深水盆地高温高压环境下的地层压力预测方法

目前,应用于南海琼东南盆地深水高温高压地区地层压力预测的方法主要为测井法、经验公式法、地震法等,由于其局限性及该区勘探对象的复杂性,致使应用效果不佳。为了提高该区地层压力预测的精度,在分析常规经验公式法原理和考虑区域压实背景多样性的基础上,结合已有的钻井资料,建立了不同区域、不同构造的正常压实速度趋势线,拟合速度与深度的关系,计算孔隙压力,形成了新的常规经验公式法(以下简称新方法);在Stephen公式的基础上,通过引入地层抗张强度、分段选择泊松比取值,计算了地层破裂压力。研究结果表明:(1)所建立的正常压实速度趋势线不需要对目标区内的岩性做出判断,可直接对目标区地层正常压实范围内的压实速度进行拟合;(2)当达到一定的地层深度后,纵波、横波速度比和泊松比基本上不受深度的影响,泊松比的这一特性可被用于分段计算破裂压力;(3)新方法预测的地层压力系数绝对误差小于0.07、相对误差小于5%。结论认为,基于新方法开发的压力预测平台可用于南海相关深水盆地高温高压环境下的地层压力预测,预测结果误差范围小、精度高,满足了设计要求、提高了工作效率,具有良好的应用前景。

A formation pressure prediction method for deepwater basins under high temperatures and high pressures

At present, the formation prediction methods used in the high-temperature and high-pressure areas of the Qiongdongnan Basin, South China Sea, mainly include the logging method, empirical formula and seismic method, etc., but their application effects are not satisfactory due to their limitations and the complexity of exploration targets. In order to increase the prediction accuracy of formation pressure in this area, we analyzed the principles of conventional empirical formula method. Considering the diversity of regional compaction settings, combined with the existing drilling data, the normal compaction velocity trend lines were established for different zones and structures to fit the velocity-depth relationship and calculate the pore pressure. Thus, a new conventional empirical formula method (hereinafter "a new method" for short) was developed. On the basis of Stephen formula, the formation fracturing pressure was calculated by introducing formation tensile strength and by selecting the value of Poisson's ratio segmentally. The following research results were obtained. First, by virtue of the normal compaction velocity trend lines established in this paper, the compaction velocity of the target formations in the normal compaction range can be fitted directly without any lithological identification of the target area. Second, when the formation depth reaches a certain value, the compression/shear wave ratio and the Poisson's ratio are hardly affected by the depth and this characteristic of Poisson's ratio can be used to calculate the fracturing pressure segmentally. Third, when this new method was used to predict the formation pressure coefficient, the absolute error was less than 0.07 and the relative error was less than 5%. In conclusion, the pressure prediction platform which is developed based on this new method can be used to predict the formation pressure of deepwater basins under high temperatures and high pressures in the South China Sea, and the prediction results are accurate with a small error range. Obviously, it satisfies the design requirement, increases the working efficiency and presents a promising application prospect.