The Use of Multilateral Well Designs for Improved Recovery in Heavy-Oil Reservoirs

Abstract

Abstract There are now a variety of ways to achieve higher recovery factors from heavy oil reservoirs, but most of them involve the injection of thermal energy or chemicals to reduce the oil viscosity. While these techniques have been highly successful, they can also be very expensive when the steam generation and/or chemical injection costs are accumulated throughout the productive life of the field. A lower cost solution, one that has been very successful in the Faja Del Orinoco of Eastern Venezuela, is to use multi-branched wells (multilaterals) to increase reservoir exposure and achieve an arguably higher recovery factor. These multilateral wells have been shown to produce more oil over a longer period of time than conventional horizontal wells without any additional operating costs. This paper will discuss the concept of using multilateral wells as an alternative to conventional Enhanced Oil Recovery (EOR) techniques in heavy oil reservoirs. It will argue that the oil recovery factor of a reservoir that is drilled with increased wellbore exposure can be comparable to thermal/chemical EOR under the right circumstances, and that the project will have a much lower operating cost. While steam injection has become the successful mainstay of most EOR projects, there are many drawbacks such as the costs of the steam generation and the emission of greenhouse gases. Multilateral wells can potentially offer an option to produce the same reservoir with lower costs while still recovering an increased percentage of the oil from the reservoir. This technique is especially applicable in inter-bedded or thin oil zones where steam injection would be costly and inefficient. Extensive background information will be presented from the Orinoco belt in Venezuela, where multilateral wells have proven to be an economical approach to develop a very challenging "extra-heavy" oil reservoir. The Petrozuata Project The Petrozuata project is a joint venture between ConocoPhillips (50.1%) and PDVSA (49.9%), to produce, upgrade, and commercialize extra heavy crude oil from the San Diego field, which is located in the Zuata region of the Orinoco Belt in eastern Venezuela. The joint venture operates under a 35-year production contract. Early blend production began in 1998 and the commercial sales of upgraded (synthetic) crude began in 2001. In 2004, Petrozuata achieved record volumes, producing more than in any year since its start-up in 1998. This increase was largely the result of extensive use of advanced drainage architecture and multilateral wells. The main oil reservoir in the ¨Faja del Orinoco¨ are the Miocene sands of the Oficina formation. In the Petrozuata area, the reservoir is described as aggrading coastal plain deposits consisting primarily of massive multistoried and/or stacked, coarse-to-fine-grained fluvial channel belts prograding from south to north. The sands are poorly- to well-sorted and unconsolidated. They have net-to-gross sand ratios of approximately 55 to 65%. Within the primary oil reservoir, the permeability ranges from 700 to 15,000 millidarcies (mD). The channel morphologies vary from highly sinuous silt-filled, laterally stacked to straighter, braided channels. The channel sands are typically 20 to 40 feet thick and as wide as 1 to 2 kilometers (3000 to 6000 feet). By their nature, they tend to be laterally discontinuous and contain a certain percentage of silts (non-reservoirs) within the channel complexes. To achieve high performance wells, the percentage of high-quality oil sand contacted along the horizontal well length must be maximized. Careful geological planning and well design can allow several disconnected sand units to be produced from a single well. Such provisions greatly improve oil recovery and increase well production. To do so, however, requires the strategic placement of horizontal sidetracks and multilateral junctions. Reservoirs range in depth from 1700 to 2500 ft TVD, with average temperatures of 120 to 125ºF. The oil gravity lies between 8.5° and 9.5º API, a range that qualifies the oil as "extra-heavy." The reservoir itself is a low pressure, viscous fluid environment that requires significant physical encouragement to get the thick oil to flow. "Drain" is actually a more appropriate term than "flow" because the wells are drilled so closely together and with such frequency that the oil is essentially drained from the sand, under the force of gravity, into wellbore tubulars from which it can be pumped to the surface.