Posteriori assessment of fracture propagation in refractured vertical oil wells by pressure transient analysis

Oil companies have realized that refracturing old wells is the more budget-friendly way to achieve cash flow than drilling and completing new wells in the down turn of the energy industry. But it has been seen that the refracturing technology does not always work equally well in all reservoir conditions. It is desirable to have a quantitative method for assessment of well refracturing design and operations. An analytical transient flow model was derived in this study to estimate the orthogonality between the old and new hydraulic fractures after refracturing. Pressure transient data from seven wells refractured in an oilfield in the southwestern China was analyzed with the model. The model implies that orthogonal fractures created in well refracturing treatments induce orthogonal reservoir linear flow in the stimulated reservoir volume. This fracture configuration can be identified by the unit slope of pressure derivative in the log-log diagnosis plot. The angle between fractures initiated from wellbore can be estimated using pressure transient data analysis technique with a log-log diagnostic plot, with extreme situations that merged or parallel fractures will show a half slope and orthogonal fractures will show a unit slope. Case studies of data from 7 wells show that orthogonal fractures were created in 3 wells, merging or parallel fractures were created in 1 well, and fractures with non-right angles were created in 3 wells. The actual well productivity data for the 7 wells are basically consistent with the expected trend implied by the estimated fracture configurations with an exception of one well. The reason is not clear and needs further investigations. More production data from refractured wells are required to further validate the technique.