Application of New Technologies to Horizontal Wells in Venezuela

Abstract

In Venezuela, at Maracaibo Lake (west) and Anzoategui-Monagas (east), there are reservoirs that have been under production for more than 30 years, which are presenting now low reservoir pressure. This situation has prompted the search for new technologies to maintain or increase the production in these zones. So far the technology that has been more successful is horizontal drilling, including the use of aerated drilling fluids and multilateral wells. Up to date there are some 700 horizontal wells in Venezuela, with an average initial production of 800–1000 BPD per well. The majority of these wells are drilled setting a casing on top of the productive zone and changing the drilling fluid to one designed to properly seal and minimized formation damage. There are areas in Lake Maracaibo where the equivalent reservoir pressure is less than the density of water; the horizontal section is drilled with a mixture of air and water based fluid to achieve 3.5 – 6.5 ppg at bottom. Multilateral horizontal technique has started with the design of wells with two arms, each in a different sand and opposite direction. Production can be commingled or set separately if needed. This paper provides details of field applications of these technologies, production data and future projections. Introduction Horizontal drilling, as an emerging technology, had made possible an improvement in production from depleted reservoirs or from remnant narrow sands. In Venezuela, horizontal wells are drilled in two main areas: Maracaibo Lake in the west region and Anzoategui-Monagas in the east region, see Fig. 1. Since the beginning of horizontal drilling in Venezuela, more than 600 wells had been drilled and over 1500 wells are expected for the year 2000 as Fig. 2 shows. These horizontal wells include mostly reentry wells to increase production of depleted reservoir from new exposed sands and new wells in narrow sands in order to incorporate remnant crude from them. From now on in this paper whenever horizontal wells are mentioned it should be taken as including both, new and reentry wells. Due to the increase in horizontal drilling, technical aspects of the application of this technology in Venezuela are presented in this paper focused mainly in Maracaibo Lake. Improvement of application of horizontal drilling with other technologies, as aerated drilling, will be included. Horizontal Wells Miocene sands are poorly consolidated and start at an average depth of 2500 to 3500 feet, with an average reservoir pressure of 1100 psi. The firsts horizontal wells drilled in these sands were done setting a casing above the productive zone and continued drilling with an excessive density of barite densified fluid until the end of the horizontal section. The production of these wells drilled up to 1994 was 1,5 times a vertical well. Laboratory Tests. Cuttings of these wells were examined using an electron diffraction microscope which showed that all the pore spaces, within certain depth, were invaded by barite. Formation damage from an inadequate formulation of drilling fluid, specially in the choice of kind and particle size of the bridging agent, was causing the low productivity of these wells. Horizontal wells have substantially longer damaging contact time with the drilling fluid. Furthermore, draw down of the productive zone may not be enough to remove the damage. While the external filter cake is formed during the drilling process, a spurt loss occurs that carries fluid and solid particles into the face of the formation, thus forming an internal filter cake. Once the filter cake is formed, external and internal, only filtrate will penetrate in the formation due to hydrostatic pressure.