Liaohe field is one of the largest heavy-oil fields in China. Its D-N Block is a deep massive ultra-heavy oil reservoir undertaking SAGD development, which will serve as a case study in this paper. Part of the dual horizontal wells had used cycle steam stimulation to start up the SAGD process but it generated severely uneven distributions of temperature, pressure, and saturation around wells due to the high reservoir heterogeneity. An improved steam circulation start-up methods instead of cycle steam stimulation is needed to overcome the problems.
We developed an optimization method by coupling the wellbore-reservoir simulation for steam circulation using a specialized thermal wellbore simulator and a commercial thermal reservoir simulation, and carefully considering the structure of the wellbore, fluid flow in the wellbore and reservoir characteristics. We found out several dominating factors on the heat loss and efficiency of steam circulation by considering the properties of tubing, annulus, liner, fluid components and formations on a conceptual model. Using an advanced criterion by monitoring the temperature distributions near the wellbore, wellbore operations including the injection rate and circulation period, the tubing strings layouts and other critical operation parameters are designed according to the features of downhole conditions to ensure a more homogeneous development of the steam chamber.
The simulation workflow and optimum circulation start-up stratergy was applied to a pilot dual-horizontal well group in the D-N Block. Steam circulating start-up process both in the wellbore and the reservoir was successfully simulated and compared with the conceptual model case. After optimization, reservoir simulation results showed a much better behavior in the start-up and the subsequent SAGD stage, mitigating the risk of steam fingering, resulting in a steady transition to SAGD stage. Monitoring data from downhole temperature sensors also suggested a favorable process. For example, temperature data in a series of time showed that the temperature between the horizontal wells kept increasing uniformly, which indicated that the steam chamber connects the two wells in a favorable manner. Comparing with the cyclic steam stimulation start-up and the steam circulation without optimization, the optimized steam circulation achieved much better recovery efficiency in SAGD process.
In this paper, a systematic workflow has been described which couples the simulation of the reservoir characteristics and wellbore conditions. Optimized operation guideline was implemented into the SAGD wells and overcame the uneven start-up of dual-horizontal wells. This achievement can be applied to other SAGD wells in Liaohe, and probably extended to other fields.