Single-Well Waterflood Strategy for Accelerating Oil Recovery

Abstract

Abstract Conventional primary production addresses from 3% (expansion of undersaturated oil) to about 15% (solution drive) of the original oil in place. An active water or gas cap drive may boost recovery significantly under ideal conditions, but otherwise pressure maintenance via water or gas injection is required. Simulations over a wide range of reservoir and fluid properties demonstrate that waterflooding from the start of production and maintaining pressure via reinjection of produced water can yield a target of 50% recovery of the oil in place in less than 5 years. The proposed single well with laterally parallel segmented horizontal branches accommodates water injection in one branch and production from the other branch with a downhole separation and reinjection system designed to reinject all produced water for up to 90% water cut in the producing branch. A simple procedure provides the optimal length of the laterals and the optimal separation between them to achieve the target recovery factor, given the reservoir and fluid properties. When closer spacing is needed to achieve sufficient pressure support for the producer or to ensure sufficient connectivity between the wells, longer laterals can be drilled to reach the same total in-place oil volume. The procedure uses previously established formulas for line drive waterflood performance adjusted for vertical to horizontal permeability anisotropy. The single-well waterflood avoids the need to lift and reinject produced water and accelerates the pore volume throughput, thus enabling the constant oil production rate (and equal surface water injection rate) characteristic of p-mode production. Also, the resultant constant reservoir pressure provides steady state conditions in the surface facility, greatly reduced risk of reservoir compaction or subsidence, and virtually no impact on the surrounding reservoir. Viability of the economics is easily demonstrated with net present value computations. Introduction Most reservoirs are exploited first through primary and then secondary recovery processes. Since the 1980's hydraulic fracturing has been employed extensively to accelerate primary production. In the 1990's horizontal wells became a widely-used method to accelerate primary production. Most recently, multilateral wells provide even another technique to accelerate primary production. In the absence of a natural drive mechanism via gas cap and/or aquifer, primary oil recovery normally includes production by liquid expansion drive (up to 5% of IOIP) and solution gas drive (up to 20% of IOIP). Higher recovery efficiencies are possible only with pressure maintenance via gas or water injection. Hydraulic fracturing and horizontal and multibranch wells accelerate production and can increase the volume of oil recovered per well, but without pressure maintenance the recovery efficiency is limited to at most about 20% of the IOIP in the well drainage volume. In a homogeneous reservoir, water flood recovery efficiency can be expected to reach or exceed 50% for oil mobility ratios ranging from 0.1 to 1.1 This paper introduces the concept of accelerated recovery. As opposed to accelerated production, which usually applies to primary production, accelerated recovery implies reaching the target recovery efficiency in significantly less time. In either case, economics such as net present value reconcile the costs and benefits of a given strategy. One added value of accelerated recovery is the reduced "timeprint" for the operation. That is, in addition to the environmental advantage of reducing the footprint of the asset development by drilling directional wells off a pad or platform, also reduced is the length of time the land is diverted for oil production or the length of time an offshore platform remains positioned against potential collision, weather and other risks.