Injection Well Rate and Pressure Analyses Suggest Practical Waterflood Improvements

Abstract

Waterflood injection wells operate at maximum efficiency when delivering maximum, non-fracturing matrix rates to highest oil water ratio (OWR) subzones. Decreasing OWR observed during the later years of a mature waterflood intensifies need for analysis and optimization of injection efficiency. Analyses of injection rates, pressures and distribution profiles, especially when combined with recently more available step rate test (SRT) data allow injector efficiencies to be quantified in terms of estimated resulting BOPD. The Los Angeles basin is home to many mature, low OWR waterfloods and a large, steadily increasing number of injection wells. In order to analyze injection efficiency over time, a prototype injection well, designed to include features common to LA basin injectors, was evaluated at key times of design, SRT, startup, profile deterioration, and finally, 100% point exit. Local DOGGR injector permitting practices were incorporated into determination of "Maximum Allowable Surface Pressure" (MASP) for the prototype well. Estimated losses of injection capacity resulting from conservative "safety" practices of frac gradient discounting and ignoring friction pressure were significant, suggesting consideration of alternative uses of SRT data. Damage/stim cycling, an unfortunately common LA basin injector operational practice, was analyzed in detail, focusing on inefficiencies resulting from damage accumulation followed by unintentional selective stimulation of lowest priority subzones. Since the industry's common goals would be advanced by a basin wide decrease of damage/stim cycling, improvements are suggested for the consideration of both operators and regulators. Operators with best record for maximizing injection efficiency reduce damage/stim cycling by maintaining appropriate water quality standards and optimized injection profiles. When coupled with aggressive production well management policies which minimize low OWR production, e.g., shutting in highest water cut wells, even very mature waterfloods have been shown to perform without rapid BOPD declines. Paper concludes with practical suggestions to accomplish DOGGR/UIC fracture-prevention goals utilizing traditional and/or modified SRT designs to generate a permitted "Maximum Allowable Rate" (MAR). In addition to avoiding unintended consequences of the traditional MASP permitting approach, waterflood management and regulation by MAR would encourage development of new technologies and operational practices.

Groundwater quality near the Montebello Oil Field, Los Angeles County, California

“…Injection well pressures, historical depletion of reservoir pressures and any resulting subsidence, and the distribution of net volumes of injection and production in oil reservoirs in the Montebello Oil Field are factors to consider in assessing the potential for fluid migration out of oil-and gas-production zones. Swanson (2018) described conditions in water-flooded oil fields in the Los Angeles Basin, including Montebello, that were operating near injection capacity. Those oil fields have many wells operating near or at the maximum allowable surface pressure, which is approximately 95 percent of the formation fracture pressure.…”

“…Those oil fields have many wells operating near or at the maximum allowable surface pressure, which is approximately 95 percent of the formation fracture pressure. Because injection well efficiencies change through time as wells become clogged (Swanson, 2018), injection rates must be reduced to keep pressures below maximum allowable values until wells are periodically stimulated, using acid washing to remove clogs. Therefore, injection rates and well efficiencies require careful management.…”