深水油气固井水合物储层物性响应与高压气水反侵研究

固井作业是能源开采过程中的一道重要工序，当深水油气固井遇到水合物地层时，固井水泥浆水化放热会引起近井壁储层中水合物分解和产生高压气水反侵，从而严重影响固井质量，甚至导致固井报废和井壁失稳，为减小和避免水合物分解的不利影响，明确不同固井工艺条件下水合物储层的物性响应和高压气水反侵规律是关键。以南海神狐海域水合物钻探工程GMGS-1中SH2站位勘探井为研究对象，建立固井二维数值模型，采用TOUGH+HYDRATE数值模拟软件再现固井水泥浆侵入和水化过程，分析过程中近井壁储层物性响应规律，得出南海水合物储层不同固井压差与水泥浆放热速率条件下高压气水反侵的临界条件判别曲线，并创新性地采用“连续分段模拟”思路解决水泥浆的动态放热问题。结果表明:水泥浆初凝之前主要可分为诱导、分解和二次水合物生成3个阶段；侵入行为主要发生在保压时期，当压力卸去后侵入深度基本不再增加；水化放热造成的温度升高导致水合物大量分解，产生的高压气水向四周运移，而压力卸去之后，高压气水向环空方向反侵的趋势更加明显；水化放热速率越大，固井压差越小，气水反侵发生的可能性越大，发生时间越早。对于浅部水合物储层，降低固井水泥浆水化热可有效减少反侵现象的发生，提高固井质量，而对于埋藏较深的储层可在破裂压力范围内同时使用较高的固井压差。本研究对水合物地层固井工艺参数优选具有良好的指导和借鉴作用。 

Hydrate reservoir physical properties response and high-pressure gas-water reverse penetration during deepwater oil and gas cementing

Well cementing is an important stage during the process of energy extraction. When deep-water oil and gas cementing encounters hydrate formations, the hydration heat of cementing slurry will cause hydrate decomposition in the reservoir near the borehole wall and reverse penetration of high-pressure gas-water. To reduce and avoid the adverse effects of hydrate decomposition, it is critical to clarify the physical response of hydrate reservoirs and the law of high-pressure gas-water reverse penetration under different cementing conditions. In this paper, a two-dimensional cementing numerical model is established according to the SH2 exploration well of GMGS-1 project in Shenhu area in the north of the South China Sea, and numerical simulation software TOUGH+HYDRATE is used to reproduce cementing slurry penetration and hydration process. Then, the response law of reservoir properties near the borehole wall during the process is analyzed, and the critical conditions for high-pressure gas-water reverse penetration under the conditions of different cementing pressure difference and heat release rate are obtained. The idea of "continuous stage simulation" is innovatively adopted to solve the dynamic heat release issue of cementing slurry. The results show that the process before initial setting of cementing slurry can be divided into three stages: induction, decomposition and secondary hydrate formation. The penetration behavior mainly occurs during the pressure holding period, and the penetration depth basically no longer increases after the holding pressure is removed. The temperature rise caused by the exothermic heat of hydration leads to the massive decomposition of hydrates, and the resulting high-pressure gas-water migrate around. After the holding pressure is released, the high-pressure gas-water present a more obvious tendency to penetrate toward the annulus. The higher the hydration heat release rate is, the smaller the cementing pressure difference is, and the greater the possibility of gas-water reverse penetration, and the earlier it will occur. For shallow hydrate reservoirs, reducing the heat of hydration of cementing slurry can effectively reduce the occurrence of reverse invasion and improve cementing quality. For deeper reservoirs, higher cementing pressure difference can be used within the fracture pressure range. This study offers a good reference for parameters optimization during the cementing process in hydrate formations.