Single Well Reservoir Management - the Ultimate Multibranch Well Challenge

Abstract

Abstract Whether due to the lure of deep-water prospects or the shear excitement to apply new technology, the recently developed multibranch wells are likely to play a crucial role in the future of petroleum exploitation. The challenge is to adjust and even develop reservoir management strategies both suited to and taking advantage of this technology. In effect, a multibranch well provides an underground gathering system. When the design includes downhole separation that provides for the production at the surface of desired fluids with reinjection of unwanted ones, the well incorporates an underground production facility. To achieve these sorts of objectives, the entire system of branches, completions, manifolds, separators, etc., must be in place and operational before the well can begin to produce. The design considerations that would enable this result rely on a modular approach to the problem and provision for contingencies for uncertainties in the reservoir description and in the reliability of the underground system. This paper outlines the issues that must be considered and provides a framework for the well design process. Introduction In a previous paper, Ehlig-Economides, et al.,1 outlined a modular approach for designing multibranch wells that is reproduced in Figure 1. This figure focused mainly on primary production considerations and on applications where a multibranch well is an option among competing alternatives. For remote or deep offshore locations, the multibranch well offers advantages that make this technology the obvious first choice. For deep-water developments, a key consideration is the need to minimize the sea floor footprint. The ultimate objective would be to develop a significant prospect with a single point of entry, that is, a single subsea wellhead. To minimize or eliminate direct intervention for the life of the project, sensors and controls enabling remote adjustments can be built into the well design. The same well may enable both primary production and pressure maintenance providing for at least 50% recovery of the original oil in place. A fully integrated design addressing planning, construction, and operation must encompass economics, appraisal, reservoir management, surface and underground facilities, branch completions, and drilling, and all with an eye on managing risks and uncertainties. Although the drivers for the design are special to deep-water and remote locations, the technologies employed may offer advantages in less hostile environments as well. Economics The economics justifying production from deep-water reservoirs must compete favorably with global project alternatives. As an example, suppose $1 billion are available for investment. Assuming a risk-loaded discount factor of 0.5, and after tax revenue per barrel of $7.5, the target oil production rate is 182 MSTB/D. In turn, production at this rate provides a 30% rate of return on investment after 3.5 years and is equivalent to an activation index of $5500/STB/D. In 3.5 years, the cumulative production is about 235 MMSTBO. If this is primary production under expansion drive, the recovery factor could be as low as 5% or less, and the initial oil in place would have to be at least 20 times the cumulative production, or 4.7 BSTBO. Globally, there are very few reservoirs of this size. Further, for a single point entry well to produce this volume, it would have to cover an enormous area. A better approach would be to design the well to produce with pressure maintenance via water injection. This exploitation scheme can deliver at least 50% of the oil in place, and the required reservoir volume would be only 470 MMSTBO. The area to be covered by the well would be less by an order of magnitude. Reservoir Management Strategy Deep-water reservoirs are typically younger deposits than land reservoirs at equivalent depths. For a reservoir depth 5000 ft below the sea floor in a 10,000 ft water depth, the overburden stress is due to the combination of the sub sea floor accumulation and the water. Normally pressured and poorly consolidated or unconsolidated formations are expected with significant light oil or gas reserves.