玛北斜坡百口泉组致密砾岩水力压裂裂缝表征

Characterization of Hydraulic Fractures in Tight Conglomerate Reservoirs in Baikouquan Formation,Mabei Slope

水力压裂是致密砾岩油气开发的主要手段,主要通过物理实验和数值模拟研究水力压裂裂缝,由于缺少现场数据的验证,增产措施的设计存在较强的不确定性。为了描述致密砾岩压裂裂缝的形态,证实储集层有效改造,在玛湖凹陷北斜坡钻取了1口以获取压裂裂缝为目的的取心井。在致密砾岩岩心观察与分析的基础上,对比岩心、成像测井、CT扫描等资料,描述了压裂裂缝形态,表征了裂缝产状和密度。致密砾岩具有粒径大、分选差、颗粒支撑以及非均质性强的特征。从长度为323 m的岩心中识别了335条压裂裂缝,主裂缝扩展以与井筒垂直的方向为主,分支裂缝较少且与主裂缝近垂直,部分层段出现破碎带,是不对称的双翼缝。裂缝具有绕砾和穿砾2种扩展方式,分别在拉张和剪切作用下形成,因此,将其分为张性缝和剪切缝。张性缝和剪切缝产状一致,均为接近90.0°的高角度倾角,近南北向的倾向,剪切缝的密度整体上大于张性缝。

Hydraulic fracturing is a main technique for developing oil and gas in tight conglomerate reservoirs.Currently,hydraulic frac⁃tures are mainly studied by means of physical experiments and numerical simulation.The study results can provide a theoretical basis for optimization of development plans,but they are not verified with field data,bringing great uncertainties to the design of stimulation mea⁃sures.In order to describe the shape of hydraulic fractures for confirming the effective stimulation in tight conglomerate reservoirs,a coring well was drilled on the north slope of Mahu sag for obtaining hydraulic fractures.Based on the observation and analysis of the tight conglom⁃erate cores,the shape,occurrence,and density of hydraulic fractures were characterized by using the core,image logging and CT scanning data.It is found that the tight conglomerate is characterized by large grain size,poor sorting,grain support,and strong heterogeneity.A total of 335 hydraulic fractures were identified in the core with the length of 323 m.Principal fractures propagate in the direction perpendicular to the wellbore;branch fractures are few and nearly perpendicular to the main fractures;crushed zones and asymmetrical double⁃wing frac⁃tures are observed in some intervals.The fractures propagate in two modes:gravel bypassing and gravel penetrating,which are formed due to tension and shear action,respectively,and are thus classified as tensional fractures and shear fractures.The tensional fractures and shear fractures are consistent in occurrence,both with a high dip angle close to 90°and a nearly south⁃north trending.The density of shear frac⁃tures is generally greater than that of tensional fractures.