakoff, acid-rock reaction, and fracture surface variation. Coupling the models above forms a new acid fracturing model, based on which extensive numerical simulation is conducted to analyze the effect of natural fracture parameter on live acid penetration distance and rough fracture surfaces. The study shows that natural fractures have remarkable effect on live acid penetration distance and rough fracture surfaces. With effect of natural fractures, the live acid penetration distance is much shorter the one predicted by conventional models, and the fracture surfaces are more rough. In acid fracturing design of naturally fractured carbonates, natural fractures are the non-negligible factor. The new model predicts more reasonable acid penetration distance. This study provides a more reliable simulation tool for acid fracturing design of naturally fractured carbonates.