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石油废井周边煤层中油层气扩散与渗流特点分析

2020-07-29 13:16:02分类:研究生论文 阅读:

 

  摘要  我国有不少煤炭资源与石油资源重叠区,先期开采石油后遗留了大量废弃石油井.来自含油地层的大量油层气沿着废弃石油井向井周围煤层扩散渗流而形成油层气富聚区,从而严重威胁到井周围煤层的安全高效开采.研究揭示废弃石油井周围煤层内油层气扩散渗流规律是对其危害进行科学有效防治的重要基础.

  本文通过煤体结构与气体运移相关理论的系统分析,得出了煤体微观孔裂隙结构特性以及煤层内油层气赋存与运移特性,认为油层气分别以吸附态与游离态赋存在煤体孔隙与裂隙系统内,并且在孔隙与裂隙系统内的运移方式分别服从菲克定律和达西定律.

  在石油井内高压驱动下,井内油层气通过渗流进入煤体裂隙系统,随后又通过扩散进入孔隙系统,基于煤体孔隙裂隙二重介质等假设,推导出了煤层中油层气的扩散与渗流方程,结合流动过程煤体渗透率动态演变特征与煤体变形机理,建立了煤层内油层气流动的扩散-渗流耦合模型.

  通过对双马煤矿煤样和石油井内油层气的实测分析,得到了油层气流动的主要参数,运用COMSOL Multiphysics软件对建立的耦合模型进行数值计算分析,发现流动过程中,石油井周围煤层内油层气压力的增加速度均呈现先加快后减慢特征,油层气压力均随着时间增加而缓慢趋于稳定,并且距离井越近,油层气压力短时间内变化越快,压力也越大.通过改变石油井内油层气的压力,得到了井内油层气压力恒定以及以不同速率线性增加时,油层气影响半径随时间的变化关系.基于模拟结果与马探31号石油井实测结果基本相吻合,对第30年时煤层内油层气流动速度与压力进行模拟,发现随着流动距离增加,油层气流动速度和压力均逐渐减小,得到了第30年时煤层内油层气流动速度与压力随着流动距离的分布关系式.

  本研究结果为双马煤矿油层气的防治及安全高效采煤提供了理论指导,对类似煤油气共生矿井安全协调开采也具有重要意义.

石油废井周边煤层中油层气扩散与渗流特点分析

  关键词:废弃石油井;油层气;扩散-渗流;数值模拟;影响半径

  ABSTRACT

  There are many areas which coexist coal and oil resources in country. And a largeamount of abandoned oil wells are left after the early exploitation of oil. The oi-bed gasaccumulation areas around abandoned oil wells are formed when much oil-bed gas from oilstrata flows along wells to the surounding coal seam, which seriously threatens the safe andefficient coal mining around the wells. The study reveals that the law of oil-bed gas diffusionand seepage in coal seam around abandoned oil well is an important basis for scientific andeffective prevention and control of its harm.

  Based on the systematic ana lysis of the theory of coal mass structure and gas migration,the characteristics of micro-pore fracture structure of coal mass and the occurrence andmigration characteristics of oil-bed gas in coal seam were obtained. It is considered thatoil-bed gas occurs in the pore and fracture system of coal body in adsorbed state and free staterespectively, and the migration mode in pore and fracture system obeys Fick's la w and Darcy'slaw respectively.

  Driven by the high pressure in the oil well, the oilbed gas in the well enters the coalfracture system through seepage, and then into the pore system through diffusion. And basedon some hypothesizes such as dual-porous media of coal mass, the diffusion and seepageequations of oil-bed gas in coal seam were derived. Combined with the dynamic variationcharacteristics of coal permeability and the deformation mechanism of coal mass, thedifusion-seepage coupling model of oi-bed gas flowing in coal seam was established.

  Through the analysis of the coal sample and oil-bed gas in well in Shuangma Coal Mine,the main parameters of oil-bed gas flowing were obtained. The COMSOL Multiphysicssoftware was used to simulate and analyze. It is found that during the flow, the increase rate ofthe oil-bed gas pressure in coal seam near oil well is frstly acce lerated and then slowed down.The gas pressure gradually stabilizes with time. The closer to the abandoned oil well, the faster the pressure change in a short time and the greater the pressure is. The relationshipbetween the influence radius of oil-bed gas and time is obtained by changing the pressure ofoil-bed gas in oil well when the oil-bed gas pressure is constant and increases linearly atdifferent rates. Based on the fact that the simulation results are in good agreement with themeasured results of Mattan 31#oil well, the oil-bed gas flow velocity and pressure in coalseam at 30th are simulated. And it is found that with the increase of the flow distance, theoil-bed gas flow velocity and pressure decrease gradually. And the distribution relationshipbetween gas flow velocity and pressure with flow distance in coal seam for 30th is obtained.

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