Abstract
The Orinoco Heavy Oil Belt (Faja) has been exploited under primary recovery techniques using mainly horizontal, fishbone and multilateral wells. This cold development can only recover between 6% and 9 % of the considerable original oil in place existing in the area. Owing to the high viscosities, widely different formation thicknesses and heterogeneities found, the implementation of different thermal recovery methods is necessary.
This project covers a feasibility study considering the Horizontal Alternating Steam Drive (HASD) process geared to increase the recovery efficiency of heavy oil within the Faja reservoirs. The process is based on a repetitive pattern using horizontal wells acting alternatively as oil producers and steam injectors. The recovery mechanism is a combination of horizontal steam flooding between wells and cyclic steam stimulation of each of the horizontal wells in the pattern. Properly implemented, HASD could be more efficient than classical cyclic steam injection and more effective than direct steam flooding.
In contrast to the Steam Assisted Gravity Drainage process (SAGD), HASD uses single horizontal wells cyclically switching between injection and production phases. The steam chamber generated while each well is injecting is laterally driven by the pressure differentials created by adjacent producers, forming a sweeping front between wells. Injectors are converted to producers (and vice versa) providing heat directly to the zones where production will occur gradually extending the steam chambers to the entire reservoir region. Thus, the impact of steam is not that of a simple well stimulation, but also achieves an effective sweep in the vicinity of the producers while decreasing oil viscosity and improving oil drainage.
This project is based on the numerical simulation results from a representative model from one of the Faja main blocks using Eclipse Thermal applied to medium thickness sands in the 20-50 net ft range.
A five-horizontal well array set up was used as the model to assess this fairly new thermal recovery technique. During the investigation, different scenarios were analyzed to obtain a generalized step-by-step optimization procedure for the process under the specified fluid and reservoir conditions.
Sensitivity analyses were performed considering the relative positioning of the horizontal well placement in the reservoir column; different injection sequences; varying the duration of each injection cycle; various injection rates; and lengths of the horizontal reach of the wells.
The results of this investigation can be used as a reference to optimize the performance of the HASD process for sand bodies of medium thickness.
