| Popis: |
A newly developed oil field requires water injection for pressure support. Due to the relatively low permeability of the sandstone reservoir (10-15 mD), it was decided to perform propped fracture stimulation on all the injection wells before starting up the water injection operations. Performing propped fracture stimulation of water injection wells before starting up long term injection has several benefits, including lower initial injection pressures, less sensitivity to poor water quality and removing the risk of not being able to inject under fracturing conditions using the normal water injection system. The initial attempts to perform fracture stimulations in the wells were not successful, as it was not possible to initiate a fracture (fracture breakdown) in 2 of the 4 wells, even after re-perforation. In the other two wells, it was possible to perform an injectivity test and a fracture treatment. In order to determine the cause of the problem, the data from the failed breakdown injection attempts for the first 2 wells was used as a welltest, and the pressure during injection and decline was history matched using a simple reservoir simulation model. This modeling confirmed that the reservoir permeability was not the problem, but that there was a significant skin present in the well, even after re-perforating. There was some type of formation damage present around the well which was deeper than the penetration depth of the perforations. Finally, performing an additional hydrochloric acid squeeze and low rate fracture initiation sequence, it was possible to stimulate all the wells. Post-frac injection tests were performed for all wells, also with the collection of pressure decline data. The injectivity tests were analyzed using a numeric reservoir simulation model which includes an explicit gridded representation of the propped fracture. History matching of the flowing and buildup periods confirmed the propped fracture dimensions and the reservoir kh, which was as expected (or better) in all cases. In the paper, the mechanisms that can prevent fracture breakdown even at pressures exceeding the overburden stress are explained in detail, as this issue occurs often enough to be of general concern. Methods of analyzing a failed breakdown injection are shown in detail, including how to diagnose whether the problem lies with the completion (no open perforations) or with the reservoir. The analysis of the fracture treatment data is shown, as well as the analysis of the post-frac water injection tests and initial injection data. Recommendations are made on the design of propped fractures for water injection well stimulation. |
