SPE paper 94569 (2005) presented the generalities, characterization and initial development plan of the Guando field, an extremely underpressured, sandstone, partially naturally fractured reservoir. This paper will present global results of the initial development program and the bases which guided to implement a reduced spacing project based on decreasing wells distance from 250 to 145 meters. Results of this new development stage are presented and discussed. This project increased field reserves in 15% and accelerated the production, allowing a 15% recovery factor in just seven years since the field exploitation started and nine years from its discovery and only with a total injected water of 0.25 volumes of its original oil in place. Introduction This field located in Colombia, South America (Fig. 1), was discovered in year 2000 by Petrobras (Operator) and its partners Ecopetrol and Nexen. Due to its extremely low initial reservoir pressure, initial field development program was based on waterflooding the reservoir since the beginning, on an inverted seven spot pattern, with a well spacing of 250 meters, complemented with peripheral injection and having as a principal component the selective injection to compensate the reservoir heterogeneity and then improve the vertical sweep efficiency. The implementation of this development program showed better results than expected, which was a motivation to plan and implement a reduced spacing pilot on the same inverted seven spot configuration, but now, with wells located at 145 m. The implementation of this phase was made in several steps:an initial pilot with three producer wells;an extended pilot with 15 new producers;drilling of 40 additional wells and;conversion of 11 producers to injectors and injection increase in 30,000 bwpd, rising the total water injection rate to 105,000 bwpd. Selected strategy and the accelerated way on which it was implemented have allowed getting a 15% recovery factor in nine years from field discovery. Review of reservoir characteristics This reservoir is a late Cretaceous (Guadalupe group), formed by fine to coarse grain sandstones, which are highly bioturated and correspond to a shallow marine to transitional depositional environment, with an average thickness of 700 ft on the main reservoir. Natural fractures exist in the majority of the field having an important effect mainly since the injection point of view, by its negative impact on sweep efficiency; fracture characterization models indicate connectivity of the system is not good for the most of the reservoir, with the exception of some areas locate close to faults (Fig. 2). Since the structural point of view, this accumulation corresponds to a sub thrust structure decapitated by Boqueron fault and with some inner faults which form indepndent reservoir blocks (Fig. 3). The reservoir is located at an average depth around 3500 ft, with the whole reservoir above the sea level. This condition combined with a thought hydrodynamic effect brings one of the more important Guando reservoir characteristic, which is its extremely low reservoir pressure that at the depth of the gas-oil contact (GOC) is only 100 psi (@1810 ft above sea level), having a maximum around 500 psi at the oil-water contact (OWC) depth.

The purpose of this paper is to present the main challenges involved in the Guando field development project, how they were addressed, the lessons learned, and in a more general sense, describe how a severely underpressured, partially naturally fractured reservoir was successfully developed using a phased approach to water injection implementation. The field appraisal and phased development approach utilized resulted in a technically and commercially successful project, despite the high degree of uncertainty that initially existed. Introduction The Guando field is located in Colombia, South America and was discovered early in the year 2000, the field is contained within the Boquerón Block with the Association Contract partners being Petrobras (operator), Nexen and Ecopetrol (Fig. 1). Initial data indicated Guando field to be the largest oil accumulation discovered in Colombia in over fifteen years, but in view of the severely underpressured nature of the reservoir (100 psi at gas/oil contact which is located roughly 1800 ft above sea level), the medium gravity oil in place with a very low gas/oil ratio (GOR), a high degree of uncertainty existed as to whether the field could be commercially developed within the contract period ending 2023. From the reservoir perspective, a numerical reservoir simulation, which integrated all available data, was employed, being functional few months after discovery. A peripheral combined with pattern (inverted seven spot) water injection scheme, utilizing selective injection completions within each wellbore, was determined to be the most effective method for waterflooding. Water injection was planned and implemented in phases in order to minimize risk with the stages being:a pattern pilot,peripheral water injection and;full field water injection (peripheral + pattern). In summary, the highly technical approach to the Guando field development has resulted in a commercial project. The step wise approach taken to the development provided flexibility to make changes as results and new information came to light, which in the end proved low impact. Reservoir Description and Reservoir Model Considered a cornerstone to the successful development of the Guando field, data acquisition received high priority throughout the field's development. Both 2D and 3D seismic information was acquired. Over 3000 ft of core was obtained from the 90+ wells which have been drilled to date and extensive core description and analysis were performed. Standard logging suites allowed for the acquisition of pressure, resistivity, density, neutron, natural spectral gamma ray and microresistivity derived image log information. Magnetic resonance and dipole sonic information was collected in the early wells. The original completions included a large number of cased hole DSTs.

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