Guando Field: Underpressured-Field Development, Part II

Abstract

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.