This paper describes an innovative thermal production scheme, named "Horizontal Alternate Steam Drive" (HASD). This process has been designed to improve the recovery of mobile heavy oils in presence of bottom aquifers, where natural depletion is inefficient because of strong water coning from the aquifer. HASD is a repetitive pattern using equi-depth horizontal drains acting alternatively as producers and steam injectors: the main recovery process is horizontal steam flooding between successive wells and is more efficient than classical cyclic steam injection. After the description of the process itself, we will illustrate the performances of HASD using simulation results on simple 2D homogeneous models. We will finally show, using 3D simulations, that HASD increases the recovery in presence of moderately connected aquifers, as compared to natural depletion, and could even improve the performances obtained by more standard thermal processes such as SAGD (Steam Assisted Gravity Drainage). Introduction In presence of mobile heavy oils (viscosity < 10000 cp), natural depletion using horizontal drains is the most commonly proposed process for field development, because of the (relatively) low cost implied for the production. However, the recoveries associated to natural depletion are low (<10%), and might even be drastically reduced in presence of an aquifer, since the producers are subject to strong water breakthroughs. Thermal processes, such as CSS (Cyclic Steam Stimulation) and SAGD can be used to improve recoveries, with the extra-cost of heat generation. CSS mainly deals with well stimulation: it will accelerate the oil production but will not significantly improve the final recovery. SAGD, as designed by R. Butler ([1]) is the process showing the highest potential for increasing recoveries in heavy oil, extra-heavy oil and bitumen fields. Field developments are mostly focused on extra-heavy oils and bitumen, but SAGD pilots have already been performed on less viscous oils ([2]). When, thanks to the initial mobility of heavy oils, production can be ensured by natural depletion, thermal processes may be regarded as a long-term issue, because of the high investment and additional operational cost they imply. This paper will however propose a new thermal process, still based on steam injection, but whose economics can be more attracting than SAGD. This process, HASD, takes advantage of the initial oil mobility, and will require fewer wells than SAGD, for comparable performances, as will be shown on simple 2D phenomenological simulations. It combines horizontal steamflooding and periodic cycling between injectors and producers. As the eventual presence of a bottom aquifer will reduce the performance of natural depletion, we will also show, on the same 2D phenomenological simulations, but also on a 3D full-field model, how thermal processes can far improve production performances. Compared performances of HASD and SAGD will be studied, in order to rank both processes according to the aquifer context. Description of the HASD process HASD is a repetitive pattern using equi-depth horizontal drains acting alternatively as producers and steam injectors. As opposed to SAGD, it does not use injector/producer pairs, but single horizontal wells, cyclically switching between injection and production. The main recovery process is horizontal steam flooding between successive wells and is more efficient than classical cyclic steam injection. Effectively, the steam chamber generated by an injector is pulled laterally by the pressure differential created by adjacent producers, forming a sweeping front between wells. Cyclically, injectors are converted to producers (and vice-versa) to extend the steam chambers to the whole reservoir region (Figure 1). Thus, the impact of steam will not be a simple well stimulation, but it will effectively sweep the reservoir, thus decreasing oil viscosity and improving oil drainage.
The Orinoco Heavy Oil Belt, located in the southern part of the Eastern Basin of Venezuela, is considered the largest deposit of heavy oil in the world. It covers an area of 14 million acres and is characterized by having crude of low API gravity (from 7 to 10º), high viscosity (from 1,000 to 10,000 cp), high porosities (from 18 to 40%) and permeabilities that can reach 30 darcies. Heterogeneity is present in the Faja, there some areas with active bottom aquifers. On these particular areas an early water breakthrough has been identified in some horizontal wells. A numerical simulation model with representative properties of an area of the Orinoco Heavy Oil Belt was defined to assess if the implementation of inflow control devices (ICDs) could reduce water production in horizontal wells. The numerical model contained a horizontal well where these completions elements were installed. The evaluation was made through a sensitivity analysis in which the configuration of the devices and some rock and fluids properties were changed. Additionally, the effect of the horizontal well length was studied as this parameter is relevant in the design and planning of horizontal wells in the Faja. The results of this investigation indicated that the use of inflow control devices can be an effective technology to delay water breakthrough in areas where there is an active bottom aquifer with a good understanding of the geological properties and reservoir behavior. On other hand, this study showed how the differential increase in the cumulative oil of the wells decreases progressively as the horizontal well length section increases. An economic model was created to compare the different simulation scenarios. This research serves as a basis for determining the feasibility of implementing inflow control devices as a water control technology and to obtain valuable information to designing the horizontal section of the wells.
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