A methodology is designed and implemented to explore the degree of impact of different operational variables of CSS on the productivity of a Colombian heavy oil field. Variables such as steam quality, steam rate, and duration of injection and soaking stages are assessed considering the geomechanical mechanisms related to the rock expansion due to temperature increases and the rock compaction due to the reduction of pore pressure in the production stage. For this purpose, a coupled modeling approach of fluid flow and geomechanics is carried out using STARS (CMG), including permeability multipliers controlled by volumetric strain. A directional permeability model and in-house software are used to obtain the relationship between permeability and volumetric deformation. Two sensitivity analyses based on experimental design and Sobol's method are performed in order to determine the influence of injection time, soaking time, injection rate, and steam quality on the cumulative oil production (Np) and oil-steam ratio (OSR). The first sensitivity analysis accounts for non-constant permeability, while for the second one, permeability is set constant. Once implemented this methodology, the operational variables that have a direct positive incidence on the Np increase are the injection time and the steam quality. The OSR behaves directly proportional to the steam quality and inversely proportional to the injection period. It is expected that the increase in Np due to the variation of operational parameters would be higher than 25%. Moreover, an analysis is accomplished to determine the degree of impact of permeability variations due to rock deformation on well productivity.
1.1. Cyclic steam stimulation
Cyclic steam stimulation (CSS) is one of the most common thermal recovery methods used for heavy oil reservoirs. Unlike other techniques based on steam injection, such as Steamflooding and Steam Assisted Gravity Drainage (SAGD), CSS requires the least logistics for its execution since it is performed in a single well, which is both injector and producer. It consists of three stages: injection, soaking, and production. In the first stage, steam at high pressure and high temperature is injected into the well for a period that can last from a few days to a few weeks. In the second stage, the steam injection is stopped, and the well is shut down for a period. In this stage, there is heat redistribution in the reservoir that can alter the process's thermal efficiency depending on its duration (Sheng, 2013). Finally, in the production stage, the well is put on production until it reaches an economic limit. The objective of CSS is to reduce residual oil saturation through several reservoir drive mechanisms, usually related to the behavior of fluids and rock under temperature and pressure changes.