泥质粉砂储层近井区域二次水合物饱和度时空演化规律研究

The periodic fluctuation of gas production rate and bottom hole temperature occurs during the first offshore natural gas hydrate production test in South China Sea, which may be attributed to the spatio-temporal evolution of reformed hydrate saturation in the near-wellbore region of clayey silt reservoir. Therefore, it is very important to clarify the spatio-temporal evolution law of reformed hydrate saturation in the near-wellbore region for realizing safe and efficient exploitation of gas hydrate in the clayey silt reservoir, but there is no research on it yet.. The influencing factors of reformed hydrate saturation in the near-wellbore region of clayey silt reservoir are studied by theoretical analysis and laboratory experiments at first; through the continuous fluid injection test with the two-dimensional radial flow test bench, the effects of gas-liquid characteristics in the near-wellbore region, permeability in the near-wellbore region and exploitation technology on the spatio-temporal evolution characteristics of reformed hydrate saturation in the near-wellbore region of clayey silt reservoir are studied respectively. Based on the test results, the spatio-temporal evolution characteristics of reformed hydrate saturation in the near-wellbore region of clayey silt reservoir and the interaction mechanism with permeability in the near-wellbore are comprehensively analyzed, and then the spatio-temporal evolution law of reformed hydrate saturation in the near-wellbore region of clayey silt reservoir.. The main innovations as follows: 1) the spatio-temporal evolution characteristics of reformed hydrate saturation in the near-wellbore region of clayey silt are studied through the continuous injection test with the two-dimensional radial flow test bench; 2) the interaction mechanism between spatio-temporal evolution characteristics of reformed hydrate saturation and the permeability in the near-wellbore region.

我国南海天然气水合物首次试采过程中出现了产气速率和井底温度周期性波动的现象，初步分析认为是由泥质粉砂储层近井区域二次水合物饱和度时空演化所致。因此，弄清泥质粉砂储层近井区域二次水合物饱和度时空演化规律对于实现我国南海泥质粉砂型水合物安全高效开采十分重要，而目前尚无该研究。. 首先通过理论分析和室内试验开展泥质粉砂储层近井区域二次水合物饱和度影响因素的研究；通过二维径向流体吹扫试验，分别研究近井区域气液特征、近井区域渗透率和开采工艺对近井区域二次水合物饱和度时空演化特征的影响；最后，综合分析近井区域二次水合物饱和度时空演化及其与近井区域渗透率相互作用机理，进而揭示泥质粉砂储层近井区域二次水合物饱和度时空演化规律。. 主要创新点：一是通过二维径向流体吹扫试验研究泥质粉砂储层近井区域二次水合物饱和度时空演化特征; 二是二次水合物饱和度时空演化与近井区域渗透率相互作用机理。

针对我国南海天然气水合物首轮试采过程中出现的产气速率和井底温度周期性波动的现象，本项目重点围绕近井储层区域温度、压力和二次水合物饱和度时空演化规律开展了大量物理模拟实验和数值模拟研究，并提出了通过井筒加热、电磁加热和气水平衡的方式进行天然气水合物的开采方法，以解决近井储层二次水合物形成影响开采效率的问题。首先通过一维反应釜研究了气体渗流、残余水、地层压力、开采压力和气体流量等对近井储层温度、压力和二次水合物饱和度影响规律。同时设计了二维径向流模拟实验装置，研究了气体吹扫条件下温度与压力分布规律。通过室内试验和数值模拟研究了气液特征和开采工艺近井储层温度、压力和二次水合物形成的影响。研究结果表明，节流膨胀效应导致的温度下降能够满足二次水合物形成条件，二次水合物倾向于在距离井壁较近的位置形成，由于形成的二次水合物堵塞了渗流通道导致地层内压力抬升，从而进一步扩大了二次水合物的形成范围，使得渗透率显著降低。通过改变开采压力、生产压差和水饱和度因素能有效缓解二次水合物形成时间和范围。一维吹扫试验和二维收敛流吹扫实验相比，在二维条件下，由于节流面积减小节流膨胀效应和水锁效应在不同位置出现了明显的差异性，即拓宽了地层中二次水合物形成机制的认识，也为实际生产的开采工艺参数提出了更严格的要求。在天然气水合物开采过程中，二次水合物的形成可造成产气波动和产量下降。人为改变开采工艺如增大井底压力、减小产气速率或通过泡沫砂浆旋喷桩工艺改造近井储层以增加开采半径，可抑制井周温度的降低，从而减小二次水合物形成。本项目的研究成果对于优化开采参数和工艺，规避开采过程中二次水合物的不利影响，实现安全高效开采天然气水合物具有重要意义。