Multilateral Field Experience in Developing an Extra Heavy Crude-Oil Reservoir
Authors: Ramirez R. , Fernandez V. , Barrios J. 
Published: December 2007

Sincor is exploiting an 8° API reservoir in Zuata field located in the Orinoco belt in southeastern Venezuela. Reservoir pressure is around 900 psi in unconsolidated sand at 1,800-ft depth. Field development plan is based on drilling extended reach horizontal wells of 7,000-ft measured depth. All of the wells are completed with a progressing cavity pumping system and piping for injection of diluent for cold production operations. Injection of diluent is required to achieve the production plan in the next 35 years. Heavy crude oil is processed at the upgrader plant located 200 Km apart from the field. Final product is 32° API oil known as Zuata Sweet. Coke and liquid sulphur are also commercial products. A total of 335 horizontal wells have been drilled and completed. Fourteen dual-lateral wells have been drilled in some instances in areas where the reservoir exhibited the poorest petrophisycal properties. Dual-laterals are expected to achieve the target oil production rate per single well by increasing the contact area between the wellbore and the reservoir. Dual lateral wells have been constructed with level 3 and level 4 junctions. Junction level is chosen on individual basis by taking into account local lithology and junction model is based on the operational capability for running a slotted liner into the horizontal drain where sidetracks for stratigraphic reasons have been drilled. Wells are also equipped with a dual completion string that allows injection of diluent downhole, which is done to reduce pressure losses and improve the well productivity. Injection point can be located at the wellhead, right above the pump, or at the well toe. This paper presents a novel and interesting combination of multilateral technology and diluent injection completion techniques to achieve project economics and oil production requirements.

Introduction

Sincor is presently developing mineral acreage in La Faja de Orinoco (The Orinoco Belt) for production of heavy crude oil.

The project is located southeastern Venezuela, between communities of Zuata and San Diego de Cabrutica (Figure 1). Current field production is 200,000 barrels of oil per day and it is expected to maintain over the next 35 years. Crude oil is moved from the producing area via a 200-km pipeline to the Jose upgrader at the Caribbean coast, where the heavy oil is upgraded to a low sulfur 32° API oil.

Drilling operations began in August 1999 with one 500 HP rig drilling vertical stratigraphic wells and surface holes. A 900 HP rig commenced batch drilling all 12 1/4" intermediate sections and then all 8 1/2" horizontal sections by September 1999. An a third 1000 HP rig started drilling production wells in February 2000.

By the end of this first drilling campaign, the company had drilled and completed 335 extended reach horizontal production drains, 14 are dual lateral drains.

General Geology

The Orinoco Belt corresponds to the southern part of the Venezuelan Oriental Basin. This east-west elongated foreland basin developed mostly from the late Oligocene to Upper Miocene periods in response to the setting of the mountains "Sierra Del Interior" which forms today the northern coast of Venezuela.

The overall structure corresponds to a low dipping monocline that exhibits dip values between 1°–2° toward the N-NE. This dip comes from two phases of tilting of the basin: first toward the north and then toward the northeast.

In the southern half of the basin, the tertiary and quaternary deposits lie unconformable on the cretaceous sediments of the Temblador Group. Toward the north, these Mesozoic sediments disappear and the tertiary deposits lie directly on the Precambrian basement (Figure 2 and 3).

Pressure and Temperature Gradients

Due to the depth of the reservoir (1500ft - 2100ft), the pressures and temperatures are relatively low.

Down hole temperature at total vertical depth is around 120°F/49°C. This corresponds to an approximated temperature gradient of 0.4°F/100 ft. Reservoir pressure ranges from 700 to 800 psi. This corresponds to a pressure gradient of 0.418 psi/ft.

General Drilling Program

In order to minimize the environmental impact of roads and sites construction, wellheads were grouped in clusters, on drilling pads. Clusters were set in the centre of a 1600m × 3400m rectangle.