Doctrines vs. Realities in Reservoir Engineering

Abstract

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 185633, “Doctrines and Realities in Reservoir Engineering,” by Euthymios Vittoratos, Petroprognostica, and Anthony Kovscek, SPE, Stanford University, prepared for the 2017 SPE Western Regional Meeting, Bakersfield, California, USA, 23–27 April. The paper has not been peer reviewed. The appreciation of empirical realities permits the improvement of commercial depletion planning and enables a greater number of projects. The concepts articulated here are applicable in almost all heavy-oil reservoirs and may be applicable also for lighter oils that possess oil chemistry typical of heavier oils (e.g., high acid content). This paper reviews the evidence for and against three doctrines in current use to develop depletion plans: (1) optimal recovery is obtained using a voidage-replacement ratio (VRR) of unity, (2) the Buckley-Leverett (BL) formulation (phase slippage) applies uniformly to heavier oils, and (3) viscous fingering dominates unstable multiphase flows. VRR=1 Doctrine: Optimal Waterflood Response Occurs With VRR=1 Reality: Periods of VRR<1 Increase Oil Recovery Though initially viewed with some suspicion, waterflooding has demonstrated multifold greater recovery than primary depletion for many reservoirs. It is, by far, the most important oil-recovery process. One visualization of the process is that of a leaky, deformable water piston displacing a fraction of the oil to the production wells. Maximum recovery occurs when the voidage created by the produced fluids (oil plus water) at reservoir conditions (pressure and temperature) is replaced fully by an equal volume of the injected water also at reservoir conditions (i.e., the waterflood VRR=1). In this way, reservoir pressure is maintained. The waterflood initial production is pure oil until water breakthrough. Subsequently, water is produced along with the oil, resulting in an increasing water/oil ratio (WOR). This simple visualization is intuitively attractive and has dominated the thinking of the industry. This “leaky, deformable piston” visualization is simplistic and neglects or sidelines complex physical and chemical properties of the reservoir. If the waterflood is operated below the oil bubblepoint pressure through a continued VRR<1, a gaseous phase forms, creating three-phase-flow conditions and more-complex displacement possibilities. Furthermore, reservoir oils can have widely different physical and chemical properties, varying by many orders of magnitude in viscosity and substantially in interfacial activity as described by metrics such as the total acid number. Despite the full recognition of these complications, over time, the now al-most universal practice developed of operating waterfloods with VRR=1. Analytical and Numerical VRR Simulations. VRR<1 activates many recovery mechanisms. Some mechanisms can be enumerated and examined, and others may exist that are yet to be identified. What is the possible upside of operating a waterflood with VRR<1? Making a rough estimate is not too difficult if we limit the number of mechanisms. With VRR<1, two dominant recovery mechanisms exist, solution-gas drive (VRR=0) and water displacement (VRR=1). As a first approximation, at an intermediary VRR<1, the two mechanisms may be additive: Oil Recovery (at Optimal VRR)=Oil Recovery by Pure Waterflood (VRR=1)+Pure Solution-Gas Drive (VRR=0).