Abstract
Abstract PVT numerical simulation of fluid inclusions was an important method to reconstruct the paleo-pressure of hydrocarbon accumulations; however, in high-maturity gas reservoirs, gas-liquid petroleum inclusions were very rare, so the traditional PVT simulation methods could not be applied. It was urgent to develop a new method. Hydrocarbon-containing inclusions were widely distributed in gas reservoirs. They could dissolve a little gaseous hydrocarbon and their trapping temperatures were close to homogenization temperatures. Using PVTsim software and combining with some parameters such as vapor/liquid ratio and homogenization temperature of hydrocarbon-containing inclusions, a new PVT simulation method was proposed to get the trapping pressure of inclusions, which was then applied to reconstruct the paleo-pressure of the Yuanba gas reservoir in the northeastern Sichuan Basin. The Xujiahe reservoir developed weak overpressure at about 160 Ma, and the pressure coefficient reached 1.11. Then the paleo-pressure increased rapidly until 148 Ma at which time the pressure coefficient reached 1.86. At about 100 Ma, the paleo-pressure reached its peak, but the pressure coefficient decreased to 1.60 because the formation burial depth was the deepest. The comprehensive analyses of hydrocarbon generation, paleo-pressure evolution and reservoir-cap combination indicated that the overpressure transferred from source rock to reservoir through the process of hydrocarbon generation and accumulation was an important mechanism for the development of overpressure in the Xujiahe reservoir.
Highlights
PVT numerical simulation of fluid inclusions was an important method to reconstruct the paleo⁃ pressure of hydrocarbon accumulations; in high⁃maturity gas reservoirs, gas⁃liquid petroleum inclusions were very rare, so the traditional PVT simulation methods could not be applied
Hydrocarbon⁃containing inclusions were widely distributed in gas reservoirs
their trapping temperatures were close to homogenization temperatures
Summary
PVT numerical simulation of fluid inclusions was an important method to reconstruct the paleo⁃ pressure of hydrocarbon accumulations; in high⁃maturity gas reservoirs, gas⁃liquid petroleum inclusions were very rare, so the traditional PVT simulation methods could not be applied. 应用[1,2,3,4,5,6,7,8] ,其中一个重要的方面就是应用流体包裹 体 PVT 数值模拟法研究油气藏中的古流体压力[9,10,11] 。 通常的方法是采用石油包裹体等容线与 同期盐水包裹体均一温度[12] ,或者石油包裹体等 容线与同期盐水包裹体等容线相交法[13] 求包裹体 捕获压力。 这 2 种方法在生油型盆地古压力恢复 中得到了广泛应用,取得了很好的效果[11-15] 。 但 是,在一些高成熟天然气藏中,往往缺乏气液两相 石油包裹体,前面的方法受到了很大的限制。 对 此,很多学者提出利用含烃盐水包裹体进行模拟, 并探讨了含烃盐水包裹体的 PVT 模拟方法。 王存 武[11] 认为含烃盐水包裹体在冷冻过程中会形成气 体水合物,其平衡温度( 冰点温度) 大于零,可以利 用包裹体的平衡温度结合气液比、均一温度等参数 求取包裹体的最小捕获压力。 但是由于包裹体在 冷冻或加温过程中存在亚稳态现象,冰点温度大于 零时包裹体并不一定是形成了气体水合物,该方法 存在一些争议。 另外,米敬奎[16] 提出运用 PVTsim 软件的“ multi⁃flash” 模块,利用包裹体在不同温压 条件下总体积不变的特性,结合气液比参数模拟包 裹体 的最小捕获压力, 该方法得到了一定的应用[9,17] 。 但是笔者研究发现,米敬奎模拟出来的含 烃盐水包裹体的成分中气态烃的摩尔含量达 15% ~ 20%,远远超出了沉积环境温压条件下气态 烃在水中的溶解度[18] 。 Mao 等[19-21] 的最新研究结 果表明,在 100 ~ 150 °C ,压力在 20 ~ 110 MPa 之间 时,甲烷在水中的溶解度在 0. 体在天然气藏中普遍存在,对于气藏古压力的恢复 具有重要意义。 本次提出的含烃盐水包裹体 PVT 模拟方法,基本上还是按照石油包裹体组分和饱和 压力(即最小捕获压力) 的模拟思路,只是进行了 局部的改进。 对于 石油包裹体的组分和饱和压力模拟, PVTsim 软件采用的是 Soave -Redich -Kwong 状态 方程,通过迭代计算使得设定的石油包裹体组成与 室温下测定的气液比达到匹配[18] 。 Aplin[13] 系统 地讲述了石油包裹体组分和饱和压力的模拟方法, 模拟过程中比较基础的一个步骤是“ 利用 PVTsim 软件计算设定石油在石油包裹体均一温度时的饱 和压力” ,这一步可以通过输入包裹体均 一 温 度 直 接计算完成。 但是对于含烃盐水包裹体,PVTsim 软件无法直接计算完成。 如何解决这一难题,笔者 分析认为,包裹体在饱和压力下达到气液均一,气 体刚好完全溶解( Vapor = 0) ,可以通过这一限制条 件求得初始成分下的饱和压力,然后结合气液比、 均一温度等参数迭代计算获得包裹体的最终成分 和该成分对应的饱和压力。 1.2 PVT 模拟方法
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