A new technique is presented which aids in maximizing water injection rates in wells recently converted to injection, thereby accelerating offset oil production response. This method is applicable to waterflood operations where injection wells are surface pressure controlled and where bottom-hole injection just below formation parting pressure (FPP) is desired. parting pressure (FPP) is desired. Two plotting methods are shown to be Instrumental in monitoring the acceleration of fill-up and average reservoir pressure growth in an actual field case. One is the Hall method: plotting a function of bottom-hole injection pressure versus cumulative water injection, and the other was Introduced by Hearn: plotting the inverse injectivity index versus cumulative water Injection.
After initiating injection into several converted wells at pressure limits well below the average offset parting pressures, periodic surface pressure increases were made at each well over a pressure increases were made at each well over a period of several months. Monitoring the Hall and period of several months. Monitoring the Hall and Hearn plots as the pressure and rates increased rendered qualitative interpretations of whether the rates were being maintained below FPP. Accelerated reservoir pressure growth was achieved which resulted In earlier than expected offset oil response.
Application of these plots also reduce, and sometimes eliminate, the need to perform periodic step-rate tests designed to determine FPP during the injection start-up period, thereby significantly reducing operating costs.
An upcoming 30-well conversion program (from producing wells to water injection wells) in a mature producing wells to water injection wells) in a mature patterned-flood presented the si situation where an patterned-flood presented the si situation where an engineer would have to devote considerable dime monitoring injection rates, pressures and volumes to decide timing for periodic step-rate tests Applying accepted guidelines for scheduling step-rate tests during conversion start-up, as introduced by Robertson and Kelm, suggests an average of three tests per well per year to keep bottom-hole injection pressures as close to FPP as poss possible. Since FPP increases as continued water poss possible. Since FPP increases as continued water injection increases average reservoir pressure, it was recognized that, ideally, injection pressures could be increased on a more frequent basis pressures could be increased on a more frequent basis (i.e., monthly) instead of three times per year during conversion start-up.
The challenge was finding a valid, cost effective method to increase injection pressures frequently and at small enough increments to remain below FPP throughout the first year after conversion. The benefits would be:
accelerated injection rate and average reservoir pressure build-up to accelerate offset oil response, and
lower operating costs from reducing the number of step-rate tests from three to one during the conversion start-up period. period.
Building and monitoring Hall and Hearn plots from daily surface injection rates and pressures allowed these small monthly pressure increases to be made for each converted well. Only one steprate test was run on each well when bottom-hole Injection pressures approached the average of the existing offset injection wells, approximately nine to twelve months after initial injection.