Reservoir Fracture Gradient Estimation and the Associated Uncertainties Using 4D Seismic and Well Data

Extraction of fluids from a reservoir results in pressure depletion and subsequent changes in the stress and strain fields. As the reservoir depletes, the fracture gradient reduces and narrows the window between wellbore stability and acceptable losses in the well. We investigate the effects of geology and subsurface conditions on fracture gradient. Spatial information from time-lapse seismic data was used to improve predrill prediction of fracture gradient and to evaluate the possible changes in fracture gradient due to depletion effects. A fast-track method to classify 4D seismic responses into different production mechanisms by analyzing both 3D and 4D AVO responses is demonstrated in this paper and applied to the Ringhorne field in the North Sea. Results are validated using depletion induced time shifts and production data from wells. We find that low stress sensitivity of the reservoir rock, coupled with strong support from the regional aquifer shows the loss of circulation recorded in wells is not attributed to depletion effects but due to an over-estimation of the pre-production fracture gradient. This is further supported by lower fracture gradient estimates in the sand intervals using well log, rock mechanics and seismic data compared to the measured LOT data.