A decline in injection rates was observed in several waterflood injection wells located in Stephens County, Oklahoma. Analysis of pressure fall-off data indicated possible damage in the fracture and/or along possible damage in the fracture and/or along the fracture face in several wells although the calculated skin effect values were negative. The large negative skin effect values were attributed to the intentionally or accidentally created fractures and highly dipping formations. After stimulation treatments, injection rates improved substantially.
A faster than expected decline in water injection rates was observed in several wells of the VESS Unit (Velma Field, Velma, Oklahoma) soon after the initiation of the waterflood operation. The injectors, in general, started at an injection rate in the 3500–5500 BWPD range at a surface pressure in the 1000–1200 psi range. In less than a year, the injection pressure had to be increased to the 2200–2500 psi range to maintain the injection rate. At this injection pressure, a slight increase in injection rate was usually observed. This increase is believed to be associated with "formation parting" or fracturing. The injection rate remained fairly constant for nearly a year. After that the injection rate started to decline from the initial 3500–5500 BWPD range to the 400–1000 BWPD range.
This decline usually occurred over a two year period and as a result of this decline in the total injected volume of water, the associated oil production from the unit also decreased.
A study was initiated in early 1976 to:
determine the reasons for the decrease in water injection rates
design a stimulation treatment to increase the injection rates
A total of fifteen wells were originally included in this study. Most of these wells were not located in critical areas where an immediate increase in injectivity was urgent, but it was felt that all of the wells in the unit were facing a similar problem - decline in water injectivity.
The study included simultaneous analysis of all available pressure fall-off data by different methods (semi-log, log-log, and Cartesian plots), chemical analysis of the injection water and backflowed samples, as well as acid reaction tests on representative core samples. In addition, all available radioactive injection profiles were studied.
In order to determine the reasons for the decline in injectivity it was felt essential to consider the various parameters which would influence the calculated skin factor values for the wells under investigation.
The skin factor value obtained from field data is the total skin factor (St). This skin factors includes the true skin factor (Sd) due to the actual condition of the wells and the pseudoskin factor (S) due to the inclination of the well or fracture:
St = Sd + S . . . . (1)
The true skin factor in itself consists of several skin factors such as those caused by partial penetration, perforations, non-Darcy flow, presence of fracture, formation or fracture face, damage, etc.
