A Practical Method to Match Trajectories of a Horizontal Well in Time Domain and in Depth in Ancient Buried Hill
Authors: Lv Zuobin , Gao Hongli , Cheng Qi , Cheng Dayong , Meng Zhiqiang 
Published: April 2019

JZS is an offshore metamorphic rock buried hill oilfield. Both horizontal and vertical velocities of the oil field change very fast. The interval velocity of the buried hill stratum is twice that of the overlying strata, and the top surface of the buried hill fluctuates greatly with a maximum height difference of 300m. In the complex buried hill reservoir, since the current professional seismic software can not realize variable time-depth relationship in horizontal direction, which leads to the error of the trajectory form and position of the horizontal well in time domain, therefore the well trajectory in time domain is not matched with that in depth.

In this paper, a new practical trajectories matching method for buried hill horizontal wells in time domain and in depth is presented. First of all, we carried on the research on the theoretical form of horizontal well trajectory in buried hill in time domain. The research shows that the theoretical trajectory form of a horizontal well in buried hill is consistent with trend of the buried hill top surface morphology. On the basis of theoretical research, by establishing the pseudo time-depth relationship of horizontal well based on measure depth (MD) and seismic reflection two way time (TWT), we realized the accurate characterization of the trajectory form and position of a horizontal well in buried hill in time domain: (1)For normal horizontal well with no more than 90 degrees inclination angle, we can respectively establish the pseudo time-depth relationship of the horizontal well in buried hill segment and in upper segment, and then merge both time-depth relationship data into a whole; (2)For the complex horizontal well with well segment whose inclination angle is more than 90 degrees, we need firstly split the well trajectory into normal well segment and complex segment according to inclination angle, then establish the pseudo time-depth relationship in normal and complex well segments respectively. More specifically, we can split the trajectory into normal trajectory segment with the inclination angle no more than 90 degrees and complex trajectory segment with the inclination angle more than 90 degrees, for normal segment, we can establish pseudo time-depth relationship like the normal horizontal well described earlier, for complex trajectory segment, we need creatively invert the top and bottom of the complex segment to convert inclination angle of the segment to within 90 degrees, and then establish pseudo time-depth relationship of the inverted segment.

Through this method, we can obtain the accurate trajectory form and position of the horizontal well in time domain and it provides a basis for accurate geological modeling based on 3D seismic attributes constrains. The real reservoir performance of JZS buried oilfield in Bohai Bay in China has proved that the 3D geological model based on the new time-depth relationship (MD&TWT) of the horizontal wells is closer to the actual reservoir.