Closing the Lab-Field Gap: A Look at Near-Wellbore Flow Regimes and Performance of 57 Field Projects

Abstract

SPE Member Abstract The goal of this paper is to link lab and field data in order to resolve a paradox that has caused the shelving of many worthwhile improved oil recovery field projects. Theoretical shear rate calculations often show that polymer or colloidal dispersion gels injected in unfractured reservoirs through perforated completions face certain destruction in terms of shear degradation. Under these conditions, calculated shear rates are often extremely high, and tend to suggest there is no point in injecting polymer, as it will be destroyed to the point of ineffectiveness. The lab-field paradox is that polymer and colloidal dispersion gels have been injected into these types of reservoirs numerous times and have proven highly successful in spite of high shear rate calculations that suggest otherwise. This paradox represents an important discrepancy between lab/theory and actual field performance. In this paper, lab and field data were combined for 62 injection wells from 57 actual field projects that utilized polymer and in-depth gels. 89% of these projects were successful improved oil recovery floods, yet theoretical shear rates in excess of 50,000 were calculated for 42% of the injection wells, using basic field assumptions. Although lab data showed that polymer would be destroyed at these high shear rates, the polymer did perform in-situ in all of the injection wells. Reasons for lab-field discrepancies are proposed and discussed. To determine which field assumptions have the strongest impact on calculated shear rates, sensitivity analyses were performed on several variables. Assumptions for apparent viscosity, injection flux area and permeability have the strongest impacts on results, and are the most poorly defined variables that go into the calculation. This work shows that shear rate calculations for field injection wells should be employed only with a strong knowledge of injection well completions and sensitivities on certain variables. Introduction It is commonly known in the improved oil recovery industry that polyacrylamides undergo inevitable mechanical degradation in the field due to stress as they are injected. Mechanical degradation is the physical "tearing apart" of a polymer molecule so that its molecular weight is reduced. Polymer effectiveness in improved oil recovery depends on flow resistance properties, which in turn depend on polymer molecular weight, so degradation can potentially have a strong adverse effect on polymer performance in the reservoir. This mechanical degradation can occur in surface equipment, such as pumps, valves and constrictions in lines, or filters. It can also occur downhole, as the polymer solution flows from the injection wellbore through the formation face. This paper is concerned with downhole degradation due to stress. Estimation of downhole stress and resulting polymer degradation depend on polymer characteristics in the field injection water and injection well flow regimes. Injection well flow regimes depend on rock properties, flow rates, completion methods and polymer resistance to flow in the injection wellbore. P. 445^