SPE-109682
SPE-109682
Key Aspects of Project Design for Polymer Flooding
Abstract
After a pilot site meets the screening qualifications for polymer flooding, the injection measures and the injection formula are key points when designing a polymer flood. This paper places special emphasis on some new design factors that were found to be important during extensive experience during polymer flooding. These factors include (1) recognizing when profile modification is needed before polymer injection and when zone isolation is of value during polymer injection, (2) establishing the optimum polymer formulations, injection rates, and individual well production allocations, and (3) time-dependent variation of the molecular weight of the polymer used in the injected slugs.
At Daqing, polymers with molecular weights from 12 to 38 million Daltons were designed and supplied to meet the requirements for different reservoir geological conditions. The optimum polymer injection volume varied around 0.7 pore volume (PV),1 depending on the water cut in the different flooding units. The average polymer concentration was designed about 1,000 mg/L, but for an individual injection station, it could be much more.2,3 The injection rate should be less than 0.2 PV/yr, depending on well spacing. Additionally, the project design should follow certain rules when allocating the injection rate and production rate for individual wells.
Introduction
Many elements have long been recognized as important during the design of a polymer flood.4-12 This paper spells out some of those elements using examples from the Daqing oilfield. Critical reservoir factors that traditionally receive consideration are the reservoir lithology, stratigraphy, important heterogeneities (such as fractures), distribution of remaining oil, well pattern, and well distance. Critical polymer properties include cost-effectiveness (e.g., cost per unit of viscosity), resistance to degradation (mechanical or shear, oxidative, thermal, microbial), tolerance of reservoir salinity and hardness, retention by rock, inaccessible pore volume, permeability dependence of performance, rheology, and compatibility with other chemicals that might be used. Issues long recognized as important for polymer bank design include bank size (volume), polymer concentration and salinity (affecting bank viscosity and mobility), and whether (and how) to grade polymer concentrations in the chase water.
At the end of 2006, oil production from polymer flooding at the Daqing Oilfield was more than 10 million tons (63 million barrels) per year (sustained for 5 years). This paper describes the design procedures that led to favorable incremental oil production and reduced water production during 12 years of successful polymer flooding in the Daqing Oil Field.
1 Zone Management before Polymer Flooding
1.1 Profile Modification before Polymer Injection
Under some circumstances, use of gel treatments or other types of “profile modification” methods may be of value before implementation of a polymer or chemical flood.13 If fractures cause severe channeling, gel treatments can greatly enhance reservoir sweep if applied before injection of large volumes of expensive polymer or surfactant formulations.14 Also, if one or more high permeability stratum are watered out, there may be considerable value in applying profile modification methods before starting the EOR project.
For some Daqing wells with layers with no crossflow, numerical simulation demonstrated that oil recovery can be enhanced 2-4 % original oil in place (OOIP) with profile modification before polymer injection.15 (10-12% OOIP was the typical EOR due to polymer flooding alone.) As expected, the benefits from profile modification decrease if it is implemented toward the middle or end of polymer injection