The main objective of this paper is show the design, implementation and results of a nitrogen + steam pilot implemented in a Colombian heavy oil reservoir. Given that the answer to the cyclic steam injection has declined significantly in some wells of the interest field due to the high number of cycles, a pilot project to assess the feasibility of injecting nitrogen accompanied cyclic steam was raised. The determination of optimum volumes and injection scheme by numerical simulation was performed in a sector model. Eleven schemes of steam plus nitrogen injection were evaluated. In all schemes, it was tested remained constant volume of injected steam, while the volume of nitrogen varied according to the scheme (pre-injection, post-injection, co-injection or a combination of the above). In all cases, it assumed the injection of nitrogen according to real drive capacity 1200 m3/hour (1017072 ft3/day). The best case corresponds to start injecting only one day with nitrogen, followed by five days of co-injection and ending with a single nitrogen day. Under this scheme an incremental production of 5642 barrels of oil a trial period of six months, with average oil production of 53 BOPD in the same period and maximum rate of 142 BOPD. According to the simulation results, it was decided to implement the pilot steam + nitrogen injection, following the best injection scheme given above; that is, a day of pre-injection (nitrogen only), five days of co-injection (steam + nitrogen) and one day post-injection (nitrogen only). The results of the pilot show that oil production has increased compared to previous cycles, reaching similar results to the numerical simulation forecast. A methodology to implement steam injection enhanced with nitrogen is proposal in this paper, which can be applied in any field of heavy crude scheme developed under cyclic steam stimulation.

Colombia is a country with important reserves of heavy oil, located mainly at the "llanos orientales" and at the "Magdalena Valley Medium" basin, the oilfields located in this basin, have been developed using cyclic steam stimulation, showing recovery factors close to 15%, indicating that a large quantity of oil has not been removed yet. Due to the above reasons, it has been thought in the steam flooding as an alternative for the extraction of remaining oil, but the intercalated shale in the deposits in question, acting as thieves, heat affects the thermal efficiency of the process, why is not clear whether the steam flooding will be viable in the deposits of the Magdalena valley, specifically in the TECA field. Given the above, it is necessary to conduct a study to determine the effect of the intercalated shale on thermal efficiency. This study was carried out using numerical simulation and analytical models in the case of a numerical simulation model was compared with intercalations of shale and a homogeneous model, we conducted a further analysis of sensitivity to the rock and fluid properties, as well as operational parameters for the implementation of a steam flooding, including a scheme of selective injection. Finally, we found that the thermal efficiency is greatly affected by the intercalated shale is decreasing by up to 30%, a value considerably higher than would seriously affect the technical and economic feasibility of the project but has been observed to implement a scheme of selective injection could increase the efficiency of the process, making this more feasible to perform.

This paper shows the evolution of the cyclic steam stimulation (CSS) process in a Colombian heavy oil field. Some challenges have been faced like: low injectivity in the first stimulation cycles, presence of swelling clays, low lateral and vertical continuity of the producing sands, steam channeling, among others. According to the oilfield complexity some strategies are currently being implemented after previous studies of numerical simulation and lab test evaluation: diesel slug to improve injectivity, reduced steam quality in the first cycles to increase steam injectivity, using clay inhibitor during steam injection, reduced spacing in the better zones according to opportunity index map, nitrogen injection in wells with more than six stimulation cycles, high frequency cycles and changes in completion design and drilling scheme among others. The set of lab test, numerical simulation and pilot test have resulted in the following: 1) Use of diesel slug before steam soaking clean the hole improving injectivity, allowing more steam volume injection. 2) Reduce steam quality in first stimulation cycles is useful to increase injectivity without significantly affecting steam oil ratio SOR. 3) Using an index map opportunity help to easily identify areas with the greatest potential for infill wells. 4) Injecting nitrogen with steam in wells with more than six stimulation cycles improve production results due to the pressurization of producing sands. Key recommendations for implementing a CSS process in heavy oil reservoirs with low lateral and vertical continuity of the producing sands, swelling clays and low net to gross are presented in this article. The recommendations range from the first injection cycles to maturity of the process and the search for new production alternatives.

In Colombia, there are several heavy oil fields where cyclic steam stimulation have been successfully applied for over 25 years and although it is assumed that the reservoir temperature is high and the conditions are ideal for making the leap to continuous steam injection, some studies apparently show that this technique is not feasible for the reservoir; in fact, between 1992 and 1994, the implementation of a pilot did not reach the expected results.From this fact raises the question: What are the key factors that ensure the success of a continuous steam injection pilot? To solve this question, a numerical simulation and analytical model based study was performed, showing that the performance of the technique lies in minimizing the energy losses and supplying only the necessary heat according to each stage of the process, these practices are known as heat management.It was found that critical aspects of heat management are: reducing well spacing, selection of open thickness to injection, perforated intervals, completion design, injector well distance from generator and injection surface line conditions, as well as constant reservoir temperature monitoring, from which the energy requirements at each stage of the process is determined and avoid an excessive injection rate which goes against project economics.For the field case it was found that: the well spacing should be reduced from 10 to 2.5 acres, the need of using selective sequential injection scheme (due the interbedded shale), the injection rate should be reduced to 46% a year after starting the process and also change the type of insulation on the surface lines. In this paper, the methodology and tools used for factor evaluation and process optimization are presented.

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