Application of Novel Techniques to Fractured Injection Diagnostics in Waterflood Developments
Authors: Nwokolo Christopher 
Published: July 2013

Controlled injection at high rates predominantly under fracture regime has been identified at the onset of most waterflood field developments as being crucial to meeting the multiple objectives of pressure maintenance, voidage replacement, sweep optimization and injector longevity. In the vast majority of these developments, a deviation from fractured injection into matrix injection mode has been identified as a primary cause for the onset of injectivity decline. Identifying and clearly delineating injection regimes therefore presents an opportunity to arrest early decline. However the identification and delineation of injection regimes has remained a big challenge.

This paper presents a high resolution and novel technique of delineating injection regimes. Traditionally LOTs, PFOs, SRTs and Mini-Fracs have been used to estimate fracture parameters in competent formations; however in soft Miocene reservoir systems that typifies the Gulf of Guinea depositional system most software and modeling workflow applied in classic fracture mechanics cannot and do not accurately model the mechanical behavior of these sediments under stress.

While the Hall Plot has been widely applied in delineating changes in injection regimes (from fractured to matrix) it has the limitation of not been able to detect subtle changes in injection regimes over short time periods. This paper showcases a novel modification to the Hall plot to provide a high resolution delineation of injection regime. The second order derivative of the plot of the cumulative work per unit time versus the cumulative injection amplifies subtle variations in the performance of injection wells not discernible with the traditional Hall plot. This diagnostic plot when combined with other equally apt diagnostic plots such as a time-lapse plot of the reciprocal of injectivity decline will provide a definite identification of the prevailing injection regime. An added advantage of these plots is the ability to provide real-time detection of the onset of injection damage and also allow a comparison of damage signature across time intervals and across injection wells which could provide additional information as to the nature of the impairment. All these combined will facilitate early identification of possible damage mechanism and a quick response to injectivity decline in terms of topsides or downhole remediation.

This technique was applied in the analysis of five injectors in a deepwater waterflood development in the Gulf of Guinea. The result was a clear and concise delineation of fracture and matrix injection regimes and the start of the transition period. Analysis indicated that early injection period had been predominantly under fracture injection for most wells (during the first 6–12months) while subsequent injection has been under brief spells of fractured injection followed by rapid transitions to predominantly matrix injection regimes which coincided with periods of injectivity decline. This diagnostic plot was also compared to the Hall plot and demonstrated that in the majority of cases, the changes in injector performance and injection regime starts way before it is detected by the Hall plot.