A significant proportion of the future oil production is expected to be driven by water injectors and in reservoirs that are sand prone. Achieving sweep efficiency and sand control in such formations is challenging. In many cases the ideal sand control is no sand control (e.g., a C&P completion) requiring rigorous sanding assessment. Sand production in injectors often go unnoticed until it is too late (sand covering the pay) making it difficult to ascertain the specific set of conditions resulting in sanding and the severity of the individual sanding episodes. Based on physics and mechanisms governing sanding, general factors can be postulated on the causes of sanding but not in a quantitative form. To provide a deeper insight into this matter, a numerical study has been undertaken to model sanding in injectors with due account of several inter-coupled factors including injection pressure, crossflow, waterhammer pressure pulses, degradation of the formation matrix resulting from repeated shut-downs and others.
This paper describes the concepts used for sand production modelling and shows application of the model to a field problem involving a C&P completion in a sand prone reservoir. The results show the mode and magnitude of sanding are influenced by the rock properties, injection operations and the equipment type and installation. The cases analyzed indicate a correspondence between the rate of shut-in and the onset of sanding. In cases involving unconsolidated sands, the waterhammer effects have a pronounced impact on sanding. Sand control can be omitted in even very weak rocks provided the injection pressure is optimized, frequency of hard shut-downs is controlled and hardware is positioned in such a manner to reduce the waterhammer pressure pulse magnitude. The proposed modeling can be used in determining the sand sump capacity required over the projected life of the well.
In comparison with the vast reported studies devoted to sand production in producers since the early 80's 1–7, very little attention has been paid to sanding in injector wells. There are considerable differences in sanding between injectors and producers. In the latter, generally high depletion and drawdown may lead to rock disaggregation that is a necessary condition for sanding. Thereafter the bean-up strategy, drawdown magnitude and watercut are the predominant factors influencing sanding 6.
In water injectors, there is no depletion, so any rock disaggregation would be the result of near wellbore degradation brought about by high injection, which may lead to fracturing and/or shear failure, and the pressure cycles associated with repeated shut-downs and waterhammer effects. Potential for sanding episodes occur only over the short periods immediately after a shut-down while crossflow and/or backflow along with any waterhammer pulses remain active.
Whereas in producers any disaggregated sand near the wellbore can become compacted with drawdown, in injectors, the near wellbore sand is likely to be under a very low effective stress state and thus quite susceptible to production. Therefore, the pressure gradient required to create sanding in injectors upon shut-down is expected to be much smaller than the same under normal production operation.
Unless flowrates are insufficient to transport sand to the surface, in producers any sand production can normally be detected by a variety of methods such as collecting samples at the surface, sand detectors and other evidences (sand in separators, erosion of tubing along the production line and other hardware such as chokes etc). This allows the operators to change strategies to mitigate sanding and monitor future activities more closely. In injectors, on the other hand, any sand will go down the bottom of the well and the first real evidence of its occurrence typically surfaces with a significant loss in the injection performance which is a strong indication of sand covering the pay interval. Owing to this, very little direct field data are available as to when sanding in injectors started and how much sand was brought in with each shut-down and which factors played the key role so to assist in developing effective sand mitigation practices. Among the rather sparse literature on sanding in injectors, are the work published by Morita et al8 and Santarelli et al9.