砾岩致密油藏直井重复压裂裂缝形态分析

Analysis of fracture geometry for refractured vertical wells in tight conglomerate reservoir

油井重复压裂是目前国内多数低渗油田恢复老井产能的主要措施之一。新疆某低渗砾岩油藏采用了不同直井重复压裂改造措施,其改造机理尚不清晰。通过建立地质力学模型、基于有限元法的孔隙压力诱导应力变化机理模型以及非结构化油藏数值模型,对重复压裂的裂缝形态及改造措施效果进行研究分析。结果表明:相较于首次压裂和普通重复压裂,体积重复压裂后裂缝形态变化显著。生产前后地应力方位变化不显著,但孔隙压力及地应力变化对水力裂缝的形态的影响较大。对于已经射开的层位,由于地层能量亏空较大,导致其裂缝扩展难度大,缝长较短,缝宽较大;而当存在补层时,由于地层能量充足,裂缝扩展较为容易。补层位置较高井可有效提高储层纵向动用程度,增产效果较好。重复压裂时机对增加产油效果影响明显,及时开展重复压裂更有助于提升单井产油能力。

Refracturing is currently one of the most used methods to rejuvenate productivity of existing wells in low-permeability reservoirs in China.Different refracturing strategies have been conducted for vertical wells in a tight conglomerate reservoir in the Xinjiang.However,the underlying mechanisms are still unclear.By establishing the geomechanical model,the mechanism model of pore pressure inducing stress change based on the finite element method and the numerical model of unstructured reservoir,the researches on the fracture geometry of refracturing and the effect of different refracturing methods have been carried out.The results are as follows.Compared with the initial fracturing and conventional refracturing,the fracture geometry changes significantly after the volume refracturing.The azimuth of the in-situ stress before and after production does not change significantly,but the changes in pore pressure and in-situ stresses have a great impact on the geometry of hydraulic fractures.For the layers that have been perforated and produced,due to the strong energy loss,the propagation of fracture is more difficult,resulting in short fracture length and wide fracture width.In the case of adding new perforation in un-developed layers,as the formation can provide enough energy,it is easier to propagate fractures.The wells which are at the higher place of the new perforation layers have better productivity after the volume refracturing.The timing of refracturing also has a significant impact on oil production.A proper choice of refracturing time can effectively improve the well productivity.